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This is gdb.info, produced by makeinfo version 6.5 from gdb.texinfo.
Copyright (C) 1988-2020 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
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any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software" and "Free Software Needs Free
Documentation", with the Front-Cover Texts being "A GNU Manual," and
with the Back-Cover Texts as in (a) below.
(a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual. Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom."
INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Gdb: (gdb). The GNU debugger.
* gdbserver: (gdb) Server. The GNU debugging server.
END-INFO-DIR-ENTRY
This file documents the GNU debugger GDB.
This is the Tenth Edition, of 'Debugging with GDB: the GNU
Source-Level Debugger' for GDB (GDB) Version 9.1.
Copyright (C) 1988-2020 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software" and "Free Software Needs Free
Documentation", with the Front-Cover Texts being "A GNU Manual," and
with the Back-Cover Texts as in (a) below.
(a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual. Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom."

File: gdb.info, Node: Pascal, Next: Rust, Prev: Fortran, Up: Supported Languages
15.4.7 Pascal
-------------
Debugging Pascal programs which use sets, subranges, file variables, or
nested functions does not currently work. GDB does not support entering
expressions, printing values, or similar features using Pascal syntax.
The Pascal-specific command 'set print pascal_static-members'
controls whether static members of Pascal objects are displayed. *Note
pascal_static-members: Print Settings.

File: gdb.info, Node: Rust, Next: Modula-2, Prev: Pascal, Up: Supported Languages
15.4.8 Rust
-----------
GDB supports the Rust Programming Language (https://www.rust-lang.org/).
Type- and value-printing, and expression parsing, are reasonably
complete. However, there are a few peculiarities and holes to be aware
of.
* Linespecs (*note Specify Location::) are never relative to the
current crate. Instead, they act as if there were a global
namespace of crates, somewhat similar to the way 'extern crate'
behaves.
That is, if GDB is stopped at a breakpoint in a function in crate
'A', module 'B', then 'break B::f' will attempt to set a breakpoint
in a function named 'f' in a crate named 'B'.
As a consequence of this approach, linespecs also cannot refer to
items using 'self::' or 'super::'.
* Because GDB implements Rust name-lookup semantics in expressions,
it will sometimes prepend the current crate to a name. For
example, if GDB is stopped at a breakpoint in the crate 'K', then
'print ::x::y' will try to find the symbol 'K::x::y'.
However, since it is useful to be able to refer to other crates
when debugging, GDB provides the 'extern' extension to circumvent
this. To use the extension, just put 'extern' before a path
expression to refer to the otherwise unavailable "global" scope.
In the above example, if you wanted to refer to the symbol 'y' in
the crate 'x', you would use 'print extern x::y'.
* The Rust expression evaluator does not support "statement-like"
expressions such as 'if' or 'match', or lambda expressions.
* Tuple expressions are not implemented.
* The Rust expression evaluator does not currently implement the
'Drop' trait. Objects that may be created by the evaluator will
never be destroyed.
* GDB does not implement type inference for generics. In order to
call generic functions or otherwise refer to generic items, you
will have to specify the type parameters manually.
* GDB currently uses the C++ demangler for Rust. In most cases this
does not cause any problems. However, in an expression context,
completing a generic function name will give syntactically invalid
results. This happens because Rust requires the '::' operator
between the function name and its generic arguments. For example,
GDB might provide a completion like 'crate::f<u32>', where the
parser would require 'crate::f::<u32>'.
* As of this writing, the Rust compiler (version 1.8) has a few holes
in the debugging information it generates. These holes prevent
certain features from being implemented by GDB:
* Method calls cannot be made via traits.
* Operator overloading is not implemented.
* When debugging in a monomorphized function, you cannot use the
generic type names.
* The type 'Self' is not available.
* 'use' statements are not available, so some names may not be
available in the crate.

File: gdb.info, Node: Modula-2, Next: Ada, Prev: Rust, Up: Supported Languages
15.4.9 Modula-2
---------------
The extensions made to GDB to support Modula-2 only support output from
the GNU Modula-2 compiler (which is currently being developed). Other
Modula-2 compilers are not currently supported, and attempting to debug
executables produced by them is most likely to give an error as GDB
reads in the executable's symbol table.
* Menu:
* M2 Operators:: Built-in operators
* Built-In Func/Proc:: Built-in functions and procedures
* M2 Constants:: Modula-2 constants
* M2 Types:: Modula-2 types
* M2 Defaults:: Default settings for Modula-2
* Deviations:: Deviations from standard Modula-2
* M2 Checks:: Modula-2 type and range checks
* M2 Scope:: The scope operators '::' and '.'
* GDB/M2:: GDB and Modula-2

File: gdb.info, Node: M2 Operators, Next: Built-In Func/Proc, Up: Modula-2
15.4.9.1 Operators
..................
Operators must be defined on values of specific types. For instance,
'+' is defined on numbers, but not on structures. Operators are often
defined on groups of types. For the purposes of Modula-2, the following
definitions hold:
* _Integral types_ consist of 'INTEGER', 'CARDINAL', and their
subranges.
* _Character types_ consist of 'CHAR' and its subranges.
* _Floating-point types_ consist of 'REAL'.
* _Pointer types_ consist of anything declared as 'POINTER TO TYPE'.
* _Scalar types_ consist of all of the above.
* _Set types_ consist of 'SET' and 'BITSET' types.
* _Boolean types_ consist of 'BOOLEAN'.
The following operators are supported, and appear in order of increasing
precedence:
','
Function argument or array index separator.
':='
Assignment. The value of VAR ':=' VALUE is VALUE.
'<, >'
Less than, greater than on integral, floating-point, or enumerated
types.
'<=, >='
Less than or equal to, greater than or equal to on integral,
floating-point and enumerated types, or set inclusion on set types.
Same precedence as '<'.
'=, <>, #'
Equality and two ways of expressing inequality, valid on scalar
types. Same precedence as '<'. In GDB scripts, only '<>' is
available for inequality, since '#' conflicts with the script
comment character.
'IN'
Set membership. Defined on set types and the types of their
members. Same precedence as '<'.
'OR'
Boolean disjunction. Defined on boolean types.
'AND, &'
Boolean conjunction. Defined on boolean types.
'@'
The GDB "artificial array" operator (*note Expressions:
Expressions.).
'+, -'
Addition and subtraction on integral and floating-point types, or
union and difference on set types.
'*'
Multiplication on integral and floating-point types, or set
intersection on set types.
'/'
Division on floating-point types, or symmetric set difference on
set types. Same precedence as '*'.
'DIV, MOD'
Integer division and remainder. Defined on integral types. Same
precedence as '*'.
'-'
Negative. Defined on 'INTEGER' and 'REAL' data.
'^'
Pointer dereferencing. Defined on pointer types.
'NOT'
Boolean negation. Defined on boolean types. Same precedence as
'^'.
'.'
'RECORD' field selector. Defined on 'RECORD' data. Same
precedence as '^'.
'[]'
Array indexing. Defined on 'ARRAY' data. Same precedence as '^'.
'()'
Procedure argument list. Defined on 'PROCEDURE' objects. Same
precedence as '^'.
'::, .'
GDB and Modula-2 scope operators.
_Warning:_ Set expressions and their operations are not yet
supported, so GDB treats the use of the operator 'IN', or the use
of operators '+', '-', '*', '/', '=', , '<>', '#', '<=', and '>='
on sets as an error.

File: gdb.info, Node: Built-In Func/Proc, Next: M2 Constants, Prev: M2 Operators, Up: Modula-2
15.4.9.2 Built-in Functions and Procedures
..........................................
Modula-2 also makes available several built-in procedures and functions.
In describing these, the following metavariables are used:
A
represents an 'ARRAY' variable.
C
represents a 'CHAR' constant or variable.
I
represents a variable or constant of integral type.
M
represents an identifier that belongs to a set. Generally used in
the same function with the metavariable S. The type of S should be
'SET OF MTYPE' (where MTYPE is the type of M).
N
represents a variable or constant of integral or floating-point
type.
R
represents a variable or constant of floating-point type.
T
represents a type.
V
represents a variable.
X
represents a variable or constant of one of many types. See the
explanation of the function for details.
All Modula-2 built-in procedures also return a result, described
below.
'ABS(N)'
Returns the absolute value of N.
'CAP(C)'
If C is a lower case letter, it returns its upper case equivalent,
otherwise it returns its argument.
'CHR(I)'
Returns the character whose ordinal value is I.
'DEC(V)'
Decrements the value in the variable V by one. Returns the new
value.
'DEC(V,I)'
Decrements the value in the variable V by I. Returns the new
value.
'EXCL(M,S)'
Removes the element M from the set S. Returns the new set.
'FLOAT(I)'
Returns the floating point equivalent of the integer I.
'HIGH(A)'
Returns the index of the last member of A.
'INC(V)'
Increments the value in the variable V by one. Returns the new
value.
'INC(V,I)'
Increments the value in the variable V by I. Returns the new
value.
'INCL(M,S)'
Adds the element M to the set S if it is not already there.
Returns the new set.
'MAX(T)'
Returns the maximum value of the type T.
'MIN(T)'
Returns the minimum value of the type T.
'ODD(I)'
Returns boolean TRUE if I is an odd number.
'ORD(X)'
Returns the ordinal value of its argument. For example, the
ordinal value of a character is its ASCII value (on machines
supporting the ASCII character set). The argument X must be of an
ordered type, which include integral, character and enumerated
types.
'SIZE(X)'
Returns the size of its argument. The argument X can be a variable
or a type.
'TRUNC(R)'
Returns the integral part of R.
'TSIZE(X)'
Returns the size of its argument. The argument X can be a variable
or a type.
'VAL(T,I)'
Returns the member of the type T whose ordinal value is I.
_Warning:_ Sets and their operations are not yet supported, so GDB
treats the use of procedures 'INCL' and 'EXCL' as an error.

File: gdb.info, Node: M2 Constants, Next: M2 Types, Prev: Built-In Func/Proc, Up: Modula-2
15.4.9.3 Constants
..................
GDB allows you to express the constants of Modula-2 in the following
ways:
* Integer constants are simply a sequence of digits. When used in an
expression, a constant is interpreted to be type-compatible with
the rest of the expression. Hexadecimal integers are specified by
a trailing 'H', and octal integers by a trailing 'B'.
* Floating point constants appear as a sequence of digits, followed
by a decimal point and another sequence of digits. An optional
exponent can then be specified, in the form 'E[+|-]NNN', where
'[+|-]NNN' is the desired exponent. All of the digits of the
floating point constant must be valid decimal (base 10) digits.
* Character constants consist of a single character enclosed by a
pair of like quotes, either single (''') or double ('"'). They may
also be expressed by their ordinal value (their ASCII value,
usually) followed by a 'C'.
* String constants consist of a sequence of characters enclosed by a
pair of like quotes, either single (''') or double ('"'). Escape
sequences in the style of C are also allowed. *Note C and C++
Constants: C Constants, for a brief explanation of escape
sequences.
* Enumerated constants consist of an enumerated identifier.
* Boolean constants consist of the identifiers 'TRUE' and 'FALSE'.
* Pointer constants consist of integral values only.
* Set constants are not yet supported.

File: gdb.info, Node: M2 Types, Next: M2 Defaults, Prev: M2 Constants, Up: Modula-2
15.4.9.4 Modula-2 Types
.......................
Currently GDB can print the following data types in Modula-2 syntax:
array types, record types, set types, pointer types, procedure types,
enumerated types, subrange types and base types. You can also print the
contents of variables declared using these type. This section gives a
number of simple source code examples together with sample GDB sessions.
The first example contains the following section of code:
VAR
s: SET OF CHAR ;
r: [20..40] ;
and you can request GDB to interrogate the type and value of 'r' and
's'.
(gdb) print s
{'A'..'C', 'Z'}
(gdb) ptype s
SET OF CHAR
(gdb) print r
21
(gdb) ptype r
[20..40]
Likewise if your source code declares 's' as:
VAR
s: SET ['A'..'Z'] ;
then you may query the type of 's' by:
(gdb) ptype s
type = SET ['A'..'Z']
Note that at present you cannot interactively manipulate set expressions
using the debugger.
The following example shows how you might declare an array in
Modula-2 and how you can interact with GDB to print its type and
contents:
VAR
s: ARRAY [-10..10] OF CHAR ;
(gdb) ptype s
ARRAY [-10..10] OF CHAR
Note that the array handling is not yet complete and although the
type is printed correctly, expression handling still assumes that all
arrays have a lower bound of zero and not '-10' as in the example above.
Here are some more type related Modula-2 examples:
TYPE
colour = (blue, red, yellow, green) ;
t = [blue..yellow] ;
VAR
s: t ;
BEGIN
s := blue ;
The GDB interaction shows how you can query the data type and value of a
variable.
(gdb) print s
$1 = blue
(gdb) ptype t
type = [blue..yellow]
In this example a Modula-2 array is declared and its contents displayed.
Observe that the contents are written in the same way as their 'C'
counterparts.
VAR
s: ARRAY [1..5] OF CARDINAL ;
BEGIN
s[1] := 1 ;
(gdb) print s
$1 = {1, 0, 0, 0, 0}
(gdb) ptype s
type = ARRAY [1..5] OF CARDINAL
The Modula-2 language interface to GDB also understands pointer types
as shown in this example:
VAR
s: POINTER TO ARRAY [1..5] OF CARDINAL ;
BEGIN
NEW(s) ;
s^[1] := 1 ;
and you can request that GDB describes the type of 's'.
(gdb) ptype s
type = POINTER TO ARRAY [1..5] OF CARDINAL
GDB handles compound types as we can see in this example. Here we
combine array types, record types, pointer types and subrange types:
TYPE
foo = RECORD
f1: CARDINAL ;
f2: CHAR ;
f3: myarray ;
END ;
myarray = ARRAY myrange OF CARDINAL ;
myrange = [-2..2] ;
VAR
s: POINTER TO ARRAY myrange OF foo ;
and you can ask GDB to describe the type of 's' as shown below.
(gdb) ptype s
type = POINTER TO ARRAY [-2..2] OF foo = RECORD
f1 : CARDINAL;
f2 : CHAR;
f3 : ARRAY [-2..2] OF CARDINAL;
END

File: gdb.info, Node: M2 Defaults, Next: Deviations, Prev: M2 Types, Up: Modula-2
15.4.9.5 Modula-2 Defaults
..........................
If type and range checking are set automatically by GDB, they both
default to 'on' whenever the working language changes to Modula-2. This
happens regardless of whether you or GDB selected the working language.
If you allow GDB to set the language automatically, then entering
code compiled from a file whose name ends with '.mod' sets the working
language to Modula-2. *Note Having GDB Infer the Source Language:
Automatically, for further details.

File: gdb.info, Node: Deviations, Next: M2 Checks, Prev: M2 Defaults, Up: Modula-2
15.4.9.6 Deviations from Standard Modula-2
..........................................
A few changes have been made to make Modula-2 programs easier to debug.
This is done primarily via loosening its type strictness:
* Unlike in standard Modula-2, pointer constants can be formed by
integers. This allows you to modify pointer variables during
debugging. (In standard Modula-2, the actual address contained in
a pointer variable is hidden from you; it can only be modified
through direct assignment to another pointer variable or expression
that returned a pointer.)
* C escape sequences can be used in strings and characters to
represent non-printable characters. GDB prints out strings with
these escape sequences embedded. Single non-printable characters
are printed using the 'CHR(NNN)' format.
* The assignment operator (':=') returns the value of its right-hand
argument.
* All built-in procedures both modify _and_ return their argument.

File: gdb.info, Node: M2 Checks, Next: M2 Scope, Prev: Deviations, Up: Modula-2
15.4.9.7 Modula-2 Type and Range Checks
.......................................
_Warning:_ in this release, GDB does not yet perform type or range
checking.
GDB considers two Modula-2 variables type equivalent if:
* They are of types that have been declared equivalent via a 'TYPE T1
= T2' statement
* They have been declared on the same line. (Note: This is true of
the GNU Modula-2 compiler, but it may not be true of other
compilers.)
As long as type checking is enabled, any attempt to combine variables
whose types are not equivalent is an error.
Range checking is done on all mathematical operations, assignment,
array index bounds, and all built-in functions and procedures.

File: gdb.info, Node: M2 Scope, Next: GDB/M2, Prev: M2 Checks, Up: Modula-2
15.4.9.8 The Scope Operators '::' and '.'
.........................................
There are a few subtle differences between the Modula-2 scope operator
('.') and the GDB scope operator ('::'). The two have similar syntax:
MODULE . ID
SCOPE :: ID
where SCOPE is the name of a module or a procedure, MODULE the name of a
module, and ID is any declared identifier within your program, except
another module.
Using the '::' operator makes GDB search the scope specified by SCOPE
for the identifier ID. If it is not found in the specified scope, then
GDB searches all scopes enclosing the one specified by SCOPE.
Using the '.' operator makes GDB search the current scope for the
identifier specified by ID that was imported from the definition module
specified by MODULE. With this operator, it is an error if the
identifier ID was not imported from definition module MODULE, or if ID
is not an identifier in MODULE.

File: gdb.info, Node: GDB/M2, Prev: M2 Scope, Up: Modula-2
15.4.9.9 GDB and Modula-2
.........................
Some GDB commands have little use when debugging Modula-2 programs.
Five subcommands of 'set print' and 'show print' apply specifically to C
and C++: 'vtbl', 'demangle', 'asm-demangle', 'object', and 'union'. The
first four apply to C++, and the last to the C 'union' type, which has
no direct analogue in Modula-2.
The '@' operator (*note Expressions: Expressions.), while available
with any language, is not useful with Modula-2. Its intent is to aid
the debugging of "dynamic arrays", which cannot be created in Modula-2
as they can in C or C++. However, because an address can be specified
by an integral constant, the construct '{TYPE}ADREXP' is still useful.
In GDB scripts, the Modula-2 inequality operator '#' is interpreted
as the beginning of a comment. Use '<>' instead.

File: gdb.info, Node: Ada, Prev: Modula-2, Up: Supported Languages
15.4.10 Ada
-----------
The extensions made to GDB for Ada only support output from the GNU Ada
(GNAT) compiler. Other Ada compilers are not currently supported, and
attempting to debug executables produced by them is most likely to be
difficult.
* Menu:
* Ada Mode Intro:: General remarks on the Ada syntax
and semantics supported by Ada mode
in GDB.
* Omissions from Ada:: Restrictions on the Ada expression syntax.
* Additions to Ada:: Extensions of the Ada expression syntax.
* Overloading support for Ada:: Support for expressions involving overloaded
subprograms.
* Stopping Before Main Program:: Debugging the program during elaboration.
* Ada Exceptions:: Ada Exceptions
* Ada Tasks:: Listing and setting breakpoints in tasks.
* Ada Tasks and Core Files:: Tasking Support when Debugging Core Files
* Ravenscar Profile:: Tasking Support when using the Ravenscar
Profile
* Ada Settings:: New settable GDB parameters for Ada.
* Ada Glitches:: Known peculiarities of Ada mode.

File: gdb.info, Node: Ada Mode Intro, Next: Omissions from Ada, Up: Ada
15.4.10.1 Introduction
......................
The Ada mode of GDB supports a fairly large subset of Ada expression
syntax, with some extensions. The philosophy behind the design of this
subset is
* That GDB should provide basic literals and access to operations for
arithmetic, dereferencing, field selection, indexing, and
subprogram calls, leaving more sophisticated computations to
subprograms written into the program (which therefore may be called
from GDB).
* That type safety and strict adherence to Ada language restrictions
are not particularly important to the GDB user.
* That brevity is important to the GDB user.
Thus, for brevity, the debugger acts as if all names declared in
user-written packages are directly visible, even if they are not visible
according to Ada rules, thus making it unnecessary to fully qualify most
names with their packages, regardless of context. Where this causes
ambiguity, GDB asks the user's intent.
The debugger will start in Ada mode if it detects an Ada main
program. As for other languages, it will enter Ada mode when stopped in
a program that was translated from an Ada source file.
While in Ada mode, you may use '--' for comments. This is useful
mostly for documenting command files. The standard GDB comment ('#')
still works at the beginning of a line in Ada mode, but not in the
middle (to allow based literals).

File: gdb.info, Node: Omissions from Ada, Next: Additions to Ada, Prev: Ada Mode Intro, Up: Ada
15.4.10.2 Omissions from Ada
............................
Here are the notable omissions from the subset:
* Only a subset of the attributes are supported:
- 'First, 'Last, and 'Length on array objects (not on types and
subtypes).
- 'Min and 'Max.
- 'Pos and 'Val.
- 'Tag.
- 'Range on array objects (not subtypes), but only as the right
operand of the membership ('in') operator.
- 'Access, 'Unchecked_Access, and 'Unrestricted_Access (a GNAT
extension).
- 'Address.
* The names in 'Characters.Latin_1' are not available and
concatenation is not implemented. Thus, escape characters in
strings are not currently available.
* Equality tests ('=' and '/=') on arrays test for bitwise equality
of representations. They will generally work correctly for strings
and arrays whose elements have integer or enumeration types. They
may not work correctly for arrays whose element types have
user-defined equality, for arrays of real values (in particular,
IEEE-conformant floating point, because of negative zeroes and
NaNs), and for arrays whose elements contain unused bits with
indeterminate values.
* The other component-by-component array operations ('and', 'or',
'xor', 'not', and relational tests other than equality) are not
implemented.
* There is limited support for array and record aggregates. They are
permitted only on the right sides of assignments, as in these
examples:
(gdb) set An_Array := (1, 2, 3, 4, 5, 6)
(gdb) set An_Array := (1, others => 0)
(gdb) set An_Array := (0|4 => 1, 1..3 => 2, 5 => 6)
(gdb) set A_2D_Array := ((1, 2, 3), (4, 5, 6), (7, 8, 9))
(gdb) set A_Record := (1, "Peter", True);
(gdb) set A_Record := (Name => "Peter", Id => 1, Alive => True)
Changing a discriminant's value by assigning an aggregate has an
undefined effect if that discriminant is used within the record.
However, you can first modify discriminants by directly assigning
to them (which normally would not be allowed in Ada), and then
performing an aggregate assignment. For example, given a variable
'A_Rec' declared to have a type such as:
type Rec (Len : Small_Integer := 0) is record
Id : Integer;
Vals : IntArray (1 .. Len);
end record;
you can assign a value with a different size of 'Vals' with two
assignments:
(gdb) set A_Rec.Len := 4
(gdb) set A_Rec := (Id => 42, Vals => (1, 2, 3, 4))
As this example also illustrates, GDB is very loose about the usual
rules concerning aggregates. You may leave out some of the
components of an array or record aggregate (such as the 'Len'
component in the assignment to 'A_Rec' above); they will retain
their original values upon assignment. You may freely use dynamic
values as indices in component associations. You may even use
overlapping or redundant component associations, although which
component values are assigned in such cases is not defined.
* Calls to dispatching subprograms are not implemented.
* The overloading algorithm is much more limited (i.e., less
selective) than that of real Ada. It makes only limited use of the
context in which a subexpression appears to resolve its meaning,
and it is much looser in its rules for allowing type matches. As a
result, some function calls will be ambiguous, and the user will be
asked to choose the proper resolution.
* The 'new' operator is not implemented.
* Entry calls are not implemented.
* Aside from printing, arithmetic operations on the native VAX
floating-point formats are not supported.
* It is not possible to slice a packed array.
* The names 'True' and 'False', when not part of a qualified name,
are interpreted as if implicitly prefixed by 'Standard', regardless
of context. Should your program redefine these names in a package
or procedure (at best a dubious practice), you will have to use
fully qualified names to access their new definitions.

File: gdb.info, Node: Additions to Ada, Next: Overloading support for Ada, Prev: Omissions from Ada, Up: Ada
15.4.10.3 Additions to Ada
..........................
As it does for other languages, GDB makes certain generic extensions to
Ada (*note Expressions::):
* If the expression E is a variable residing in memory (typically a
local variable or array element) and N is a positive integer, then
'E@N' displays the values of E and the N-1 adjacent variables
following it in memory as an array. In Ada, this operator is
generally not necessary, since its prime use is in displaying parts
of an array, and slicing will usually do this in Ada. However,
there are occasional uses when debugging programs in which certain
debugging information has been optimized away.
* 'B::VAR' means "the variable named VAR that appears in function or
file B." When B is a file name, you must typically surround it in
single quotes.
* The expression '{TYPE} ADDR' means "the variable of type TYPE that
appears at address ADDR."
* A name starting with '$' is a convenience variable (*note
Convenience Vars::) or a machine register (*note Registers::).
In addition, GDB provides a few other shortcuts and outright
additions specific to Ada:
* The assignment statement is allowed as an expression, returning its
right-hand operand as its value. Thus, you may enter
(gdb) set x := y + 3
(gdb) print A(tmp := y + 1)
* The semicolon is allowed as an "operator," returning as its value
the value of its right-hand operand. This allows, for example,
complex conditional breaks:
(gdb) break f
(gdb) condition 1 (report(i); k += 1; A(k) > 100)
* Rather than use catenation and symbolic character names to
introduce special characters into strings, one may instead use a
special bracket notation, which is also used to print strings. A
sequence of characters of the form '["XX"]' within a string or
character literal denotes the (single) character whose numeric
encoding is XX in hexadecimal. The sequence of characters '["""]'
also denotes a single quotation mark in strings. For example,
"One line.["0a"]Next line.["0a"]"
contains an ASCII newline character ('Ada.Characters.Latin_1.LF')
after each period.
* The subtype used as a prefix for the attributes 'Pos, 'Min, and
'Max is optional (and is ignored in any case). For example, it is
valid to write
(gdb) print 'max(x, y)
* When printing arrays, GDB uses positional notation when the array
has a lower bound of 1, and uses a modified named notation
otherwise. For example, a one-dimensional array of three integers
with a lower bound of 3 might print as
(3 => 10, 17, 1)
That is, in contrast to valid Ada, only the first component has a
'=>' clause.
* You may abbreviate attributes in expressions with any unique,
multi-character subsequence of their names (an exact match gets
preference). For example, you may use a'len, a'gth, or a'lh in
place of a'length.
* Since Ada is case-insensitive, the debugger normally maps
identifiers you type to lower case. The GNAT compiler uses
upper-case characters for some of its internal identifiers, which
are normally of no interest to users. For the rare occasions when
you actually have to look at them, enclose them in angle brackets
to avoid the lower-case mapping. For example,
(gdb) print <JMPBUF_SAVE>[0]
* Printing an object of class-wide type or dereferencing an
access-to-class-wide value will display all the components of the
object's specific type (as indicated by its run-time tag).
Likewise, component selection on such a value will operate on the
specific type of the object.

File: gdb.info, Node: Overloading support for Ada, Next: Stopping Before Main Program, Prev: Additions to Ada, Up: Ada
15.4.10.4 Overloading support for Ada
.....................................
The debugger supports limited overloading. Given a subprogram call in
which the function symbol has multiple definitions, it will use the
number of actual parameters and some information about their types to
attempt to narrow the set of definitions. It also makes very limited
use of context, preferring procedures to functions in the context of the
'call' command, and functions to procedures elsewhere.
If, after narrowing, the set of matching definitions still contains
more than one definition, GDB will display a menu to query which one it
should use, for instance:
(gdb) print f(1)
Multiple matches for f
[0] cancel
[1] foo.f (integer) return boolean at foo.adb:23
[2] foo.f (foo.new_integer) return boolean at foo.adb:28
>
In this case, just select one menu entry either to cancel expression
evaluation (type '0' and press <RET>) or to continue evaluation with a
specific instance (type the corresponding number and press <RET>).
Here are a couple of commands to customize GDB's behavior in this
case:
'set ada print-signatures'
Control whether parameter types and return types are displayed in
overloads selection menus. It is 'on' by default. *Note
Overloading support for Ada::.
'show ada print-signatures'
Show the current setting for displaying parameter types and return
types in overloads selection menu. *Note Overloading support for
Ada::.

File: gdb.info, Node: Stopping Before Main Program, Next: Ada Exceptions, Prev: Overloading support for Ada, Up: Ada
15.4.10.5 Stopping at the Very Beginning
........................................
It is sometimes necessary to debug the program during elaboration, and
before reaching the main procedure. As defined in the Ada Reference
Manual, the elaboration code is invoked from a procedure called
'adainit'. To run your program up to the beginning of elaboration,
simply use the following two commands: 'tbreak adainit' and 'run'.

File: gdb.info, Node: Ada Exceptions, Next: Ada Tasks, Prev: Stopping Before Main Program, Up: Ada
15.4.10.6 Ada Exceptions
........................
A command is provided to list all Ada exceptions:
'info exceptions'
'info exceptions REGEXP'
The 'info exceptions' command allows you to list all Ada exceptions
defined within the program being debugged, as well as their
addresses. With a regular expression, REGEXP, as argument, only
those exceptions whose names match REGEXP are listed.
Below is a small example, showing how the command can be used, first
without argument, and next with a regular expression passed as an
argument.
(gdb) info exceptions
All defined Ada exceptions:
constraint_error: 0x613da0
program_error: 0x613d20
storage_error: 0x613ce0
tasking_error: 0x613ca0
const.aint_global_e: 0x613b00
(gdb) info exceptions const.aint
All Ada exceptions matching regular expression "const.aint":
constraint_error: 0x613da0
const.aint_global_e: 0x613b00
It is also possible to ask GDB to stop your program's execution when
an exception is raised. For more details, see *note Set Catchpoints::.

File: gdb.info, Node: Ada Tasks, Next: Ada Tasks and Core Files, Prev: Ada Exceptions, Up: Ada
15.4.10.7 Extensions for Ada Tasks
..................................
Support for Ada tasks is analogous to that for threads (*note
Threads::). GDB provides the following task-related commands:
'info tasks'
This command shows a list of current Ada tasks, as in the following
example:
(gdb) info tasks
ID TID P-ID Pri State Name
1 8088000 0 15 Child Activation Wait main_task
2 80a4000 1 15 Accept Statement b
3 809a800 1 15 Child Activation Wait a
* 4 80ae800 3 15 Runnable c
In this listing, the asterisk before the last task indicates it to
be the task currently being inspected.
ID
Represents GDB's internal task number.
TID
The Ada task ID.
P-ID
The parent's task ID (GDB's internal task number).
Pri
The base priority of the task.
State
Current state of the task.
'Unactivated'
The task has been created but has not been activated. It
cannot be executing.
'Runnable'
The task is not blocked for any reason known to Ada. (It
may be waiting for a mutex, though.) It is conceptually
"executing" in normal mode.
'Terminated'
The task is terminated, in the sense of ARM 9.3 (5). Any
dependents that were waiting on terminate alternatives
have been awakened and have terminated themselves.
'Child Activation Wait'
The task is waiting for created tasks to complete
activation.
'Accept Statement'
The task is waiting on an accept or selective wait
statement.
'Waiting on entry call'
The task is waiting on an entry call.
'Async Select Wait'
The task is waiting to start the abortable part of an
asynchronous select statement.
'Delay Sleep'
The task is waiting on a select statement with only a
delay alternative open.
'Child Termination Wait'
The task is sleeping having completed a master within
itself, and is waiting for the tasks dependent on that
master to become terminated or waiting on a terminate
Phase.
'Wait Child in Term Alt'
The task is sleeping waiting for tasks on terminate
alternatives to finish terminating.
'Accepting RV with TASKNO'
The task is accepting a rendez-vous with the task TASKNO.
Name
Name of the task in the program.
'info task TASKNO'
This command shows detailed informations on the specified task, as
in the following example:
(gdb) info tasks
ID TID P-ID Pri State Name
1 8077880 0 15 Child Activation Wait main_task
* 2 807c468 1 15 Runnable task_1
(gdb) info task 2
Ada Task: 0x807c468
Name: "task_1"
Thread: 0
LWP: 0x1fac
Parent: 1 ("main_task")
Base Priority: 15
State: Runnable
'task'
This command prints the ID and name of the current task.
(gdb) info tasks
ID TID P-ID Pri State Name
1 8077870 0 15 Child Activation Wait main_task
* 2 807c458 1 15 Runnable some_task
(gdb) task
[Current task is 2 "some_task"]
'task TASKNO'
This command is like the 'thread THREAD-ID' command (*note
Threads::). It switches the context of debugging from the current
task to the given task.
(gdb) info tasks
ID TID P-ID Pri State Name
1 8077870 0 15 Child Activation Wait main_task
* 2 807c458 1 15 Runnable some_task
(gdb) task 1
[Switching to task 1 "main_task"]
#0 0x8067726 in pthread_cond_wait ()
(gdb) bt
#0 0x8067726 in pthread_cond_wait ()
#1 0x8056714 in system.os_interface.pthread_cond_wait ()
#2 0x805cb63 in system.task_primitives.operations.sleep ()
#3 0x806153e in system.tasking.stages.activate_tasks ()
#4 0x804aacc in un () at un.adb:5
'break LOCATION task TASKNO'
'break LOCATION task TASKNO if ...'
These commands are like the 'break ... thread ...' command (*note
Thread Stops::). The LOCATION argument specifies source lines, as
described in *note Specify Location::.
Use the qualifier 'task TASKNO' with a breakpoint command to
specify that you only want GDB to stop the program when a
particular Ada task reaches this breakpoint. The TASKNO is one of
the numeric task identifiers assigned by GDB, shown in the first
column of the 'info tasks' display.
If you do not specify 'task TASKNO' when you set a breakpoint, the
breakpoint applies to _all_ tasks of your program.
You can use the 'task' qualifier on conditional breakpoints as
well; in this case, place 'task TASKNO' before the breakpoint
condition (before the 'if').
For example,
(gdb) info tasks
ID TID P-ID Pri State Name
1 140022020 0 15 Child Activation Wait main_task
2 140045060 1 15 Accept/Select Wait t2
3 140044840 1 15 Runnable t1
* 4 140056040 1 15 Runnable t3
(gdb) b 15 task 2
Breakpoint 5 at 0x120044cb0: file test_task_debug.adb, line 15.
(gdb) cont
Continuing.
task # 1 running
task # 2 running
Breakpoint 5, test_task_debug () at test_task_debug.adb:15
15 flush;
(gdb) info tasks
ID TID P-ID Pri State Name
1 140022020 0 15 Child Activation Wait main_task
* 2 140045060 1 15 Runnable t2
3 140044840 1 15 Runnable t1
4 140056040 1 15 Delay Sleep t3

File: gdb.info, Node: Ada Tasks and Core Files, Next: Ravenscar Profile, Prev: Ada Tasks, Up: Ada
15.4.10.8 Tasking Support when Debugging Core Files
...................................................
When inspecting a core file, as opposed to debugging a live program,
tasking support may be limited or even unavailable, depending on the
platform being used. For instance, on x86-linux, the list of tasks is
available, but task switching is not supported.
On certain platforms, the debugger needs to perform some memory
writes in order to provide Ada tasking support. When inspecting a core
file, this means that the core file must be opened with read-write
privileges, using the command '"set write on"' (*note Patching::).
Under these circumstances, you should make a backup copy of the core
file before inspecting it with GDB.

File: gdb.info, Node: Ravenscar Profile, Next: Ada Settings, Prev: Ada Tasks and Core Files, Up: Ada
15.4.10.9 Tasking Support when using the Ravenscar Profile
..........................................................
The "Ravenscar Profile" is a subset of the Ada tasking features,
specifically designed for systems with safety-critical real-time
requirements.
'set ravenscar task-switching on'
Allows task switching when debugging a program that uses the
Ravenscar Profile. This is the default.
'set ravenscar task-switching off'
Turn off task switching when debugging a program that uses the
Ravenscar Profile. This is mostly intended to disable the code
that adds support for the Ravenscar Profile, in case a bug in
either GDB or in the Ravenscar runtime is preventing GDB from
working properly. To be effective, this command should be run
before the program is started.
'show ravenscar task-switching'
Show whether it is possible to switch from task to task in a
program using the Ravenscar Profile.

File: gdb.info, Node: Ada Settings, Next: Ada Glitches, Prev: Ravenscar Profile, Up: Ada
15.4.10.10 Ada Settings
.......................
'set varsize-limit SIZE'
Prevent GDB from attempting to evaluate objects whose size is above
the given limit (SIZE) when those sizes are computed from run-time
quantities. This is typically the case when the object has a
variable size, such as an array whose bounds are not known at
compile time for example. Setting SIZE to 'unlimited' removes the
size limitation. By default, the limit is about 65KB.
The purpose of having such a limit is to prevent GDB from trying to
grab enormous chunks of virtual memory when asked to evaluate a
quantity whose bounds have been corrupted or have not yet been
fully initialized. The limit applies to the results of some
subexpressions as well as to complete expressions. For example, an
expression denoting a simple integer component, such as 'x.y.z',
may fail if the size of 'x.y' is variable and exceeds 'size'. On
the other hand, GDB is sometimes clever; the expression 'A(i)',
where 'A' is an array variable with non-constant size, will
generally succeed regardless of the bounds on 'A', as long as the
component size is less than SIZE.
'show varsize-limit'
Show the limit on types whose size is determined by run-time
quantities.

File: gdb.info, Node: Ada Glitches, Prev: Ada Settings, Up: Ada
15.4.10.11 Known Peculiarities of Ada Mode
..........................................
Besides the omissions listed previously (*note Omissions from Ada::), we
know of several problems with and limitations of Ada mode in GDB, some
of which will be fixed with planned future releases of the debugger and
the GNU Ada compiler.
* Static constants that the compiler chooses not to materialize as
objects in storage are invisible to the debugger.
* Named parameter associations in function argument lists are ignored
(the argument lists are treated as positional).
* Many useful library packages are currently invisible to the
debugger.
* Fixed-point arithmetic, conversions, input, and output is carried
out using floating-point arithmetic, and may give results that only
approximate those on the host machine.
* The GNAT compiler never generates the prefix 'Standard' for any of
the standard symbols defined by the Ada language. GDB knows about
this: it will strip the prefix from names when you use it, and will
never look for a name you have so qualified among local symbols,
nor match against symbols in other packages or subprograms. If you
have defined entities anywhere in your program other than
parameters and local variables whose simple names match names in
'Standard', GNAT's lack of qualification here can cause confusion.
When this happens, you can usually resolve the confusion by
qualifying the problematic names with package 'Standard'
explicitly.
Older versions of the compiler sometimes generate erroneous debugging
information, resulting in the debugger incorrectly printing the value of
affected entities. In some cases, the debugger is able to work around
an issue automatically. In other cases, the debugger is able to work
around the issue, but the work-around has to be specifically enabled.
'set ada trust-PAD-over-XVS on'
Configure GDB to strictly follow the GNAT encoding when computing
the value of Ada entities, particularly when 'PAD' and 'PAD___XVS'
types are involved (see 'ada/exp_dbug.ads' in the GCC sources for a
complete description of the encoding used by the GNAT compiler).
This is the default.
'set ada trust-PAD-over-XVS off'
This is related to the encoding using by the GNAT compiler. If GDB
sometimes prints the wrong value for certain entities, changing
'ada trust-PAD-over-XVS' to 'off' activates a work-around which may
fix the issue. It is always safe to set 'ada trust-PAD-over-XVS'
to 'off', but this incurs a slight performance penalty, so it is
recommended to leave this setting to 'on' unless necessary.
Internally, the debugger also relies on the compiler following a
number of conventions known as the 'GNAT Encoding', all documented in
'gcc/ada/exp_dbug.ads' in the GCC sources. This encoding describes how
the debugging information should be generated for certain types. In
particular, this convention makes use of "descriptive types", which are
artificial types generated purely to help the debugger.
These encodings were defined at a time when the debugging information
format used was not powerful enough to describe some of the more complex
types available in Ada. Since DWARF allows us to express nearly all Ada
features, the long-term goal is to slowly replace these descriptive
types by their pure DWARF equivalent. To facilitate that transition, a
new maintenance option is available to force the debugger to ignore
those descriptive types. It allows the user to quickly evaluate how
well GDB works without them.
'maintenance ada set ignore-descriptive-types [on|off]'
Control whether the debugger should ignore descriptive types. The
default is not to ignore descriptives types ('off').
'maintenance ada show ignore-descriptive-types'
Show if descriptive types are ignored by GDB.

File: gdb.info, Node: Unsupported Languages, Prev: Supported Languages, Up: Languages
15.5 Unsupported Languages
==========================
In addition to the other fully-supported programming languages, GDB also
provides a pseudo-language, called 'minimal'. It does not represent a
real programming language, but provides a set of capabilities close to
what the C or assembly languages provide. This should allow most simple
operations to be performed while debugging an application that uses a
language currently not supported by GDB.
If the language is set to 'auto', GDB will automatically select this
language if the current frame corresponds to an unsupported language.

File: gdb.info, Node: Symbols, Next: Altering, Prev: Languages, Up: Top
16 Examining the Symbol Table
*****************************
The commands described in this chapter allow you to inquire about the
symbols (names of variables, functions and types) defined in your
program. This information is inherent in the text of your program and
does not change as your program executes. GDB finds it in your
program's symbol table, in the file indicated when you started GDB
(*note Choosing Files: File Options.), or by one of the file-management
commands (*note Commands to Specify Files: Files.).
Occasionally, you may need to refer to symbols that contain unusual
characters, which GDB ordinarily treats as word delimiters. The most
frequent case is in referring to static variables in other source files
(*note Program Variables: Variables.). File names are recorded in
object files as debugging symbols, but GDB would ordinarily parse a
typical file name, like 'foo.c', as the three words 'foo' '.' 'c'. To
allow GDB to recognize 'foo.c' as a single symbol, enclose it in single
quotes; for example,
p 'foo.c'::x
looks up the value of 'x' in the scope of the file 'foo.c'.
'set case-sensitive on'
'set case-sensitive off'
'set case-sensitive auto'
Normally, when GDB looks up symbols, it matches their names with
case sensitivity determined by the current source language.
Occasionally, you may wish to control that. The command 'set
case-sensitive' lets you do that by specifying 'on' for
case-sensitive matches or 'off' for case-insensitive ones. If you
specify 'auto', case sensitivity is reset to the default suitable
for the source language. The default is case-sensitive matches for
all languages except for Fortran, for which the default is
case-insensitive matches.
'show case-sensitive'
This command shows the current setting of case sensitivity for
symbols lookups.
'set print type methods'
'set print type methods on'
'set print type methods off'
Normally, when GDB prints a class, it displays any methods declared
in that class. You can control this behavior either by passing the
appropriate flag to 'ptype', or using 'set print type methods'.
Specifying 'on' will cause GDB to display the methods; this is the
default. Specifying 'off' will cause GDB to omit the methods.
'show print type methods'
This command shows the current setting of method display when
printing classes.
'set print type nested-type-limit LIMIT'
'set print type nested-type-limit unlimited'
Set the limit of displayed nested types that the type printer will
show. A LIMIT of 'unlimited' or '-1' will show all nested
definitions. By default, the type printer will not show any nested
types defined in classes.
'show print type nested-type-limit'
This command shows the current display limit of nested types when
printing classes.
'set print type typedefs'
'set print type typedefs on'
'set print type typedefs off'
Normally, when GDB prints a class, it displays any typedefs defined
in that class. You can control this behavior either by passing the
appropriate flag to 'ptype', or using 'set print type typedefs'.
Specifying 'on' will cause GDB to display the typedef definitions;
this is the default. Specifying 'off' will cause GDB to omit the
typedef definitions. Note that this controls whether the typedef
definition itself is printed, not whether typedef names are
substituted when printing other types.
'show print type typedefs'
This command shows the current setting of typedef display when
printing classes.
'info address SYMBOL'
Describe where the data for SYMBOL is stored. For a register
variable, this says which register it is kept in. For a
non-register local variable, this prints the stack-frame offset at
which the variable is always stored.
Note the contrast with 'print &SYMBOL', which does not work at all
for a register variable, and for a stack local variable prints the
exact address of the current instantiation of the variable.
'info symbol ADDR'
Print the name of a symbol which is stored at the address ADDR. If
no symbol is stored exactly at ADDR, GDB prints the nearest symbol
and an offset from it:
(gdb) info symbol 0x54320
_initialize_vx + 396 in section .text
This is the opposite of the 'info address' command. You can use it
to find out the name of a variable or a function given its address.
For dynamically linked executables, the name of executable or
shared library containing the symbol is also printed:
(gdb) info symbol 0x400225
_start + 5 in section .text of /tmp/a.out
(gdb) info symbol 0x2aaaac2811cf
__read_nocancel + 6 in section .text of /usr/lib64/libc.so.6
'demangle [-l LANGUAGE] [--] NAME'
Demangle NAME. If LANGUAGE is provided it is the name of the
language to demangle NAME in. Otherwise NAME is demangled in the
current language.
The '--' option specifies the end of options, and is useful when
NAME begins with a dash.
The parameter 'demangle-style' specifies how to interpret the kind
of mangling used. *Note Print Settings::.
'whatis[/FLAGS] [ARG]'
Print the data type of ARG, which can be either an expression or a
name of a data type. With no argument, print the data type of '$',
the last value in the value history.
If ARG is an expression (*note Expressions: Expressions.), it is
not actually evaluated, and any side-effecting operations (such as
assignments or function calls) inside it do not take place.
If ARG is a variable or an expression, 'whatis' prints its literal
type as it is used in the source code. If the type was defined
using a 'typedef', 'whatis' will _not_ print the data type
underlying the 'typedef'. If the type of the variable or the
expression is a compound data type, such as 'struct' or 'class',
'whatis' never prints their fields or methods. It just prints the
'struct'/'class' name (a.k.a. its "tag"). If you want to see the
members of such a compound data type, use 'ptype'.
If ARG is a type name that was defined using 'typedef', 'whatis'
"unrolls" only one level of that 'typedef'. Unrolling means that
'whatis' will show the underlying type used in the 'typedef'
declaration of ARG. However, if that underlying type is also a
'typedef', 'whatis' will not unroll it.
For C code, the type names may also have the form 'class
CLASS-NAME', 'struct STRUCT-TAG', 'union UNION-TAG' or 'enum
ENUM-TAG'.
FLAGS can be used to modify how the type is displayed. Available
flags are:
'r'
Display in "raw" form. Normally, GDB substitutes template
parameters and typedefs defined in a class when printing the
class' members. The '/r' flag disables this.
'm'
Do not print methods defined in the class.
'M'
Print methods defined in the class. This is the default, but
the flag exists in case you change the default with 'set print
type methods'.
't'
Do not print typedefs defined in the class. Note that this
controls whether the typedef definition itself is printed, not
whether typedef names are substituted when printing other
types.
'T'
Print typedefs defined in the class. This is the default, but
the flag exists in case you change the default with 'set print
type typedefs'.
'o'
Print the offsets and sizes of fields in a struct, similar to
what the 'pahole' tool does. This option implies the '/tm'
flags.
For example, given the following declarations:
struct tuv
{
int a1;
char *a2;
int a3;
};
struct xyz
{
int f1;
char f2;
void *f3;
struct tuv f4;
};
union qwe
{
struct tuv fff1;
struct xyz fff2;
};
struct tyu
{
int a1 : 1;
int a2 : 3;
int a3 : 23;
char a4 : 2;
int64_t a5;
int a6 : 5;
int64_t a7 : 3;
};
Issuing a 'ptype /o struct tuv' command would print:
(gdb) ptype /o struct tuv
/* offset | size */ type = struct tuv {
/* 0 | 4 */ int a1;
/* XXX 4-byte hole */
/* 8 | 8 */ char *a2;
/* 16 | 4 */ int a3;
/* total size (bytes): 24 */
}
Notice the format of the first column of comments. There, you
can find two parts separated by the '|' character: the
_offset_, which indicates where the field is located inside
the struct, in bytes, and the _size_ of the field. Another
interesting line is the marker of a _hole_ in the struct,
indicating that it may be possible to pack the struct and make
it use less space by reorganizing its fields.
It is also possible to print offsets inside an union:
(gdb) ptype /o union qwe
/* offset | size */ type = union qwe {
/* 24 */ struct tuv {
/* 0 | 4 */ int a1;
/* XXX 4-byte hole */
/* 8 | 8 */ char *a2;
/* 16 | 4 */ int a3;
/* total size (bytes): 24 */
} fff1;
/* 40 */ struct xyz {
/* 0 | 4 */ int f1;
/* 4 | 1 */ char f2;
/* XXX 3-byte hole */
/* 8 | 8 */ void *f3;
/* 16 | 24 */ struct tuv {
/* 16 | 4 */ int a1;
/* XXX 4-byte hole */
/* 24 | 8 */ char *a2;
/* 32 | 4 */ int a3;
/* total size (bytes): 24 */
} f4;
/* total size (bytes): 40 */
} fff2;
/* total size (bytes): 40 */
}
In this case, since 'struct tuv' and 'struct xyz' occupy the
same space (because we are dealing with an union), the offset
is not printed for them. However, you can still examine the
offset of each of these structures' fields.
Another useful scenario is printing the offsets of a struct
containing bitfields:
(gdb) ptype /o struct tyu
/* offset | size */ type = struct tyu {
/* 0:31 | 4 */ int a1 : 1;
/* 0:28 | 4 */ int a2 : 3;
/* 0: 5 | 4 */ int a3 : 23;
/* 3: 3 | 1 */ signed char a4 : 2;
/* XXX 3-bit hole */
/* XXX 4-byte hole */
/* 8 | 8 */ int64_t a5;
/* 16: 0 | 4 */ int a6 : 5;
/* 16: 5 | 8 */ int64_t a7 : 3;
"/* XXX 7-byte padding */
/* total size (bytes): 24 */
}
Note how the offset information is now extended to also
include the first bit of the bitfield.
'ptype[/FLAGS] [ARG]'
'ptype' accepts the same arguments as 'whatis', but prints a
detailed description of the type, instead of just the name of the
type. *Note Expressions: Expressions.
Contrary to 'whatis', 'ptype' always unrolls any 'typedef's in its
argument declaration, whether the argument is a variable,
expression, or a data type. This means that 'ptype' of a variable
or an expression will not print literally its type as present in
the source code--use 'whatis' for that. 'typedef's at the pointer
or reference targets are also unrolled. Only 'typedef's of fields,
methods and inner 'class typedef's of 'struct's, 'class'es and
'union's are not unrolled even with 'ptype'.
For example, for this variable declaration:
typedef double real_t;
struct complex { real_t real; double imag; };
typedef struct complex complex_t;
complex_t var;
real_t *real_pointer_var;
the two commands give this output:
(gdb) whatis var
type = complex_t
(gdb) ptype var
type = struct complex {
real_t real;
double imag;
}
(gdb) whatis complex_t
type = struct complex
(gdb) whatis struct complex
type = struct complex
(gdb) ptype struct complex
type = struct complex {
real_t real;
double imag;
}
(gdb) whatis real_pointer_var
type = real_t *
(gdb) ptype real_pointer_var
type = double *
As with 'whatis', using 'ptype' without an argument refers to the
type of '$', the last value in the value history.
Sometimes, programs use opaque data types or incomplete
specifications of complex data structure. If the debug information
included in the program does not allow GDB to display a full
declaration of the data type, it will say '<incomplete type>'. For
example, given these declarations:
struct foo;
struct foo *fooptr;
but no definition for 'struct foo' itself, GDB will say:
(gdb) ptype foo
$1 = <incomplete type>
"Incomplete type" is C terminology for data types that are not
completely specified.
Othertimes, information about a variable's type is completely
absent from the debug information included in the program. This
most often happens when the program or library where the variable
is defined includes no debug information at all. GDB knows the
variable exists from inspecting the linker/loader symbol table
(e.g., the ELF dynamic symbol table), but such symbols do not
contain type information. Inspecting the type of a (global)
variable for which GDB has no type information shows:
(gdb) ptype var
type = <data variable, no debug info>
*Note no debug info variables: Variables, for how to print the
values of such variables.
'info types [-q] [REGEXP]'
Print a brief description of all types whose names match the
regular expression REGEXP (or all types in your program, if you
supply no argument). Each complete typename is matched as though
it were a complete line; thus, 'i type value' gives information on
all types in your program whose names include the string 'value',
but 'i type ^value$' gives information only on types whose complete
name is 'value'.
In programs using different languages, GDB chooses the syntax to
print the type description according to the 'set language' value:
using 'set language auto' (see *note Set Language Automatically:
Automatically.) means to use the language of the type, other values
mean to use the manually specified language (see *note Set Language
Manually: Manually.).
This command differs from 'ptype' in two ways: first, like
'whatis', it does not print a detailed description; second, it
lists all source files and line numbers where a type is defined.
The output from 'into types' is proceeded with a header line
describing what types are being listed. The optional flag '-q',
which stands for 'quiet', disables printing this header
information.
'info type-printers'
Versions of GDB that ship with Python scripting enabled may have
"type printers" available. When using 'ptype' or 'whatis', these
printers are consulted when the name of a type is needed. *Note
Type Printing API::, for more information on writing type printers.
'info type-printers' displays all the available type printers.
'enable type-printer NAME...'
'disable type-printer NAME...'
These commands can be used to enable or disable type printers.
'info scope LOCATION'
List all the variables local to a particular scope. This command
accepts a LOCATION argument--a function name, a source line, or an
address preceded by a '*', and prints all the variables local to
the scope defined by that location. (*Note Specify Location::, for
details about supported forms of LOCATION.) For example:
(gdb) info scope command_line_handler
Scope for command_line_handler:
Symbol rl is an argument at stack/frame offset 8, length 4.
Symbol linebuffer is in static storage at address 0x150a18, length 4.
Symbol linelength is in static storage at address 0x150a1c, length 4.
Symbol p is a local variable in register $esi, length 4.
Symbol p1 is a local variable in register $ebx, length 4.
Symbol nline is a local variable in register $edx, length 4.
Symbol repeat is a local variable at frame offset -8, length 4.
This command is especially useful for determining what data to
collect during a "trace experiment", see *note collect: Tracepoint
Actions.
'info source'
Show information about the current source file--that is, the source
file for the function containing the current point of execution:
* the name of the source file, and the directory containing it,
* the directory it was compiled in,
* its length, in lines,
* which programming language it is written in,
* if the debug information provides it, the program that
compiled the file (which may include, e.g., the compiler
version and command line arguments),
* whether the executable includes debugging information for that
file, and if so, what format the information is in (e.g.,
STABS, Dwarf 2, etc.), and
* whether the debugging information includes information about
preprocessor macros.
'info sources'
Print the names of all source files in your program for which there
is debugging information, organized into two lists: files whose
symbols have already been read, and files whose symbols will be
read when needed.
'info sources [-dirname | -basename] [--] [REGEXP]'
Like 'info sources', but only print the names of the files matching
the provided REGEXP. By default, the REGEXP is used to match
anywhere in the filename. If '-dirname', only files having a
dirname matching REGEXP are shown. If '-basename', only files
having a basename matching REGEXP are shown. The matching is
case-sensitive, except on operating systems that have
case-insensitive filesystem (e.g., MS-Windows).
'info functions [-q] [-n]'
Print the names and data types of all defined functions. Similarly
to 'info types', this command groups its output by source files and
annotates each function definition with its source line number.
In programs using different languages, GDB chooses the syntax to
print the function name and type according to the 'set language'
value: using 'set language auto' (see *note Set Language
Automatically: Automatically.) means to use the language of the
function, other values mean to use the manually specified language
(see *note Set Language Manually: Manually.).
The '-n' flag excludes "non-debugging symbols" from the results. A
non-debugging symbol is a symbol that comes from the executable's
symbol table, not from the debug information (for example, DWARF)
associated with the executable.
The optional flag '-q', which stands for 'quiet', disables printing
header information and messages explaining why no functions have
been printed.
'info functions [-q] [-n] [-t TYPE_REGEXP] [REGEXP]'
Like 'info functions', but only print the names and data types of
the functions selected with the provided regexp(s).
If REGEXP is provided, print only the functions whose names match
the regular expression REGEXP. Thus, 'info fun step' finds all
functions whose names include 'step'; 'info fun ^step' finds those
whose names start with 'step'. If a function name contains
characters that conflict with the regular expression language (e.g.
'operator*()'), they may be quoted with a backslash.
If TYPE_REGEXP is provided, print only the functions whose types,
as printed by the 'whatis' command, match the regular expression
TYPE_REGEXP. If TYPE_REGEXP contains space(s), it should be
enclosed in quote characters. If needed, use backslash to escape
the meaning of special characters or quotes. Thus, 'info fun -t
'^int ('' finds the functions that return an integer; 'info fun -t
'(.*int.*'' finds the functions that have an argument type
containing int; 'info fun -t '^int (' ^step' finds the functions
whose names start with 'step' and that return int.
If both REGEXP and TYPE_REGEXP are provided, a function is printed
only if its name matches REGEXP and its type matches TYPE_REGEXP.
'info variables [-q] [-n]'
Print the names and data types of all variables that are defined
outside of functions (i.e. excluding local variables). The printed
variables are grouped by source files and annotated with their
respective source line numbers.
In programs using different languages, GDB chooses the syntax to
print the variable name and type according to the 'set language'
value: using 'set language auto' (see *note Set Language
Automatically: Automatically.) means to use the language of the
variable, other values mean to use the manually specified language
(see *note Set Language Manually: Manually.).
The '-n' flag excludes non-debugging symbols from the results.
The optional flag '-q', which stands for 'quiet', disables printing
header information and messages explaining why no variables have
been printed.
'info variables [-q] [-n] [-t TYPE_REGEXP] [REGEXP]'
Like 'info variables', but only print the variables selected with
the provided regexp(s).
If REGEXP is provided, print only the variables whose names match
the regular expression REGEXP.
If TYPE_REGEXP is provided, print only the variables whose types,
as printed by the 'whatis' command, match the regular expression
TYPE_REGEXP. If TYPE_REGEXP contains space(s), it should be
enclosed in quote characters. If needed, use backslash to escape
the meaning of special characters or quotes.
If both REGEXP and TYPE_REGEXP are provided, an argument is printed
only if its name matches REGEXP and its type matches TYPE_REGEXP.
'info modules [-q] [REGEXP]'
List all Fortran modules in the program, or all modules matching
the optional regular expression REGEXP.
The optional flag '-q', which stands for 'quiet', disables printing
header information and messages explaining why no modules have been
printed.
'info module functions [-q] [-m MODULE-REGEXP] [-t TYPE-REGEXP] [REGEXP]'
'info module variables [-q] [-m MODULE-REGEXP] [-t TYPE-REGEXP] [REGEXP]'
List all functions or variables within all Fortran modules. The
set of functions or variables listed can be limited by providing
some or all of the optional regular expressions. If MODULE-REGEXP
is provided, then only Fortran modules matching MODULE-REGEXP will
be searched. Only functions or variables whose type matches the
optional regular expression TYPE-REGEXP will be listed. And only
functions or variables whose name matches the optional regular
expression REGEXP will be listed.
The optional flag '-q', which stands for 'quiet', disables printing
header information and messages explaining why no functions or
variables have been printed.
'info classes'
'info classes REGEXP'
Display all Objective-C classes in your program, or (with the
REGEXP argument) all those matching a particular regular
expression.
'info selectors'
'info selectors REGEXP'
Display all Objective-C selectors in your program, or (with the
REGEXP argument) all those matching a particular regular
expression.
'set opaque-type-resolution on'
Tell GDB to resolve opaque types. An opaque type is a type
declared as a pointer to a 'struct', 'class', or 'union'--for
example, 'struct MyType *'--that is used in one source file
although the full declaration of 'struct MyType' is in another
source file. The default is on.
A change in the setting of this subcommand will not take effect
until the next time symbols for a file are loaded.
'set opaque-type-resolution off'
Tell GDB not to resolve opaque types. In this case, the type is
printed as follows:
{<no data fields>}
'show opaque-type-resolution'
Show whether opaque types are resolved or not.
'set print symbol-loading'
'set print symbol-loading full'
'set print symbol-loading brief'
'set print symbol-loading off'
The 'set print symbol-loading' command allows you to control the
printing of messages when GDB loads symbol information. By default
a message is printed for the executable and one for each shared
library, and normally this is what you want. However, when
debugging apps with large numbers of shared libraries these
messages can be annoying. When set to 'brief' a message is printed
for each executable, and when GDB loads a collection of shared
libraries at once it will only print one message regardless of the
number of shared libraries. When set to 'off' no messages are
printed.
'show print symbol-loading'
Show whether messages will be printed when a GDB command entered
from the keyboard causes symbol information to be loaded.
'maint print symbols [-pc ADDRESS] [FILENAME]'
'maint print symbols [-objfile OBJFILE] [-source SOURCE] [--] [FILENAME]'
'maint print psymbols [-objfile OBJFILE] [-pc ADDRESS] [--] [FILENAME]'
'maint print psymbols [-objfile OBJFILE] [-source SOURCE] [--] [FILENAME]'
'maint print msymbols [-objfile OBJFILE] [--] [FILENAME]'
Write a dump of debugging symbol data into the file FILENAME or the
terminal if FILENAME is unspecified. If '-objfile OBJFILE' is
specified, only dump symbols for that objfile. If '-pc ADDRESS' is
specified, only dump symbols for the file with code at that
address. Note that ADDRESS may be a symbol like 'main'. If
'-source SOURCE' is specified, only dump symbols for that source
file.
These commands are used to debug the GDB symbol-reading code.
These commands do not modify internal GDB state, therefore 'maint
print symbols' will only print symbols for already expanded symbol
tables. You can use the command 'info sources' to find out which
files these are. If you use 'maint print psymbols' instead, the
dump shows information about symbols that GDB only knows
partially--that is, symbols defined in files that GDB has skimmed,
but not yet read completely. Finally, 'maint print msymbols' just
dumps "minimal symbols", e.g., "ELF symbols".
*Note Commands to Specify Files: Files, for a discussion of how GDB
reads symbols (in the description of 'symbol-file').
'maint info symtabs [ REGEXP ]'
'maint info psymtabs [ REGEXP ]'
List the 'struct symtab' or 'struct partial_symtab' structures
whose names match REGEXP. If REGEXP is not given, list them all.
The output includes expressions which you can copy into a GDB
debugging this one to examine a particular structure in more
detail. For example:
(gdb) maint info psymtabs dwarf2read
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ psymtab /home/gnu/src/gdb/dwarf2read.c
((struct partial_symtab *) 0x8474b10)
readin no
fullname (null)
text addresses 0x814d3c8 -- 0x8158074
globals (* (struct partial_symbol **) 0x8507a08 @ 9)
statics (* (struct partial_symbol **) 0x40e95b78 @ 2882)
dependencies (none)
}
}
(gdb) maint info symtabs
(gdb)
We see that there is one partial symbol table whose filename
contains the string 'dwarf2read', belonging to the 'gdb'
executable; and we see that GDB has not read in any symtabs yet at
all. If we set a breakpoint on a function, that will cause GDB to
read the symtab for the compilation unit containing that function:
(gdb) break dwarf2_psymtab_to_symtab
Breakpoint 1 at 0x814e5da: file /home/gnu/src/gdb/dwarf2read.c,
line 1574.
(gdb) maint info symtabs
{ objfile /home/gnu/build/gdb/gdb
((struct objfile *) 0x82e69d0)
{ symtab /home/gnu/src/gdb/dwarf2read.c
((struct symtab *) 0x86c1f38)
dirname (null)
fullname (null)
blockvector ((struct blockvector *) 0x86c1bd0) (primary)
linetable ((struct linetable *) 0x8370fa0)
debugformat DWARF 2
}
}
(gdb)
'maint info line-table [ REGEXP ]'
List the 'struct linetable' from all 'struct symtab' instances
whose name matches REGEXP. If REGEXP is not given, list the
'struct linetable' from all 'struct symtab'.
'maint set symbol-cache-size SIZE'
Set the size of the symbol cache to SIZE. The default size is
intended to be good enough for debugging most applications. This
option exists to allow for experimenting with different sizes.
'maint show symbol-cache-size'
Show the size of the symbol cache.
'maint print symbol-cache'
Print the contents of the symbol cache. This is useful when
debugging symbol cache issues.
'maint print symbol-cache-statistics'
Print symbol cache usage statistics. This helps determine how well
the cache is being utilized.
'maint flush-symbol-cache'
Flush the contents of the symbol cache, all entries are removed.
This command is useful when debugging the symbol cache. It is also
useful when collecting performance data.

File: gdb.info, Node: Altering, Next: GDB Files, Prev: Symbols, Up: Top
17 Altering Execution
*********************
Once you think you have found an error in your program, you might want
to find out for certain whether correcting the apparent error would lead
to correct results in the rest of the run. You can find the answer by
experiment, using the GDB features for altering execution of the
program.
For example, you can store new values into variables or memory
locations, give your program a signal, restart it at a different
address, or even return prematurely from a function.
* Menu:
* Assignment:: Assignment to variables
* Jumping:: Continuing at a different address
* Signaling:: Giving your program a signal
* Returning:: Returning from a function
* Calling:: Calling your program's functions
* Patching:: Patching your program
* Compiling and Injecting Code:: Compiling and injecting code in GDB

File: gdb.info, Node: Assignment, Next: Jumping, Up: Altering
17.1 Assignment to Variables
============================
To alter the value of a variable, evaluate an assignment expression.
*Note Expressions: Expressions. For example,
print x=4
stores the value 4 into the variable 'x', and then prints the value of
the assignment expression (which is 4). *Note Using GDB with Different
Languages: Languages, for more information on operators in supported
languages.
If you are not interested in seeing the value of the assignment, use
the 'set' command instead of the 'print' command. 'set' is really the
same as 'print' except that the expression's value is not printed and is
not put in the value history (*note Value History: Value History.). The
expression is evaluated only for its effects.
If the beginning of the argument string of the 'set' command appears
identical to a 'set' subcommand, use the 'set variable' command instead
of just 'set'. This command is identical to 'set' except for its lack
of subcommands. For example, if your program has a variable 'width',
you get an error if you try to set a new value with just 'set width=13',
because GDB has the command 'set width':
(gdb) whatis width
type = double
(gdb) p width
$4 = 13
(gdb) set width=47
Invalid syntax in expression.
The invalid expression, of course, is '=47'. In order to actually set
the program's variable 'width', use
(gdb) set var width=47
Because the 'set' command has many subcommands that can conflict with
the names of program variables, it is a good idea to use the 'set
variable' command instead of just 'set'. For example, if your program
has a variable 'g', you run into problems if you try to set a new value
with just 'set g=4', because GDB has the command 'set gnutarget',
abbreviated 'set g':
(gdb) whatis g
type = double
(gdb) p g
$1 = 1
(gdb) set g=4
(gdb) p g
$2 = 1
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/smith/cc_progs/a.out
"/home/smith/cc_progs/a.out": can't open to read symbols:
Invalid bfd target.
(gdb) show g
The current BFD target is "=4".
The program variable 'g' did not change, and you silently set the
'gnutarget' to an invalid value. In order to set the variable 'g', use
(gdb) set var g=4
GDB allows more implicit conversions in assignments than C; you can
freely store an integer value into a pointer variable or vice versa, and
you can convert any structure to any other structure that is the same
length or shorter.
To store values into arbitrary places in memory, use the '{...}'
construct to generate a value of specified type at a specified address
(*note Expressions: Expressions.). For example, '{int}0x83040' refers
to memory location '0x83040' as an integer (which implies a certain size
and representation in memory), and
set {int}0x83040 = 4
stores the value 4 into that memory location.

File: gdb.info, Node: Jumping, Next: Signaling, Prev: Assignment, Up: Altering
17.2 Continuing at a Different Address
======================================
Ordinarily, when you continue your program, you do so at the place where
it stopped, with the 'continue' command. You can instead continue at an
address of your own choosing, with the following commands:
'jump LOCATION'
'j LOCATION'
Resume execution at LOCATION. Execution stops again immediately if
there is a breakpoint there. *Note Specify Location::, for a
description of the different forms of LOCATION. It is common
practice to use the 'tbreak' command in conjunction with 'jump'.
*Note Setting Breakpoints: Set Breaks.
The 'jump' command does not change the current stack frame, or the
stack pointer, or the contents of any memory location or any
register other than the program counter. If LOCATION is in a
different function from the one currently executing, the results
may be bizarre if the two functions expect different patterns of
arguments or of local variables. For this reason, the 'jump'
command requests confirmation if the specified line is not in the
function currently executing. However, even bizarre results are
predictable if you are well acquainted with the machine-language
code of your program.
On many systems, you can get much the same effect as the 'jump'
command by storing a new value into the register '$pc'. The difference
is that this does not start your program running; it only changes the
address of where it _will_ run when you continue. For example,
set $pc = 0x485
makes the next 'continue' command or stepping command execute at address
'0x485', rather than at the address where your program stopped. *Note
Continuing and Stepping: Continuing and Stepping.
The most common occasion to use the 'jump' command is to back
up--perhaps with more breakpoints set--over a portion of a program that
has already executed, in order to examine its execution in more detail.

File: gdb.info, Node: Signaling, Next: Returning, Prev: Jumping, Up: Altering
17.3 Giving your Program a Signal
=================================
'signal SIGNAL'
Resume execution where your program is stopped, but immediately
give it the signal SIGNAL. The SIGNAL can be the name or the
number of a signal. For example, on many systems 'signal 2' and
'signal SIGINT' are both ways of sending an interrupt signal.
Alternatively, if SIGNAL is zero, continue execution without giving
a signal. This is useful when your program stopped on account of a
signal and would ordinarily see the signal when resumed with the
'continue' command; 'signal 0' causes it to resume without a
signal.
_Note:_ When resuming a multi-threaded program, SIGNAL is delivered
to the currently selected thread, not the thread that last reported
a stop. This includes the situation where a thread was stopped due
to a signal. So if you want to continue execution suppressing the
signal that stopped a thread, you should select that same thread
before issuing the 'signal 0' command. If you issue the 'signal 0'
command with another thread as the selected one, GDB detects that
and asks for confirmation.
Invoking the 'signal' command is not the same as invoking the
'kill' utility from the shell. Sending a signal with 'kill' causes
GDB to decide what to do with the signal depending on the signal
handling tables (*note Signals::). The 'signal' command passes the
signal directly to your program.
'signal' does not repeat when you press <RET> a second time after
executing the command.
'queue-signal SIGNAL'
Queue SIGNAL to be delivered immediately to the current thread when
execution of the thread resumes. The SIGNAL can be the name or the
number of a signal. For example, on many systems 'signal 2' and
'signal SIGINT' are both ways of sending an interrupt signal. The
handling of the signal must be set to pass the signal to the
program, otherwise GDB will report an error. You can control the
handling of signals from GDB with the 'handle' command (*note
Signals::).
Alternatively, if SIGNAL is zero, any currently queued signal for
the current thread is discarded and when execution resumes no
signal will be delivered. This is useful when your program stopped
on account of a signal and would ordinarily see the signal when
resumed with the 'continue' command.
This command differs from the 'signal' command in that the signal
is just queued, execution is not resumed. And 'queue-signal'
cannot be used to pass a signal whose handling state has been set
to 'nopass' (*note Signals::).
*Note stepping into signal handlers::, for information on how
stepping commands behave when the thread has a signal queued.

File: gdb.info, Node: Returning, Next: Calling, Prev: Signaling, Up: Altering
17.4 Returning from a Function
==============================
'return'
'return EXPRESSION'
You can cancel execution of a function call with the 'return'
command. If you give an EXPRESSION argument, its value is used as
the function's return value.
When you use 'return', GDB discards the selected stack frame (and all
frames within it). You can think of this as making the discarded frame
return prematurely. If you wish to specify a value to be returned, give
that value as the argument to 'return'.
This pops the selected stack frame (*note Selecting a Frame:
Selection.), and any other frames inside of it, leaving its caller as
the innermost remaining frame. That frame becomes selected. The
specified value is stored in the registers used for returning values of
functions.
The 'return' command does not resume execution; it leaves the program
stopped in the state that would exist if the function had just returned.
In contrast, the 'finish' command (*note Continuing and Stepping:
Continuing and Stepping.) resumes execution until the selected stack
frame returns naturally.
GDB needs to know how the EXPRESSION argument should be set for the
inferior. The concrete registers assignment depends on the OS ABI and
the type being returned by the selected stack frame. For example it is
common for OS ABI to return floating point values in FPU registers while
integer values in CPU registers. Still some ABIs return even floating
point values in CPU registers. Larger integer widths (such as 'long
long int') also have specific placement rules. GDB already knows the OS
ABI from its current target so it needs to find out also the type being
returned to make the assignment into the right register(s).
Normally, the selected stack frame has debug info. GDB will always
use the debug info instead of the implicit type of EXPRESSION when the
debug info is available. For example, if you type 'return -1', and the
function in the current stack frame is declared to return a 'long long
int', GDB transparently converts the implicit 'int' value of -1 into a
'long long int':
Breakpoint 1, func () at gdb.base/return-nodebug.c:29
29 return 31;
(gdb) return -1
Make func return now? (y or n) y
#0 0x004004f6 in main () at gdb.base/return-nodebug.c:43
43 printf ("result=%lld\n", func ());
(gdb)
However, if the selected stack frame does not have a debug info,
e.g., if the function was compiled without debug info, GDB has to find
out the type to return from user. Specifying a different type by
mistake may set the value in different inferior registers than the
caller code expects. For example, typing 'return -1' with its implicit
type 'int' would set only a part of a 'long long int' result for a debug
info less function (on 32-bit architectures). Therefore the user is
required to specify the return type by an appropriate cast explicitly:
Breakpoint 2, 0x0040050b in func ()
(gdb) return -1
Return value type not available for selected stack frame.
Please use an explicit cast of the value to return.
(gdb) return (long long int) -1
Make selected stack frame return now? (y or n) y
#0 0x00400526 in main ()
(gdb)

File: gdb.info, Node: Calling, Next: Patching, Prev: Returning, Up: Altering
17.5 Calling Program Functions
==============================
'print EXPR'
Evaluate the expression EXPR and display the resulting value. The
expression may include calls to functions in the program being
debugged.
'call EXPR'
Evaluate the expression EXPR without displaying 'void' returned
values.
You can use this variant of the 'print' command if you want to
execute a function from your program that does not return anything
(a.k.a. "a void function"), but without cluttering the output with
'void' returned values that GDB will otherwise print. If the
result is not void, it is printed and saved in the value history.
It is possible for the function you call via the 'print' or 'call'
command to generate a signal (e.g., if there's a bug in the function, or
if you passed it incorrect arguments). What happens in that case is
controlled by the 'set unwindonsignal' command.
Similarly, with a C++ program it is possible for the function you
call via the 'print' or 'call' command to generate an exception that is
not handled due to the constraints of the dummy frame. In this case,
any exception that is raised in the frame, but has an out-of-frame
exception handler will not be found. GDB builds a dummy-frame for the
inferior function call, and the unwinder cannot seek for exception
handlers outside of this dummy-frame. What happens in that case is
controlled by the 'set unwind-on-terminating-exception' command.
'set unwindonsignal'
Set unwinding of the stack if a signal is received while in a
function that GDB called in the program being debugged. If set to
on, GDB unwinds the stack it created for the call and restores the
context to what it was before the call. If set to off (the
default), GDB stops in the frame where the signal was received.
'show unwindonsignal'
Show the current setting of stack unwinding in the functions called
by GDB.
'set unwind-on-terminating-exception'
Set unwinding of the stack if a C++ exception is raised, but left
unhandled while in a function that GDB called in the program being
debugged. If set to on (the default), GDB unwinds the stack it
created for the call and restores the context to what it was before
the call. If set to off, GDB the exception is delivered to the
default C++ exception handler and the inferior terminated.
'show unwind-on-terminating-exception'
Show the current setting of stack unwinding in the functions called
by GDB.
'set may-call-functions'
Set permission to call functions in the program. This controls
whether GDB will attempt to call functions in the program, such as
with expressions in the 'print' command. It defaults to 'on'.
To call a function in the program, GDB has to temporarily modify
the state of the inferior. This has potentially undesired side
effects. Also, having GDB call nested functions is likely to be
erroneous and may even crash the program being debugged. You can
avoid such hazards by forbidding GDB from calling functions in the
program being debugged. If calling functions in the program is
forbidden, GDB will throw an error when a command (such as printing
an expression) starts a function call in the program.
'show may-call-functions'
Show permission to call functions in the program.
17.5.1 Calling functions with no debug info
-------------------------------------------
Sometimes, a function you wish to call is missing debug information. In
such case, GDB does not know the type of the function, including the
types of the function's parameters. To avoid calling the inferior
function incorrectly, which could result in the called function
functioning erroneously and even crash, GDB refuses to call the function
unless you tell it the type of the function.
For prototyped (i.e. ANSI/ISO style) functions, there are two ways to
do that. The simplest is to cast the call to the function's declared
return type. For example:
(gdb) p getenv ("PATH")
'getenv' has unknown return type; cast the call to its declared return type
(gdb) p (char *) getenv ("PATH")
$1 = 0x7fffffffe7ba "/usr/local/bin:/"...
Casting the return type of a no-debug function is equivalent to
casting the function to a pointer to a prototyped function that has a
prototype that matches the types of the passed-in arguments, and calling
that. I.e., the call above is equivalent to:
(gdb) p ((char * (*) (const char *)) getenv) ("PATH")
and given this prototyped C or C++ function with float parameters:
float multiply (float v1, float v2) { return v1 * v2; }
these calls are equivalent:
(gdb) p (float) multiply (2.0f, 3.0f)
(gdb) p ((float (*) (float, float)) multiply) (2.0f, 3.0f)
If the function you wish to call is declared as unprototyped (i.e.
old K&R style), you must use the cast-to-function-pointer syntax, so
that GDB knows that it needs to apply default argument promotions
(promote float arguments to double). *Note float promotion: ABI. For
example, given this unprototyped C function with float parameters, and
no debug info:
float
multiply_noproto (v1, v2)
float v1, v2;
{
return v1 * v2;
}
you call it like this:
(gdb) p ((float (*) ()) multiply_noproto) (2.0f, 3.0f)

File: gdb.info, Node: Patching, Next: Compiling and Injecting Code, Prev: Calling, Up: Altering
17.6 Patching Programs
======================
By default, GDB opens the file containing your program's executable code
(or the corefile) read-only. This prevents accidental alterations to
machine code; but it also prevents you from intentionally patching your
program's binary.
If you'd like to be able to patch the binary, you can specify that
explicitly with the 'set write' command. For example, you might want to
turn on internal debugging flags, or even to make emergency repairs.
'set write on'
'set write off'
If you specify 'set write on', GDB opens executable and core files
for both reading and writing; if you specify 'set write off' (the
default), GDB opens them read-only.
If you have already loaded a file, you must load it again (using
the 'exec-file' or 'core-file' command) after changing 'set write',
for your new setting to take effect.
'show write'
Display whether executable files and core files are opened for
writing as well as reading.

File: gdb.info, Node: Compiling and Injecting Code, Prev: Patching, Up: Altering
17.7 Compiling and injecting code in GDB
========================================
GDB supports on-demand compilation and code injection into programs
running under GDB. GCC 5.0 or higher built with 'libcc1.so' must be
installed for this functionality to be enabled. This functionality is
implemented with the following commands.
'compile code SOURCE-CODE'
'compile code -raw -- SOURCE-CODE'
Compile SOURCE-CODE with the compiler language found as the current
language in GDB (*note Languages::). If compilation and injection
is not supported with the current language specified in GDB, or the
compiler does not support this feature, an error message will be
printed. If SOURCE-CODE compiles and links successfully, GDB will
load the object-code emitted, and execute it within the context of
the currently selected inferior. It is important to note that the
compiled code is executed immediately. After execution, the
compiled code is removed from GDB and any new types or variables
you have defined will be deleted.
The command allows you to specify SOURCE-CODE in two ways. The
simplest method is to provide a single line of code to the command.
E.g.:
compile code printf ("hello world\n");
If you specify options on the command line as well as source code,
they may conflict. The '--' delimiter can be used to separate
options from actual source code. E.g.:
compile code -r -- printf ("hello world\n");
Alternatively you can enter source code as multiple lines of text.
To enter this mode, invoke the 'compile code' command without any
text following the command. This will start the multiple-line
editor and allow you to type as many lines of source code as
required. When you have completed typing, enter 'end' on its own
line to exit the editor.
compile code
>printf ("hello\n");
>printf ("world\n");
>end
Specifying '-raw', prohibits GDB from wrapping the provided
SOURCE-CODE in a callable scope. In this case, you must specify
the entry point of the code by defining a function named
'_gdb_expr_'. The '-raw' code cannot access variables of the
inferior. Using '-raw' option may be needed for example when
SOURCE-CODE requires '#include' lines which may conflict with
inferior symbols otherwise.
'compile file FILENAME'
'compile file -raw FILENAME'
Like 'compile code', but take the source code from FILENAME.
compile file /home/user/example.c
'compile print [[OPTIONS] --] EXPR'
'compile print [[OPTIONS] --] /F EXPR'
Compile and execute EXPR with the compiler language found as the
current language in GDB (*note Languages::). By default the value
of EXPR is printed in a format appropriate to its data type; you
can choose a different format by specifying '/F', where F is a
letter specifying the format; see *note Output Formats: Output
Formats. The 'compile print' command accepts the same options as
the 'print' command; see *note print options::.
'compile print [[OPTIONS] --]'
'compile print [[OPTIONS] --] /F'
Alternatively you can enter the expression (source code producing
it) as multiple lines of text. To enter this mode, invoke the
'compile print' command without any text following the command.
This will start the multiple-line editor.
The process of compiling and injecting the code can be inspected using:
'set debug compile'
Turns on or off display of GDB process of compiling and injecting
the code. The default is off.
'show debug compile'
Displays the current state of displaying GDB process of compiling
and injecting the code.
'set debug compile-cplus-types'
Turns on or off the display of C++ type conversion debugging
information. The default is off.
'show debug compile-cplus-types'
Displays the current state of displaying debugging information for
C++ type conversion.
17.7.1 Compilation options for the 'compile' command
----------------------------------------------------
GDB needs to specify the right compilation options for the code to be
injected, in part to make its ABI compatible with the inferior and in
part to make the injected code compatible with GDB's injecting process.
The options used, in increasing precedence:
target architecture and OS options ('gdbarch')
These options depend on target processor type and target operating
system, usually they specify at least 32-bit ('-m32') or 64-bit
('-m64') compilation option.
compilation options recorded in the target
GCC (since version 4.7) stores the options used for compilation
into 'DW_AT_producer' part of DWARF debugging information according
to the GCC option '-grecord-gcc-switches'. One has to explicitly
specify '-g' during inferior compilation otherwise GCC produces no
DWARF. This feature is only relevant for platforms where '-g'
produces DWARF by default, otherwise one may try to enforce DWARF
by using '-gdwarf-4'.
compilation options set by 'set compile-args'
You can override compilation options using the following command:
'set compile-args'
Set compilation options used for compiling and injecting code with
the 'compile' commands. These options override any conflicting
ones from the target architecture and/or options stored during
inferior compilation.
'show compile-args'
Displays the current state of compilation options override. This
does not show all the options actually used during compilation, use
*note set debug compile:: for that.
17.7.2 Caveats when using the 'compile' command
-----------------------------------------------
There are a few caveats to keep in mind when using the 'compile'
command. As the caveats are different per language, the table below
highlights specific issues on a per language basis.
C code examples and caveats
When the language in GDB is set to 'C', the compiler will attempt
to compile the source code with a 'C' compiler. The source code
provided to the 'compile' command will have much the same access to
variables and types as it normally would if it were part of the
program currently being debugged in GDB.
Below is a sample program that forms the basis of the examples that
follow. This program has been compiled and loaded into GDB, much
like any other normal debugging session.
void function1 (void)
{
int i = 42;
printf ("function 1\n");
}
void function2 (void)
{
int j = 12;
function1 ();
}
int main(void)
{
int k = 6;
int *p;
function2 ();
return 0;
}
For the purposes of the examples in this section, the program above
has been compiled, loaded into GDB, stopped at the function 'main',
and GDB is awaiting input from the user.
To access variables and types for any program in GDB, the program
must be compiled and packaged with debug information. The
'compile' command is not an exception to this rule. Without debug
information, you can still use the 'compile' command, but you will
be very limited in what variables and types you can access.
So with that in mind, the example above has been compiled with
debug information enabled. The 'compile' command will have access
to all variables and types (except those that may have been
optimized out). Currently, as GDB has stopped the program in the
'main' function, the 'compile' command would have access to the
variable 'k'. You could invoke the 'compile' command and type some
source code to set the value of 'k'. You can also read it, or do
anything with that variable you would normally do in 'C'. Be aware
that changes to inferior variables in the 'compile' command are
persistent. In the following example:
compile code k = 3;
the variable 'k' is now 3. It will retain that value until
something else in the example program changes it, or another
'compile' command changes it.
Normal scope and access rules apply to source code compiled and
injected by the 'compile' command. In the example, the variables
'j' and 'k' are not accessible yet, because the program is
currently stopped in the 'main' function, where these variables are
not in scope. Therefore, the following command
compile code j = 3;
will result in a compilation error message.
Once the program is continued, execution will bring these variables
in scope, and they will become accessible; then the code you
specify via the 'compile' command will be able to access them.
You can create variables and types with the 'compile' command as
part of your source code. Variables and types that are created as
part of the 'compile' command are not visible to the rest of the
program for the duration of its run. This example is valid:
compile code int ff = 5; printf ("ff is %d\n", ff);
However, if you were to type the following into GDB after that
command has completed:
compile code printf ("ff is %d\n'', ff);
a compiler error would be raised as the variable 'ff' no longer
exists. Object code generated and injected by the 'compile'
command is removed when its execution ends. Caution is advised
when assigning to program variables values of variables created by
the code submitted to the 'compile' command. This example is
valid:
compile code int ff = 5; k = ff;
The value of the variable 'ff' is assigned to 'k'. The variable
'k' does not require the existence of 'ff' to maintain the value it
has been assigned. However, pointers require particular care in
assignment. If the source code compiled with the 'compile' command
changed the address of a pointer in the example program, perhaps to
a variable created in the 'compile' command, that pointer would
point to an invalid location when the command exits. The following
example would likely cause issues with your debugged program:
compile code int ff = 5; p = &ff;
In this example, 'p' would point to 'ff' when the 'compile' command
is executing the source code provided to it. However, as variables
in the (example) program persist with their assigned values, the
variable 'p' would point to an invalid location when the command
exists. A general rule should be followed in that you should
either assign 'NULL' to any assigned pointers, or restore a valid
location to the pointer before the command exits.
Similar caution must be exercised with any structs, unions, and
typedefs defined in 'compile' command. Types defined in the
'compile' command will no longer be available in the next 'compile'
command. Therefore, if you cast a variable to a type defined in
the 'compile' command, care must be taken to ensure that any future
need to resolve the type can be achieved.
(gdb) compile code static struct a { int a; } v = { 42 }; argv = &v;
(gdb) compile code printf ("%d\n", ((struct a *) argv)->a);
gdb command line:1:36: error: dereferencing pointer to incomplete type ‘struct a’
Compilation failed.
(gdb) compile code struct a { int a; }; printf ("%d\n", ((struct a *) argv)->a);
42
Variables that have been optimized away by the compiler are not
accessible to the code submitted to the 'compile' command. Access
to those variables will generate a compiler error which GDB will
print to the console.
17.7.3 Compiler search for the 'compile' command
------------------------------------------------
GDB needs to find GCC for the inferior being debugged which may not be
obvious for remote targets of different architecture than where GDB is
running. Environment variable 'PATH' on GDB host is searched for GCC
binary matching the target architecture and operating system. This
search can be overriden by 'set compile-gcc' GDB command below. 'PATH'
is taken from shell that executed GDB, it is not the value set by GDB
command 'set environment'). *Note Environment::.
Specifically 'PATH' is searched for binaries matching regular
expression 'ARCH(-[^-]*)?-OS-gcc' according to the inferior target being
debugged. ARCH is processor name -- multiarch is supported, so for
example both 'i386' and 'x86_64' targets look for pattern
'(x86_64|i.86)' and both 's390' and 's390x' targets look for pattern
's390x?'. OS is currently supported only for pattern 'linux(-gnu)?'.
On Posix hosts the compiler driver GDB needs to find also shared
library 'libcc1.so' from the compiler. It is searched in default shared
library search path (overridable with usual environment variable
'LD_LIBRARY_PATH'), unrelated to 'PATH' or 'set compile-gcc' settings.
Contrary to it 'libcc1plugin.so' is found according to the installation
of the found compiler -- as possibly specified by the 'set compile-gcc'
command.
'set compile-gcc'
Set compilation command used for compiling and injecting code with
the 'compile' commands. If this option is not set (it is set to an
empty string), the search described above will occur -- that is the
default.
'show compile-gcc'
Displays the current compile command GCC driver filename. If set,
it is the main command 'gcc', found usually for example under name
'x86_64-linux-gnu-gcc'.

File: gdb.info, Node: GDB Files, Next: Targets, Prev: Altering, Up: Top
18 GDB Files
************
GDB needs to know the file name of the program to be debugged, both in
order to read its symbol table and in order to start your program. To
debug a core dump of a previous run, you must also tell GDB the name of
the core dump file.
* Menu:
* Files:: Commands to specify files
* File Caching:: Information about GDB's file caching
* Separate Debug Files:: Debugging information in separate files
* MiniDebugInfo:: Debugging information in a special section
* Index Files:: Index files speed up GDB
* Symbol Errors:: Errors reading symbol files
* Data Files:: GDB data files

File: gdb.info, Node: Files, Next: File Caching, Up: GDB Files
18.1 Commands to Specify Files
==============================
You may want to specify executable and core dump file names. The usual
way to do this is at start-up time, using the arguments to GDB's
start-up commands (*note Getting In and Out of GDB: Invocation.).
Occasionally it is necessary to change to a different file during a
GDB session. Or you may run GDB and forget to specify a file you want
to use. Or you are debugging a remote target via 'gdbserver' (*note
file: Server.). In these situations the GDB commands to specify new
files are useful.
'file FILENAME'
Use FILENAME as the program to be debugged. It is read for its
symbols and for the contents of pure memory. It is also the
program executed when you use the 'run' command. If you do not
specify a directory and the file is not found in the GDB working
directory, GDB uses the environment variable 'PATH' as a list of
directories to search, just as the shell does when looking for a
program to run. You can change the value of this variable, for
both GDB and your program, using the 'path' command.
You can load unlinked object '.o' files into GDB using the 'file'
command. You will not be able to "run" an object file, but you can
disassemble functions and inspect variables. Also, if the
underlying BFD functionality supports it, you could use 'gdb
-write' to patch object files using this technique. Note that GDB
can neither interpret nor modify relocations in this case, so
branches and some initialized variables will appear to go to the
wrong place. But this feature is still handy from time to time.
'file'
'file' with no argument makes GDB discard any information it has on
both executable file and the symbol table.
'exec-file [ FILENAME ]'
Specify that the program to be run (but not the symbol table) is
found in FILENAME. GDB searches the environment variable 'PATH' if
necessary to locate your program. Omitting FILENAME means to
discard information on the executable file.
'symbol-file [ FILENAME [ -o OFFSET ]]'
Read symbol table information from file FILENAME. 'PATH' is
searched when necessary. Use the 'file' command to get both symbol
table and program to run from the same file.
If an optional OFFSET is specified, it is added to the start
address of each section in the symbol file. This is useful if the
program is relocated at runtime, such as the Linux kernel with
kASLR enabled.
'symbol-file' with no argument clears out GDB information on your
program's symbol table.
The 'symbol-file' command causes GDB to forget the contents of some
breakpoints and auto-display expressions. This is because they may
contain pointers to the internal data recording symbols and data
types, which are part of the old symbol table data being discarded
inside GDB.
'symbol-file' does not repeat if you press <RET> again after
executing it once.
When GDB is configured for a particular environment, it understands
debugging information in whatever format is the standard generated
for that environment; you may use either a GNU compiler, or other
compilers that adhere to the local conventions. Best results are
usually obtained from GNU compilers; for example, using 'GCC' you
can generate debugging information for optimized code.
For most kinds of object files, with the exception of old SVR3
systems using COFF, the 'symbol-file' command does not normally
read the symbol table in full right away. Instead, it scans the
symbol table quickly to find which source files and which symbols
are present. The details are read later, one source file at a
time, as they are needed.
The purpose of this two-stage reading strategy is to make GDB start
up faster. For the most part, it is invisible except for
occasional pauses while the symbol table details for a particular
source file are being read. (The 'set verbose' command can turn
these pauses into messages if desired. *Note Optional Warnings and
Messages: Messages/Warnings.)
We have not implemented the two-stage strategy for COFF yet. When
the symbol table is stored in COFF format, 'symbol-file' reads the
symbol table data in full right away. Note that "stabs-in-COFF"
still does the two-stage strategy, since the debug info is actually
in stabs format.
'symbol-file [ -readnow ] FILENAME'
'file [ -readnow ] FILENAME'
You can override the GDB two-stage strategy for reading symbol
tables by using the '-readnow' option with any of the commands that
load symbol table information, if you want to be sure GDB has the
entire symbol table available.
'symbol-file [ -readnever ] FILENAME'
'file [ -readnever ] FILENAME'
You can instruct GDB to never read the symbolic information
contained in FILENAME by using the '-readnever' option. *Note
--readnever::.
'core-file [FILENAME]'
'core'
Specify the whereabouts of a core dump file to be used as the
"contents of memory". Traditionally, core files contain only some
parts of the address space of the process that generated them; GDB
can access the executable file itself for other parts.
'core-file' with no argument specifies that no core file is to be
used.
Note that the core file is ignored when your program is actually
running under GDB. So, if you have been running your program and
you wish to debug a core file instead, you must kill the subprocess
in which the program is running. To do this, use the 'kill'
command (*note Killing the Child Process: Kill Process.).
'add-symbol-file FILENAME [ -readnow | -readnever ] [ -o OFFSET ] [ TEXTADDRESS ] [ -s SECTION ADDRESS ... ]'
The 'add-symbol-file' command reads additional symbol table
information from the file FILENAME. You would use this command
when FILENAME has been dynamically loaded (by some other means)
into the program that is running. The TEXTADDRESS parameter gives
the memory address at which the file's text section has been
loaded. You can additionally specify the base address of other
sections using an arbitrary number of '-s SECTION ADDRESS' pairs.
If a section is omitted, GDB will use its default addresses as
found in FILENAME. Any ADDRESS or TEXTADDRESS can be given as an
expression.
If an optional OFFSET is specified, it is added to the start
address of each section, except those for which the address was
specified explicitly.
The symbol table of the file FILENAME is added to the symbol table
originally read with the 'symbol-file' command. You can use the
'add-symbol-file' command any number of times; the new symbol data
thus read is kept in addition to the old.
Changes can be reverted using the command 'remove-symbol-file'.
Although FILENAME is typically a shared library file, an executable
file, or some other object file which has been fully relocated for
loading into a process, you can also load symbolic information from
relocatable '.o' files, as long as:
* the file's symbolic information refers only to linker symbols
defined in that file, not to symbols defined by other object
files,
* every section the file's symbolic information refers to has
actually been loaded into the inferior, as it appears in the
file, and
* you can determine the address at which every section was
loaded, and provide these to the 'add-symbol-file' command.
Some embedded operating systems, like Sun Chorus and VxWorks, can
load relocatable files into an already running program; such
systems typically make the requirements above easy to meet.
However, it's important to recognize that many native systems use
complex link procedures ('.linkonce' section factoring and C++
constructor table assembly, for example) that make the requirements
difficult to meet. In general, one cannot assume that using
'add-symbol-file' to read a relocatable object file's symbolic
information will have the same effect as linking the relocatable
object file into the program in the normal way.
'add-symbol-file' does not repeat if you press <RET> after using
it.
'remove-symbol-file FILENAME'
'remove-symbol-file -a ADDRESS'
Remove a symbol file added via the 'add-symbol-file' command. The
file to remove can be identified by its FILENAME or by an ADDRESS
that lies within the boundaries of this symbol file in memory.
Example:
(gdb) add-symbol-file /home/user/gdb/mylib.so 0x7ffff7ff9480
add symbol table from file "/home/user/gdb/mylib.so" at
.text_addr = 0x7ffff7ff9480
(y or n) y
Reading symbols from /home/user/gdb/mylib.so...done.
(gdb) remove-symbol-file -a 0x7ffff7ff9480
Remove symbol table from file "/home/user/gdb/mylib.so"? (y or n) y
(gdb)
'remove-symbol-file' does not repeat if you press <RET> after using
it.
'add-symbol-file-from-memory ADDRESS'
Load symbols from the given ADDRESS in a dynamically loaded object
file whose image is mapped directly into the inferior's memory.
For example, the Linux kernel maps a 'syscall DSO' into each
process's address space; this DSO provides kernel-specific code for
some system calls. The argument can be any expression whose
evaluation yields the address of the file's shared object file
header. For this command to work, you must have used 'symbol-file'
or 'exec-file' commands in advance.
'section SECTION ADDR'
The 'section' command changes the base address of the named SECTION
of the exec file to ADDR. This can be used if the exec file does
not contain section addresses, (such as in the 'a.out' format), or
when the addresses specified in the file itself are wrong. Each
section must be changed separately. The 'info files' command,
described below, lists all the sections and their addresses.
'info files'
'info target'
'info files' and 'info target' are synonymous; both print the
current target (*note Specifying a Debugging Target: Targets.),
including the names of the executable and core dump files currently
in use by GDB, and the files from which symbols were loaded. The
command 'help target' lists all possible targets rather than
current ones.
'maint info sections'
Another command that can give you extra information about program
sections is 'maint info sections'. In addition to the section
information displayed by 'info files', this command displays the
flags and file offset of each section in the executable and core
dump files. In addition, 'maint info sections' provides the
following command options (which may be arbitrarily combined):
'ALLOBJ'
Display sections for all loaded object files, including shared
libraries.
'SECTIONS'
Display info only for named SECTIONS.
'SECTION-FLAGS'
Display info only for sections for which SECTION-FLAGS are
true. The section flags that GDB currently knows about are:
'ALLOC'
Section will have space allocated in the process when
loaded. Set for all sections except those containing
debug information.
'LOAD'
Section will be loaded from the file into the child
process memory. Set for pre-initialized code and data,
clear for '.bss' sections.
'RELOC'
Section needs to be relocated before loading.
'READONLY'
Section cannot be modified by the child process.
'CODE'
Section contains executable code only.
'DATA'
Section contains data only (no executable code).
'ROM'
Section will reside in ROM.
'CONSTRUCTOR'
Section contains data for constructor/destructor lists.
'HAS_CONTENTS'
Section is not empty.
'NEVER_LOAD'
An instruction to the linker to not output the section.
'COFF_SHARED_LIBRARY'
A notification to the linker that the section contains
COFF shared library information.
'IS_COMMON'
Section contains common symbols.
'set trust-readonly-sections on'
Tell GDB that readonly sections in your object file really are
read-only (i.e. that their contents will not change). In that
case, GDB can fetch values from these sections out of the object
file, rather than from the target program. For some targets
(notably embedded ones), this can be a significant enhancement to
debugging performance.
The default is off.
'set trust-readonly-sections off'
Tell GDB not to trust readonly sections. This means that the
contents of the section might change while the program is running,
and must therefore be fetched from the target when needed.
'show trust-readonly-sections'
Show the current setting of trusting readonly sections.
All file-specifying commands allow both absolute and relative file
names as arguments. GDB always converts the file name to an absolute
file name and remembers it that way.
GDB supports GNU/Linux, MS-Windows, SunOS, Darwin/Mach-O, SVr4, IBM
RS/6000 AIX, QNX Neutrino, FDPIC (FR-V), and DSBT (TIC6X) shared
libraries.
On MS-Windows GDB must be linked with the Expat library to support
shared libraries. *Note Expat::.
GDB automatically loads symbol definitions from shared libraries when
you use the 'run' command, or when you examine a core file. (Before you
issue the 'run' command, GDB does not understand references to a
function in a shared library, however--unless you are debugging a core
file).
There are times, however, when you may wish to not automatically load
symbol definitions from shared libraries, such as when they are
particularly large or there are many of them.
To control the automatic loading of shared library symbols, use the
commands:
'set auto-solib-add MODE'
If MODE is 'on', symbols from all shared object libraries will be
loaded automatically when the inferior begins execution, you attach
to an independently started inferior, or when the dynamic linker
informs GDB that a new library has been loaded. If MODE is 'off',
symbols must be loaded manually, using the 'sharedlibrary' command.
The default value is 'on'.
If your program uses lots of shared libraries with debug info that
takes large amounts of memory, you can decrease the GDB memory
footprint by preventing it from automatically loading the symbols
from shared libraries. To that end, type 'set auto-solib-add off'
before running the inferior, then load each library whose debug
symbols you do need with 'sharedlibrary REGEXP', where REGEXP is a
regular expression that matches the libraries whose symbols you
want to be loaded.
'show auto-solib-add'
Display the current autoloading mode.
To explicitly load shared library symbols, use the 'sharedlibrary'
command:
'info share REGEX'
'info sharedlibrary REGEX'
Print the names of the shared libraries which are currently loaded
that match REGEX. If REGEX is omitted then print all shared
libraries that are loaded.
'info dll REGEX'
This is an alias of 'info sharedlibrary'.
'sharedlibrary REGEX'
'share REGEX'
Load shared object library symbols for files matching a Unix
regular expression. As with files loaded automatically, it only
loads shared libraries required by your program for a core file or
after typing 'run'. If REGEX is omitted all shared libraries
required by your program are loaded.
'nosharedlibrary'
Unload all shared object library symbols. This discards all
symbols that have been loaded from all shared libraries. Symbols
from shared libraries that were loaded by explicit user requests
are not discarded.
Sometimes you may wish that GDB stops and gives you control when any
of shared library events happen. The best way to do this is to use
'catch load' and 'catch unload' (*note Set Catchpoints::).
GDB also supports the the 'set stop-on-solib-events' command for
this. This command exists for historical reasons. It is less useful
than setting a catchpoint, because it does not allow for conditions or
commands as a catchpoint does.
'set stop-on-solib-events'
This command controls whether GDB should give you control when the
dynamic linker notifies it about some shared library event. The
most common event of interest is loading or unloading of a new
shared library.
'show stop-on-solib-events'
Show whether GDB stops and gives you control when shared library
events happen.
Shared libraries are also supported in many cross or remote debugging
configurations. GDB needs to have access to the target's libraries;
this can be accomplished either by providing copies of the libraries on
the host system, or by asking GDB to automatically retrieve the
libraries from the target. If copies of the target libraries are
provided, they need to be the same as the target libraries, although the
copies on the target can be stripped as long as the copies on the host
are not.
For remote debugging, you need to tell GDB where the target libraries
are, so that it can load the correct copies--otherwise, it may try to
load the host's libraries. GDB has two variables to specify the search
directories for target libraries.
'set sysroot PATH'
Use PATH as the system root for the program being debugged. Any
absolute shared library paths will be prefixed with PATH; many
runtime loaders store the absolute paths to the shared library in
the target program's memory. When starting processes remotely, and
when attaching to already-running processes (local or remote),
their executable filenames will be prefixed with PATH if reported
to GDB as absolute by the operating system. If you use 'set
sysroot' to find executables and shared libraries, they need to be
laid out in the same way that they are on the target, with e.g. a
'/bin', '/lib' and '/usr/lib' hierarchy under PATH.
If PATH starts with the sequence 'target:' and the target system is
remote then GDB will retrieve the target binaries from the remote
system. This is only supported when using a remote target that
supports the 'remote get' command (*note Sending files to a remote
system: File Transfer.). The part of PATH following the initial
'target:' (if present) is used as system root prefix on the remote
file system. If PATH starts with the sequence 'remote:' this is
converted to the sequence 'target:' by 'set sysroot'(1). If you
want to specify a local system root using a directory that happens
to be named 'target:' or 'remote:', you need to use some equivalent
variant of the name like './target:'.
For targets with an MS-DOS based filesystem, such as MS-Windows and
SymbianOS, GDB tries prefixing a few variants of the target
absolute file name with PATH. But first, on Unix hosts, GDB
converts all backslash directory separators into forward slashes,
because the backslash is not a directory separator on Unix:
c:\foo\bar.dll => c:/foo/bar.dll
Then, GDB attempts prefixing the target file name with PATH, and
looks for the resulting file name in the host file system:
c:/foo/bar.dll => /path/to/sysroot/c:/foo/bar.dll
If that does not find the binary, GDB tries removing the ':'
character from the drive spec, both for convenience, and, for the
case of the host file system not supporting file names with colons:
c:/foo/bar.dll => /path/to/sysroot/c/foo/bar.dll
This makes it possible to have a system root that mirrors a target
with more than one drive. E.g., you may want to setup your local
copies of the target system shared libraries like so (note 'c' vs
'z'):
/path/to/sysroot/c/sys/bin/foo.dll
/path/to/sysroot/c/sys/bin/bar.dll
/path/to/sysroot/z/sys/bin/bar.dll
and point the system root at '/path/to/sysroot', so that GDB can
find the correct copies of both 'c:\sys\bin\foo.dll', and
'z:\sys\bin\bar.dll'.
If that still does not find the binary, GDB tries removing the
whole drive spec from the target file name:
c:/foo/bar.dll => /path/to/sysroot/foo/bar.dll
This last lookup makes it possible to not care about the drive
name, if you don't want or need to.
The 'set solib-absolute-prefix' command is an alias for 'set
sysroot'.
You can set the default system root by using the configure-time
'--with-sysroot' option. If the system root is inside GDB's
configured binary prefix (set with '--prefix' or '--exec-prefix'),
then the default system root will be updated automatically if the
installed GDB is moved to a new location.
'show sysroot'
Display the current executable and shared library prefix.
'set solib-search-path PATH'
If this variable is set, PATH is a colon-separated list of
directories to search for shared libraries. 'solib-search-path' is
used after 'sysroot' fails to locate the library, or if the path to
the library is relative instead of absolute. If you want to use
'solib-search-path' instead of 'sysroot', be sure to set 'sysroot'
to a nonexistent directory to prevent GDB from finding your host's
libraries. 'sysroot' is preferred; setting it to a nonexistent
directory may interfere with automatic loading of shared library
symbols.
'show solib-search-path'
Display the current shared library search path.
'set target-file-system-kind KIND'
Set assumed file system kind for target reported file names.
Shared library file names as reported by the target system may not
make sense as is on the system GDB is running on. For example,
when remote debugging a target that has MS-DOS based file system
semantics, from a Unix host, the target may be reporting to GDB a
list of loaded shared libraries with file names such as
'c:\Windows\kernel32.dll'. On Unix hosts, there's no concept of
drive letters, so the 'c:\' prefix is not normally understood as
indicating an absolute file name, and neither is the backslash
normally considered a directory separator character. In that case,
the native file system would interpret this whole absolute file
name as a relative file name with no directory components. This
would make it impossible to point GDB at a copy of the remote
target's shared libraries on the host using 'set sysroot', and
impractical with 'set solib-search-path'. Setting
'target-file-system-kind' to 'dos-based' tells GDB to interpret
such file names similarly to how the target would, and to map them
to file names valid on GDB's native file system semantics. The
value of KIND can be '"auto"', in addition to one of the supported
file system kinds. In that case, GDB tries to determine the
appropriate file system variant based on the current target's
operating system (*note Configuring the Current ABI: ABI.). The
supported file system settings are:
'unix'
Instruct GDB to assume the target file system is of Unix kind.
Only file names starting the forward slash ('/') character are
considered absolute, and the directory separator character is
also the forward slash.
'dos-based'
Instruct GDB to assume the target file system is DOS based.
File names starting with either a forward slash, or a drive
letter followed by a colon (e.g., 'c:'), are considered
absolute, and both the slash ('/') and the backslash ('\\')
characters are considered directory separators.
'auto'
Instruct GDB to use the file system kind associated with the
target operating system (*note Configuring the Current ABI:
ABI.). This is the default.
When processing file names provided by the user, GDB frequently needs
to compare them to the file names recorded in the program's debug info.
Normally, GDB compares just the "base names" of the files as strings,
which is reasonably fast even for very large programs. (The base name
of a file is the last portion of its name, after stripping all the
leading directories.) This shortcut in comparison is based upon the
assumption that files cannot have more than one base name. This is
usually true, but references to files that use symlinks or similar
filesystem facilities violate that assumption. If your program records
files using such facilities, or if you provide file names to GDB using
symlinks etc., you can set 'basenames-may-differ' to 'true' to instruct
GDB to completely canonicalize each pair of file names it needs to
compare. This will make file-name comparisons accurate, but at a price
of a significant slowdown.
'set basenames-may-differ'
Set whether a source file may have multiple base names.
'show basenames-may-differ'
Show whether a source file may have multiple base names.
---------- Footnotes ----------
(1) Historically the functionality to retrieve binaries from the
remote system was provided by prefixing PATH with 'remote:'

File: gdb.info, Node: File Caching, Next: Separate Debug Files, Prev: Files, Up: GDB Files
18.2 File Caching
=================
To speed up file loading, and reduce memory usage, GDB will reuse the
'bfd' objects used to track open files. *Note BFD: (bfd)Top. The
following commands allow visibility and control of the caching behavior.
'maint info bfds'
This prints information about each 'bfd' object that is known to
GDB.
'maint set bfd-sharing'
'maint show bfd-sharing'
Control whether 'bfd' objects can be shared. When sharing is
enabled GDB reuses already open 'bfd' objects rather than reopening
the same file. Turning sharing off does not cause already shared
'bfd' objects to be unshared, but all future files that are opened
will create a new 'bfd' object. Similarly, re-enabling sharing
does not cause multiple existing 'bfd' objects to be collapsed into
a single shared 'bfd' object.
'set debug bfd-cache LEVEL'
Turns on debugging of the bfd cache, setting the level to LEVEL.
'show debug bfd-cache'
Show the current debugging level of the bfd cache.

File: gdb.info, Node: Separate Debug Files, Next: MiniDebugInfo, Prev: File Caching, Up: GDB Files
18.3 Debugging Information in Separate Files
============================================
GDB allows you to put a program's debugging information in a file
separate from the executable itself, in a way that allows GDB to find
and load the debugging information automatically. Since debugging
information can be very large--sometimes larger than the executable code
itself--some systems distribute debugging information for their
executables in separate files, which users can install only when they
need to debug a problem.
GDB supports two ways of specifying the separate debug info file:
* The executable contains a "debug link" that specifies the name of
the separate debug info file. The separate debug file's name is
usually 'EXECUTABLE.debug', where EXECUTABLE is the name of the
corresponding executable file without leading directories (e.g.,
'ls.debug' for '/usr/bin/ls'). In addition, the debug link
specifies a 32-bit "Cyclic Redundancy Check" (CRC) checksum for the
debug file, which GDB uses to validate that the executable and the
debug file came from the same build.
* The executable contains a "build ID", a unique bit string that is
also present in the corresponding debug info file. (This is
supported only on some operating systems, when using the ELF or PE
file formats for binary files and the GNU Binutils.) For more
details about this feature, see the description of the '--build-id'
command-line option in *note Command Line Options: (ld)Options.
The debug info file's name is not specified explicitly by the build
ID, but can be computed from the build ID, see below.
Depending on the way the debug info file is specified, GDB uses two
different methods of looking for the debug file:
* For the "debug link" method, GDB looks up the named file in the
directory of the executable file, then in a subdirectory of that
directory named '.debug', and finally under each one of the global
debug directories, in a subdirectory whose name is identical to the
leading directories of the executable's absolute file name. (On
MS-Windows/MS-DOS, the drive letter of the executable's leading
directories is converted to a one-letter subdirectory, i.e.
'd:/usr/bin/' is converted to '/d/usr/bin/', because Windows
filesystems disallow colons in file names.)
* For the "build ID" method, GDB looks in the '.build-id'
subdirectory of each one of the global debug directories for a file
named 'NN/NNNNNNNN.debug', where NN are the first 2 hex characters
of the build ID bit string, and NNNNNNNN are the rest of the bit
string. (Real build ID strings are 32 or more hex characters, not
10.)
So, for example, suppose you ask GDB to debug '/usr/bin/ls', which
has a debug link that specifies the file 'ls.debug', and a build ID
whose value in hex is 'abcdef1234'. If the list of the global debug
directories includes '/usr/lib/debug', then GDB will look for the
following debug information files, in the indicated order:
- '/usr/lib/debug/.build-id/ab/cdef1234.debug'
- '/usr/bin/ls.debug'
- '/usr/bin/.debug/ls.debug'
- '/usr/lib/debug/usr/bin/ls.debug'.
Global debugging info directories default to what is set by GDB
configure option '--with-separate-debug-dir'. During GDB run you can
also set the global debugging info directories, and view the list GDB is
currently using.
'set debug-file-directory DIRECTORIES'
Set the directories which GDB searches for separate debugging
information files to DIRECTORY. Multiple path components can be
set concatenating them by a path separator.
'show debug-file-directory'
Show the directories GDB searches for separate debugging
information files.
A debug link is a special section of the executable file named
'.gnu_debuglink'. The section must contain:
* A filename, with any leading directory components removed, followed
by a zero byte,
* zero to three bytes of padding, as needed to reach the next
four-byte boundary within the section, and
* a four-byte CRC checksum, stored in the same endianness used for
the executable file itself. The checksum is computed on the
debugging information file's full contents by the function given
below, passing zero as the CRC argument.
Any executable file format can carry a debug link, as long as it can
contain a section named '.gnu_debuglink' with the contents described
above.
The build ID is a special section in the executable file (and in
other ELF binary files that GDB may consider). This section is often
named '.note.gnu.build-id', but that name is not mandatory. It contains
unique identification for the built files--the ID remains the same
across multiple builds of the same build tree. The default algorithm
SHA1 produces 160 bits (40 hexadecimal characters) of the content for
the build ID string. The same section with an identical value is
present in the original built binary with symbols, in its stripped
variant, and in the separate debugging information file.
The debugging information file itself should be an ordinary
executable, containing a full set of linker symbols, sections, and
debugging information. The sections of the debugging information file
should have the same names, addresses, and sizes as the original file,
but they need not contain any data--much like a '.bss' section in an
ordinary executable.
The GNU binary utilities (Binutils) package includes the 'objcopy'
utility that can produce the separated executable / debugging
information file pairs using the following commands:
objcopy --only-keep-debug foo foo.debug
strip -g foo
These commands remove the debugging information from the executable file
'foo' and place it in the file 'foo.debug'. You can use the first,
second or both methods to link the two files:
* The debug link method needs the following additional command to
also leave behind a debug link in 'foo':
objcopy --add-gnu-debuglink=foo.debug foo
Ulrich Drepper's 'elfutils' package, starting with version 0.53,
contains a version of the 'strip' command such that the command
'strip foo -f foo.debug' has the same functionality as the two
'objcopy' commands and the 'ln -s' command above, together.
* Build ID gets embedded into the main executable using 'ld
--build-id' or the GCC counterpart 'gcc -Wl,--build-id'. Build ID
support plus compatibility fixes for debug files separation are
present in GNU binary utilities (Binutils) package since version
2.18.
The CRC used in '.gnu_debuglink' is the CRC-32 defined in IEEE 802.3
using the polynomial:
x^{32} + x^{26} + x^{23} + x^{22} + x^{16} + x^{12} + x^{11}
+ x^{10} + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
The function is computed byte at a time, taking the least significant
bit of each byte first. The initial pattern '0xffffffff' is used, to
ensure leading zeros affect the CRC and the final result is inverted to
ensure trailing zeros also affect the CRC.
_Note:_ This is the same CRC polynomial as used in handling the
"Remote Serial Protocol" 'qCRC' packet (*note qCRC packet::). However
in the case of the Remote Serial Protocol, the CRC is computed _most_
significant bit first, and the result is not inverted, so trailing zeros
have no effect on the CRC value.
To complete the description, we show below the code of the function
which produces the CRC used in '.gnu_debuglink'. Inverting the
initially supplied 'crc' argument means that an initial call to this
function passing in zero will start computing the CRC using
'0xffffffff'.
unsigned long
gnu_debuglink_crc32 (unsigned long crc,
unsigned char *buf, size_t len)
{
static const unsigned long crc32_table[256] =
{
0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
0x2d02ef8d
};
unsigned char *end;
crc = ~crc & 0xffffffff;
for (end = buf + len; buf < end; ++buf)
crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
return ~crc & 0xffffffff;
}
This computation does not apply to the "build ID" method.

File: gdb.info, Node: MiniDebugInfo, Next: Index Files, Prev: Separate Debug Files, Up: GDB Files
18.4 Debugging information in a special section
===============================================
Some systems ship pre-built executables and libraries that have a
special '.gnu_debugdata' section. This feature is called
"MiniDebugInfo". This section holds an LZMA-compressed object and is
used to supply extra symbols for backtraces.
The intent of this section is to provide extra minimal debugging
information for use in simple backtraces. It is not intended to be a
replacement for full separate debugging information (*note Separate
Debug Files::). The example below shows the intended use; however, GDB
does not currently put restrictions on what sort of debugging
information might be included in the section.
GDB has support for this extension. If the section exists, then it
is used provided that no other source of debugging information can be
found, and that GDB was configured with LZMA support.
This section can be easily created using 'objcopy' and other standard
utilities:
# Extract the dynamic symbols from the main binary, there is no need
# to also have these in the normal symbol table.
nm -D BINARY --format=posix --defined-only \
| awk '{ print $1 }' | sort > dynsyms
# Extract all the text (i.e. function) symbols from the debuginfo.
# (Note that we actually also accept "D" symbols, for the benefit
# of platforms like PowerPC64 that use function descriptors.)
nm BINARY --format=posix --defined-only \
| awk '{ if ($2 == "T" || $2 == "t" || $2 == "D") print $1 }' \
| sort > funcsyms
# Keep all the function symbols not already in the dynamic symbol
# table.
comm -13 dynsyms funcsyms > keep_symbols
# Separate full debug info into debug binary.
objcopy --only-keep-debug BINARY debug
# Copy the full debuginfo, keeping only a minimal set of symbols and
# removing some unnecessary sections.
objcopy -S --remove-section .gdb_index --remove-section .comment \
--keep-symbols=keep_symbols debug mini_debuginfo
# Drop the full debug info from the original binary.
strip --strip-all -R .comment BINARY
# Inject the compressed data into the .gnu_debugdata section of the
# original binary.
xz mini_debuginfo
objcopy --add-section .gnu_debugdata=mini_debuginfo.xz BINARY

File: gdb.info, Node: Index Files, Next: Symbol Errors, Prev: MiniDebugInfo, Up: GDB Files
18.5 Index Files Speed Up GDB
=============================
When GDB finds a symbol file, it scans the symbols in the file in order
to construct an internal symbol table. This lets most GDB operations
work quickly--at the cost of a delay early on. For large programs, this
delay can be quite lengthy, so GDB provides a way to build an index,
which speeds up startup.
For convenience, GDB comes with a program, 'gdb-add-index', which can
be used to add the index to a symbol file. It takes the symbol file as
its only argument:
$ gdb-add-index symfile
*Note gdb-add-index::.
It is also possible to do the work manually. Here is what
'gdb-add-index' does behind the curtains.
The index is stored as a section in the symbol file. GDB can write
the index to a file, then you can put it into the symbol file using
'objcopy'.
To create an index file, use the 'save gdb-index' command:
'save gdb-index [-dwarf-5] DIRECTORY'
Create index files for all symbol files currently known by GDB.
For each known SYMBOL-FILE, this command by default creates it
produces a single file 'SYMBOL-FILE.gdb-index'. If you invoke this
command with the '-dwarf-5' option, it produces 2 files:
'SYMBOL-FILE.debug_names' and 'SYMBOL-FILE.debug_str'. The files
are created in the given DIRECTORY.
Once you have created an index file you can merge it into your symbol
file, here named 'symfile', using 'objcopy':
$ objcopy --add-section .gdb_index=symfile.gdb-index \
--set-section-flags .gdb_index=readonly symfile symfile
Or for '-dwarf-5':
$ objcopy --dump-section .debug_str=symfile.debug_str.new symfile
$ cat symfile.debug_str >>symfile.debug_str.new
$ objcopy --add-section .debug_names=symfile.gdb-index \
--set-section-flags .debug_names=readonly \
--update-section .debug_str=symfile.debug_str.new symfile symfile
GDB will normally ignore older versions of '.gdb_index' sections that
have been deprecated. Usually they are deprecated because they are
missing a new feature or have performance issues. To tell GDB to use a
deprecated index section anyway specify 'set
use-deprecated-index-sections on'. The default is 'off'. This can
speed up startup, but may result in some functionality being lost.
*Note Index Section Format::.
_Warning:_ Setting 'use-deprecated-index-sections' to 'on' must be
done before gdb reads the file. The following will not work:
$ gdb -ex "set use-deprecated-index-sections on" <program>
Instead you must do, for example,
$ gdb -iex "set use-deprecated-index-sections on" <program>
There are currently some limitation on indices. They only work when
using DWARF debugging information, not stabs. And, only the '-dwarf-5'
index works for programs using Ada.
18.5.1 Automatic symbol index cache
-----------------------------------
It is possible for GDB to automatically save a copy of this index in a
cache on disk and retrieve it from there when loading the same binary in
the future. This feature can be turned on with 'set index-cache on'.
The following commands can be used to tweak the behavior of the index
cache.
'set index-cache on'
'set index-cache off'
Enable or disable the use of the symbol index cache.
'set index-cache directory DIRECTORY'
'show index-cache directory'
Set/show the directory where index files will be saved.
The default value for this directory depends on the host platform.
On most systems, the index is cached in the 'gdb' subdirectory of
the directory pointed to by the 'XDG_CACHE_HOME' environment
variable, if it is defined, else in the '.cache/gdb' subdirectory
of your home directory. However, on some systems, the default may
differ according to local convention.
There is no limit on the disk space used by index cache. It is
perfectly safe to delete the content of that directory to free up
disk space.
'show index-cache stats'
Print the number of cache hits and misses since the launch of GDB.

File: gdb.info, Node: Symbol Errors, Next: Data Files, Prev: Index Files, Up: GDB Files
18.6 Errors Reading Symbol Files
================================
While reading a symbol file, GDB occasionally encounters problems, such
as symbol types it does not recognize, or known bugs in compiler output.
By default, GDB does not notify you of such problems, since they are
relatively common and primarily of interest to people debugging
compilers. If you are interested in seeing information about
ill-constructed symbol tables, you can either ask GDB to print only one
message about each such type of problem, no matter how many times the
problem occurs; or you can ask GDB to print more messages, to see how
many times the problems occur, with the 'set complaints' command (*note
Optional Warnings and Messages: Messages/Warnings.).
The messages currently printed, and their meanings, include:
'inner block not inside outer block in SYMBOL'
The symbol information shows where symbol scopes begin and end
(such as at the start of a function or a block of statements).
This error indicates that an inner scope block is not fully
contained in its outer scope blocks.
GDB circumvents the problem by treating the inner block as if it
had the same scope as the outer block. In the error message,
SYMBOL may be shown as "'(don't know)'" if the outer block is not a
function.
'block at ADDRESS out of order'
The symbol information for symbol scope blocks should occur in
order of increasing addresses. This error indicates that it does
not do so.
GDB does not circumvent this problem, and has trouble locating
symbols in the source file whose symbols it is reading. (You can
often determine what source file is affected by specifying 'set
verbose on'. *Note Optional Warnings and Messages:
Messages/Warnings.)
'bad block start address patched'
The symbol information for a symbol scope block has a start address
smaller than the address of the preceding source line. This is
known to occur in the SunOS 4.1.1 (and earlier) C compiler.
GDB circumvents the problem by treating the symbol scope block as
starting on the previous source line.
'bad string table offset in symbol N'
Symbol number N contains a pointer into the string table which is
larger than the size of the string table.
GDB circumvents the problem by considering the symbol to have the
name 'foo', which may cause other problems if many symbols end up
with this name.
'unknown symbol type 0xNN'
The symbol information contains new data types that GDB does not
yet know how to read. '0xNN' is the symbol type of the
uncomprehended information, in hexadecimal.
GDB circumvents the error by ignoring this symbol information.
This usually allows you to debug your program, though certain
symbols are not accessible. If you encounter such a problem and
feel like debugging it, you can debug 'gdb' with itself, breakpoint
on 'complain', then go up to the function 'read_dbx_symtab' and
examine '*bufp' to see the symbol.
'stub type has NULL name'
GDB could not find the full definition for a struct or class.
'const/volatile indicator missing (ok if using g++ v1.x), got...'
The symbol information for a C++ member function is missing some
information that recent versions of the compiler should have output
for it.
'info mismatch between compiler and debugger'
GDB could not parse a type specification output by the compiler.

File: gdb.info, Node: Data Files, Prev: Symbol Errors, Up: GDB Files
18.7 GDB Data Files
===================
GDB will sometimes read an auxiliary data file. These files are kept in
a directory known as the "data directory".
You can set the data directory's name, and view the name GDB is
currently using.
'set data-directory DIRECTORY'
Set the directory which GDB searches for auxiliary data files to
DIRECTORY.
'show data-directory'
Show the directory GDB searches for auxiliary data files.
You can set the default data directory by using the configure-time
'--with-gdb-datadir' option. If the data directory is inside GDB's
configured binary prefix (set with '--prefix' or '--exec-prefix'), then
the default data directory will be updated automatically if the
installed GDB is moved to a new location.
The data directory may also be specified with the '--data-directory'
command line option. *Note Mode Options::.

File: gdb.info, Node: Targets, Next: Remote Debugging, Prev: GDB Files, Up: Top
19 Specifying a Debugging Target
********************************
A "target" is the execution environment occupied by your program.
Often, GDB runs in the same host environment as your program; in that
case, the debugging target is specified as a side effect when you use
the 'file' or 'core' commands. When you need more flexibility--for
example, running GDB on a physically separate host, or controlling a
standalone system over a serial port or a realtime system over a TCP/IP
connection--you can use the 'target' command to specify one of the
target types configured for GDB (*note Commands for Managing Targets:
Target Commands.).
It is possible to build GDB for several different "target
architectures". When GDB is built like that, you can choose one of the
available architectures with the 'set architecture' command.
'set architecture ARCH'
This command sets the current target architecture to ARCH. The
value of ARCH can be '"auto"', in addition to one of the supported
architectures.
'show architecture'
Show the current target architecture.
'set processor'
'processor'
These are alias commands for, respectively, 'set architecture' and
'show architecture'.
* Menu:
* Active Targets:: Active targets
* Target Commands:: Commands for managing targets
* Byte Order:: Choosing target byte order

File: gdb.info, Node: Active Targets, Next: Target Commands, Up: Targets
19.1 Active Targets
===================
There are multiple classes of targets such as: processes, executable
files or recording sessions. Core files belong to the process class,
making core file and process mutually exclusive. Otherwise, GDB can
work concurrently on multiple active targets, one in each class. This
allows you to (for example) start a process and inspect its activity,
while still having access to the executable file after the process
finishes. Or if you start process recording (*note Reverse Execution::)
and 'reverse-step' there, you are presented a virtual layer of the
recording target, while the process target remains stopped at the
chronologically last point of the process execution.
Use the 'core-file' and 'exec-file' commands to select a new core
file or executable target (*note Commands to Specify Files: Files.). To
specify as a target a process that is already running, use the 'attach'
command (*note Debugging an Already-running Process: Attach.).

File: gdb.info, Node: Target Commands, Next: Byte Order, Prev: Active Targets, Up: Targets
19.2 Commands for Managing Targets
==================================
'target TYPE PARAMETERS'
Connects the GDB host environment to a target machine or process.
A target is typically a protocol for talking to debugging
facilities. You use the argument TYPE to specify the type or
protocol of the target machine.
Further PARAMETERS are interpreted by the target protocol, but
typically include things like device names or host names to connect
with, process numbers, and baud rates.
The 'target' command does not repeat if you press <RET> again after
executing the command.
'help target'
Displays the names of all targets available. To display targets
currently selected, use either 'info target' or 'info files' (*note
Commands to Specify Files: Files.).
'help target NAME'
Describe a particular target, including any parameters necessary to
select it.
'set gnutarget ARGS'
GDB uses its own library BFD to read your files. GDB knows whether
it is reading an "executable", a "core", or a ".o" file; however,
you can specify the file format with the 'set gnutarget' command.
Unlike most 'target' commands, with 'gnutarget' the 'target' refers
to a program, not a machine.
_Warning:_ To specify a file format with 'set gnutarget', you
must know the actual BFD name.
*Note Commands to Specify Files: Files.
'show gnutarget'
Use the 'show gnutarget' command to display what file format
'gnutarget' is set to read. If you have not set 'gnutarget', GDB
will determine the file format for each file automatically, and
'show gnutarget' displays 'The current BFD target is "auto"'.
Here are some common targets (available, or not, depending on the GDB
configuration):
'target exec PROGRAM'
An executable file. 'target exec PROGRAM' is the same as
'exec-file PROGRAM'.
'target core FILENAME'
A core dump file. 'target core FILENAME' is the same as 'core-file
FILENAME'.
'target remote MEDIUM'
A remote system connected to GDB via a serial line or network
connection. This command tells GDB to use its own remote protocol
over MEDIUM for debugging. *Note Remote Debugging::.
For example, if you have a board connected to '/dev/ttya' on the
machine running GDB, you could say:
target remote /dev/ttya
'target remote' supports the 'load' command. This is only useful
if you have some other way of getting the stub to the target
system, and you can put it somewhere in memory where it won't get
clobbered by the download.
'target sim [SIMARGS] ...'
Builtin CPU simulator. GDB includes simulators for most
architectures. In general,
target sim
load
run
works; however, you cannot assume that a specific memory map,
device drivers, or even basic I/O is available, although some
simulators do provide these. For info about any processor-specific
simulator details, see the appropriate section in *note Embedded
Processors: Embedded Processors.
'target native'
Setup for local/native process debugging. Useful to make the 'run'
command spawn native processes (likewise 'attach', etc.) even when
'set auto-connect-native-target' is 'off' (*note set
auto-connect-native-target::).
Different targets are available on different configurations of GDB;
your configuration may have more or fewer targets.
Many remote targets require you to download the executable's code
once you've successfully established a connection. You may wish to
control various aspects of this process.
'set hash'
This command controls whether a hash mark '#' is displayed while
downloading a file to the remote monitor. If on, a hash mark is
displayed after each S-record is successfully downloaded to the
monitor.
'show hash'
Show the current status of displaying the hash mark.
'set debug monitor'
Enable or disable display of communications messages between GDB
and the remote monitor.
'show debug monitor'
Show the current status of displaying communications between GDB
and the remote monitor.
'load FILENAME OFFSET'
Depending on what remote debugging facilities are configured into
GDB, the 'load' command may be available. Where it exists, it is
meant to make FILENAME (an executable) available for debugging on
the remote system--by downloading, or dynamic linking, for example.
'load' also records the FILENAME symbol table in GDB, like the
'add-symbol-file' command.
If your GDB does not have a 'load' command, attempting to execute
it gets the error message "'You can't do that when your target is
...'"
The file is loaded at whatever address is specified in the
executable. For some object file formats, you can specify the load
address when you link the program; for other formats, like a.out,
the object file format specifies a fixed address.
It is also possible to tell GDB to load the executable file at a
specific offset described by the optional argument OFFSET. When
OFFSET is provided, FILENAME must also be provided.
Depending on the remote side capabilities, GDB may be able to load
programs into flash memory.
'load' does not repeat if you press <RET> again after using it.
'flash-erase'
Erases all known flash memory regions on the target.

File: gdb.info, Node: Byte Order, Prev: Target Commands, Up: Targets
19.3 Choosing Target Byte Order
===============================
Some types of processors, such as the MIPS, PowerPC, and Renesas SH,
offer the ability to run either big-endian or little-endian byte orders.
Usually the executable or symbol will include a bit to designate the
endian-ness, and you will not need to worry about which to use.
However, you may still find it useful to adjust GDB's idea of processor
endian-ness manually.
'set endian big'
Instruct GDB to assume the target is big-endian.
'set endian little'
Instruct GDB to assume the target is little-endian.
'set endian auto'
Instruct GDB to use the byte order associated with the executable.
'show endian'
Display GDB's current idea of the target byte order.
If the 'set endian auto' mode is in effect and no executable has been
selected, then the endianness used is the last one chosen either by one
of the 'set endian big' and 'set endian little' commands or by inferring
from the last executable used. If no endianness has been previously
chosen, then the default for this mode is inferred from the target GDB
has been built for, and is 'little' if the name of the target CPU has an
'el' suffix and 'big' otherwise.
Note that these commands merely adjust interpretation of symbolic
data on the host, and that they have absolutely no effect on the target
system.

File: gdb.info, Node: Remote Debugging, Next: Configurations, Prev: Targets, Up: Top
20 Debugging Remote Programs
****************************
If you are trying to debug a program running on a machine that cannot
run GDB in the usual way, it is often useful to use remote debugging.
For example, you might use remote debugging on an operating system
kernel, or on a small system which does not have a general purpose
operating system powerful enough to run a full-featured debugger.
Some configurations of GDB have special serial or TCP/IP interfaces
to make this work with particular debugging targets. In addition, GDB
comes with a generic serial protocol (specific to GDB, but not specific
to any particular target system) which you can use if you write the
remote stubs--the code that runs on the remote system to communicate
with GDB.
Other remote targets may be available in your configuration of GDB;
use 'help target' to list them.
* Menu:
* Connecting:: Connecting to a remote target
* File Transfer:: Sending files to a remote system
* Server:: Using the gdbserver program
* Remote Configuration:: Remote configuration
* Remote Stub:: Implementing a remote stub

File: gdb.info, Node: Connecting, Next: File Transfer, Up: Remote Debugging
20.1 Connecting to a Remote Target
==================================
This section describes how to connect to a remote target, including the
types of connections and their differences, how to set up executable and
symbol files on the host and target, and the commands used for
connecting to and disconnecting from the remote target.
20.1.1 Types of Remote Connections
----------------------------------
GDB supports two types of remote connections, 'target remote' mode and
'target extended-remote' mode. Note that many remote targets support
only 'target remote' mode. There are several major differences between
the two types of connections, enumerated here:
Result of detach or program exit
*With target remote mode:* When the debugged program exits or you
detach from it, GDB disconnects from the target. When using
'gdbserver', 'gdbserver' will exit.
*With target extended-remote mode:* When the debugged program exits
or you detach from it, GDB remains connected to the target, even
though no program is running. You can rerun the program, attach to
a running program, or use 'monitor' commands specific to the
target.
When using 'gdbserver' in this case, it does not exit unless it was
invoked using the '--once' option. If the '--once' option was not
used, you can ask 'gdbserver' to exit using the 'monitor exit'
command (*note Monitor Commands for gdbserver::).
Specifying the program to debug
For both connection types you use the 'file' command to specify the
program on the host system. If you are using 'gdbserver' there are
some differences in how to specify the location of the program on
the target.
*With target remote mode:* You must either specify the program to
debug on the 'gdbserver' command line or use the '--attach' option
(*note Attaching to a Running Program: Attaching to a program.).
*With target extended-remote mode:* You may specify the program to
debug on the 'gdbserver' command line, or you can load the program
or attach to it using GDB commands after connecting to 'gdbserver'.
You can start 'gdbserver' without supplying an initial command to
run or process ID to attach. To do this, use the '--multi' command
line option. Then you can connect using 'target extended-remote'
and start the program you want to debug (see below for details on
using the 'run' command in this scenario). Note that the
conditions under which 'gdbserver' terminates depend on how GDB
connects to it ('target remote' or 'target extended-remote'). The
'--multi' option to 'gdbserver' has no influence on that.
The 'run' command
*With target remote mode:* The 'run' command is not supported.
Once a connection has been established, you can use all the usual
GDB commands to examine and change data. The remote program is
already running, so you can use commands like 'step' and
'continue'.
*With target extended-remote mode:* The 'run' command is supported.
The 'run' command uses the value set by 'set remote exec-file'
(*note set remote exec-file::) to select the program to run.
Command line arguments are supported, except for wildcard expansion
and I/O redirection (*note Arguments::).
If you specify the program to debug on the command line, then the
'run' command is not required to start execution, and you can
resume using commands like 'step' and 'continue' as with 'target
remote' mode.
Attaching
*With target remote mode:* The GDB command 'attach' is not
supported. To attach to a running program using 'gdbserver', you
must use the '--attach' option (*note Running gdbserver::).
*With target extended-remote mode:* To attach to a running program,
you may use the 'attach' command after the connection has been
established. If you are using 'gdbserver', you may also invoke
'gdbserver' using the '--attach' option (*note Running
gdbserver::).
20.1.2 Host and Target Files
----------------------------
GDB, running on the host, needs access to symbol and debugging
information for your program running on the target. This requires
access to an unstripped copy of your program, and possibly any
associated symbol files. Note that this section applies equally to both
'target remote' mode and 'target extended-remote' mode.
Some remote targets (*note qXfer executable filename read::, and
*note Host I/O Packets::) allow GDB to access program files over the
same connection used to communicate with GDB. With such a target, if
the remote program is unstripped, the only command you need is 'target
remote' (or 'target extended-remote').
If the remote program is stripped, or the target does not support
remote program file access, start up GDB using the name of the local
unstripped copy of your program as the first argument, or use the 'file'
command. Use 'set sysroot' to specify the location (on the host) of
target libraries (unless your GDB was compiled with the correct sysroot
using '--with-sysroot'). Alternatively, you may use 'set
solib-search-path' to specify how GDB locates target libraries.
The symbol file and target libraries must exactly match the
executable and libraries on the target, with one exception: the files on
the host system should not be stripped, even if the files on the target
system are. Mismatched or missing files will lead to confusing results
during debugging. On GNU/Linux targets, mismatched or missing files may
also prevent 'gdbserver' from debugging multi-threaded programs.
20.1.3 Remote Connection Commands
---------------------------------
GDB can communicate with the target over a serial line, a local Unix
domain socket, or over an IP network using TCP or UDP. In each case,
GDB uses the same protocol for debugging your program; only the medium
carrying the debugging packets varies. The 'target remote' and 'target
extended-remote' commands establish a connection to the target. Both
commands accept the same arguments, which indicate the medium to use:
'target remote SERIAL-DEVICE'
'target extended-remote SERIAL-DEVICE'
Use SERIAL-DEVICE to communicate with the target. For example, to
use a serial line connected to the device named '/dev/ttyb':
target remote /dev/ttyb
If you're using a serial line, you may want to give GDB the
'--baud' option, or use the 'set serial baud' command (*note set
serial baud: Remote Configuration.) before the 'target' command.
'target remote LOCAL-SOCKET'
'target extended-remote LOCAL-SOCKET'
Use LOCAL-SOCKET to communicate with the target. For example, to
use a local Unix domain socket bound to the file system entry
'/tmp/gdb-socket0':
target remote /tmp/gdb-socket0
Note that this command has the same form as the command to connect
to a serial line. GDB will automatically determine which kind of
file you have specified and will make the appropriate kind of
connection. This feature is not available if the host system does
not support Unix domain sockets.
'target remote HOST:PORT'
'target remote [HOST]:PORT'
'target remote tcp:HOST:PORT'
'target remote tcp:[HOST]:PORT'
'target remote tcp4:HOST:PORT'
'target remote tcp6:HOST:PORT'
'target remote tcp6:[HOST]:PORT'
'target extended-remote HOST:PORT'
'target extended-remote [HOST]:PORT'
'target extended-remote tcp:HOST:PORT'
'target extended-remote tcp:[HOST]:PORT'
'target extended-remote tcp4:HOST:PORT'
'target extended-remote tcp6:HOST:PORT'
'target extended-remote tcp6:[HOST]:PORT'
Debug using a TCP connection to PORT on HOST. The HOST may be
either a host name, a numeric IPv4 address, or a numeric IPv6
address (with or without the square brackets to separate the
address from the port); PORT must be a decimal number. The HOST
could be the target machine itself, if it is directly connected to
the net, or it might be a terminal server which in turn has a
serial line to the target.
For example, to connect to port 2828 on a terminal server named
'manyfarms':
target remote manyfarms:2828
To connect to port 2828 on a terminal server whose address is
'2001:0db8:85a3:0000:0000:8a2e:0370:7334', you can either use the
square bracket syntax:
target remote [2001:0db8:85a3:0000:0000:8a2e:0370:7334]:2828
or explicitly specify the IPv6 protocol:
target remote tcp6:2001:0db8:85a3:0000:0000:8a2e:0370:7334:2828
This last example may be confusing to the reader, because there is
no visible separation between the hostname and the port number.
Therefore, we recommend the user to provide IPv6 addresses using
square brackets for clarity. However, it is important to mention
that for GDB there is no ambiguity: the number after the last colon
is considered to be the port number.
If your remote target is actually running on the same machine as
your debugger session (e.g. a simulator for your target running on
the same host), you can omit the hostname. For example, to connect
to port 1234 on your local machine:
target remote :1234
Note that the colon is still required here.
'target remote udp:HOST:PORT'
'target remote udp:[HOST]:PORT'
'target remote udp4:HOST:PORT'
'target remote udp6:[HOST]:PORT'
'target extended-remote udp:HOST:PORT'
'target extended-remote udp:HOST:PORT'
'target extended-remote udp:[HOST]:PORT'
'target extended-remote udp4:HOST:PORT'
'target extended-remote udp6:HOST:PORT'
'target extended-remote udp6:[HOST]:PORT'
Debug using UDP packets to PORT on HOST. For example, to connect
to UDP port 2828 on a terminal server named 'manyfarms':
target remote udp:manyfarms:2828
When using a UDP connection for remote debugging, you should keep
in mind that the 'U' stands for "Unreliable". UDP can silently
drop packets on busy or unreliable networks, which will cause havoc
with your debugging session.
'target remote | COMMAND'
'target extended-remote | COMMAND'
Run COMMAND in the background and communicate with it using a pipe.
The COMMAND is a shell command, to be parsed and expanded by the
system's command shell, '/bin/sh'; it should expect remote protocol
packets on its standard input, and send replies on its standard
output. You could use this to run a stand-alone simulator that
speaks the remote debugging protocol, to make net connections using
programs like 'ssh', or for other similar tricks.
If COMMAND closes its standard output (perhaps by exiting), GDB
will try to send it a 'SIGTERM' signal. (If the program has
already exited, this will have no effect.)
Whenever GDB is waiting for the remote program, if you type the
interrupt character (often 'Ctrl-c'), GDB attempts to stop the program.
This may or may not succeed, depending in part on the hardware and the
serial drivers the remote system uses. If you type the interrupt
character once again, GDB displays this prompt:
Interrupted while waiting for the program.
Give up (and stop debugging it)? (y or n)
In 'target remote' mode, if you type 'y', GDB abandons the remote
debugging session. (If you decide you want to try again later, you can
use 'target remote' again to connect once more.) If you type 'n', GDB
goes back to waiting.
In 'target extended-remote' mode, typing 'n' will leave GDB connected
to the target.
'detach'
When you have finished debugging the remote program, you can use
the 'detach' command to release it from GDB control. Detaching
from the target normally resumes its execution, but the results
will depend on your particular remote stub. After the 'detach'
command in 'target remote' mode, GDB is free to connect to another
target. In 'target extended-remote' mode, GDB is still connected
to the target.
'disconnect'
The 'disconnect' command closes the connection to the target, and
the target is generally not resumed. It will wait for GDB (this
instance or another one) to connect and continue debugging. After
the 'disconnect' command, GDB is again free to connect to another
target.
'monitor CMD'
This command allows you to send arbitrary commands directly to the
remote monitor. Since GDB doesn't care about the commands it sends
like this, this command is the way to extend GDB--you can add new
commands that only the external monitor will understand and
implement.

File: gdb.info, Node: File Transfer, Next: Server, Prev: Connecting, Up: Remote Debugging
20.2 Sending files to a remote system
=====================================
Some remote targets offer the ability to transfer files over the same
connection used to communicate with GDB. This is convenient for targets
accessible through other means, e.g. GNU/Linux systems running
'gdbserver' over a network interface. For other targets, e.g. embedded
devices with only a single serial port, this may be the only way to
upload or download files.
Not all remote targets support these commands.
'remote put HOSTFILE TARGETFILE'
Copy file HOSTFILE from the host system (the machine running GDB)
to TARGETFILE on the target system.
'remote get TARGETFILE HOSTFILE'
Copy file TARGETFILE from the target system to HOSTFILE on the host
system.
'remote delete TARGETFILE'
Delete TARGETFILE from the target system.

File: gdb.info, Node: Server, Next: Remote Configuration, Prev: File Transfer, Up: Remote Debugging
20.3 Using the 'gdbserver' Program
==================================
'gdbserver' is a control program for Unix-like systems, which allows you
to connect your program with a remote GDB via 'target remote' or 'target
extended-remote'--but without linking in the usual debugging stub.
'gdbserver' is not a complete replacement for the debugging stubs,
because it requires essentially the same operating-system facilities
that GDB itself does. In fact, a system that can run 'gdbserver' to
connect to a remote GDB could also run GDB locally! 'gdbserver' is
sometimes useful nevertheless, because it is a much smaller program than
GDB itself. It is also easier to port than all of GDB, so you may be
able to get started more quickly on a new system by using 'gdbserver'.
Finally, if you develop code for real-time systems, you may find that
the tradeoffs involved in real-time operation make it more convenient to
do as much development work as possible on another system, for example
by cross-compiling. You can use 'gdbserver' to make a similar choice
for debugging.
GDB and 'gdbserver' communicate via either a serial line or a TCP
connection, using the standard GDB remote serial protocol.
_Warning:_ 'gdbserver' does not have any built-in security. Do not
run 'gdbserver' connected to any public network; a GDB connection
to 'gdbserver' provides access to the target system with the same
privileges as the user running 'gdbserver'.
20.3.1 Running 'gdbserver'
--------------------------
Run 'gdbserver' on the target system. You need a copy of the program
you want to debug, including any libraries it requires. 'gdbserver'
does not need your program's symbol table, so you can strip the program
if necessary to save space. GDB on the host system does all the symbol
handling.
To use the server, you must tell it how to communicate with GDB; the
name of your program; and the arguments for your program. The usual
syntax is:
target> gdbserver COMM PROGRAM [ ARGS ... ]
COMM is either a device name (to use a serial line), or a TCP
hostname and portnumber, or '-' or 'stdio' to use stdin/stdout of
'gdbserver'. For example, to debug Emacs with the argument 'foo.txt'
and communicate with GDB over the serial port '/dev/com1':
target> gdbserver /dev/com1 emacs foo.txt
'gdbserver' waits passively for the host GDB to communicate with it.
To use a TCP connection instead of a serial line:
target> gdbserver host:2345 emacs foo.txt
The only difference from the previous example is the first argument,
specifying that you are communicating with the host GDB via TCP. The
'host:2345' argument means that 'gdbserver' is to expect a TCP
connection from machine 'host' to local TCP port 2345. (Currently, the
'host' part is ignored.) You can choose any number you want for the
port number as long as it does not conflict with any TCP ports already
in use on the target system (for example, '23' is reserved for
'telnet').(1) You must use the same port number with the host GDB
'target remote' command.
The 'stdio' connection is useful when starting 'gdbserver' with ssh:
(gdb) target remote | ssh -T hostname gdbserver - hello
The '-T' option to ssh is provided because we don't need a remote
pty, and we don't want escape-character handling. Ssh does this by
default when a command is provided, the flag is provided to make it
explicit. You could elide it if you want to.
Programs started with stdio-connected gdbserver have '/dev/null' for
'stdin', and 'stdout','stderr' are sent back to gdb for display through
a pipe connected to gdbserver. Both 'stdout' and 'stderr' use the same
pipe.
20.3.1.1 Attaching to a Running Program
.......................................
On some targets, 'gdbserver' can also attach to running programs. This
is accomplished via the '--attach' argument. The syntax is:
target> gdbserver --attach COMM PID
PID is the process ID of a currently running process. It isn't
necessary to point 'gdbserver' at a binary for the running process.
In 'target extended-remote' mode, you can also attach using the GDB
attach command (*note Attaching in Types of Remote Connections::).
You can debug processes by name instead of process ID if your target
has the 'pidof' utility:
target> gdbserver --attach COMM `pidof PROGRAM`
In case more than one copy of PROGRAM is running, or PROGRAM has
multiple threads, most versions of 'pidof' support the '-s' option to
only return the first process ID.
20.3.1.2 TCP port allocation lifecycle of 'gdbserver'
.....................................................
This section applies only when 'gdbserver' is run to listen on a TCP
port.
'gdbserver' normally terminates after all of its debugged processes
have terminated in 'target remote' mode. On the other hand, for 'target
extended-remote', 'gdbserver' stays running even with no processes left.
GDB normally terminates the spawned debugged process on its exit, which
normally also terminates 'gdbserver' in the 'target remote' mode.
Therefore, when the connection drops unexpectedly, and GDB cannot ask
'gdbserver' to kill its debugged processes, 'gdbserver' stays running
even in the 'target remote' mode.
When 'gdbserver' stays running, GDB can connect to it again later.
Such reconnecting is useful for features like *note disconnected
tracing::. For completeness, at most one GDB can be connected at a
time.
By default, 'gdbserver' keeps the listening TCP port open, so that
subsequent connections are possible. However, if you start 'gdbserver'
with the '--once' option, it will stop listening for any further
connection attempts after connecting to the first GDB session. This
means no further connections to 'gdbserver' will be possible after the
first one. It also means 'gdbserver' will terminate after the first
connection with remote GDB has closed, even for unexpectedly closed
connections and even in the 'target extended-remote' mode. The '--once'
option allows reusing the same port number for connecting to multiple
instances of 'gdbserver' running on the same host, since each instance
closes its port after the first connection.
20.3.1.3 Other Command-Line Arguments for 'gdbserver'
.....................................................
You can use the '--multi' option to start 'gdbserver' without specifying
a program to debug or a process to attach to. Then you can attach in
'target extended-remote' mode and run or attach to a program. For more
information, *note --multi Option in Types of Remote Connnections::.
The '--debug' option tells 'gdbserver' to display extra status
information about the debugging process. The '--remote-debug' option
tells 'gdbserver' to display remote protocol debug output. The
'--debug-file=FILENAME' option tells 'gdbserver' to write any debug
output to the given FILENAME. These options are intended for
'gdbserver' development and for bug reports to the developers.
The '--debug-format=option1[,option2,...]' option tells 'gdbserver'
to include additional information in each output. Possible options are:
'none'
Turn off all extra information in debugging output.
'all'
Turn on all extra information in debugging output.
'timestamps'
Include a timestamp in each line of debugging output.
Options are processed in order. Thus, for example, if 'none' appears
last then no additional information is added to debugging output.
The '--wrapper' option specifies a wrapper to launch programs for
debugging. The option should be followed by the name of the wrapper,
then any command-line arguments to pass to the wrapper, then '--'
indicating the end of the wrapper arguments.
'gdbserver' runs the specified wrapper program with a combined
command line including the wrapper arguments, then the name of the
program to debug, then any arguments to the program. The wrapper runs
until it executes your program, and then GDB gains control.
You can use any program that eventually calls 'execve' with its
arguments as a wrapper. Several standard Unix utilities do this, e.g.
'env' and 'nohup'. Any Unix shell script ending with 'exec "$@"' will
also work.
For example, you can use 'env' to pass an environment variable to the
debugged program, without setting the variable in 'gdbserver''s
environment:
$ gdbserver --wrapper env LD_PRELOAD=libtest.so -- :2222 ./testprog
The '--selftest' option runs the self tests in 'gdbserver':
$ gdbserver --selftest
Ran 2 unit tests, 0 failed
These tests are disabled in release.
20.3.2 Connecting to 'gdbserver'
--------------------------------
The basic procedure for connecting to the remote target is:
* Run GDB on the host system.
* Make sure you have the necessary symbol files (*note Host and
target files::). Load symbols for your application using the
'file' command before you connect. Use 'set sysroot' to locate
target libraries (unless your GDB was compiled with the correct
sysroot using '--with-sysroot').
* Connect to your target (*note Connecting to a Remote Target:
Connecting.). For TCP connections, you must start up 'gdbserver'
prior to using the 'target' command. Otherwise you may get an
error whose text depends on the host system, but which usually
looks something like 'Connection refused'. Don't use the 'load'
command in GDB when using 'target remote' mode, since the program
is already on the target.
20.3.3 Monitor Commands for 'gdbserver'
---------------------------------------
During a GDB session using 'gdbserver', you can use the 'monitor'
command to send special requests to 'gdbserver'. Here are the available
commands.
'monitor help'
List the available monitor commands.
'monitor set debug 0'
'monitor set debug 1'
Disable or enable general debugging messages.
'monitor set remote-debug 0'
'monitor set remote-debug 1'
Disable or enable specific debugging messages associated with the
remote protocol (*note Remote Protocol::).
'monitor set debug-file filename'
'monitor set debug-file'
Send any debug output to the given file, or to stderr.
'monitor set debug-format option1[,option2,...]'
Specify additional text to add to debugging messages. Possible
options are:
'none'
Turn off all extra information in debugging output.
'all'
Turn on all extra information in debugging output.
'timestamps'
Include a timestamp in each line of debugging output.
Options are processed in order. Thus, for example, if 'none'
appears last then no additional information is added to debugging
output.
'monitor set libthread-db-search-path [PATH]'
When this command is issued, PATH is a colon-separated list of
directories to search for 'libthread_db' (*note set
libthread-db-search-path: Threads.). If you omit PATH,
'libthread-db-search-path' will be reset to its default value.
The special entry '$pdir' for 'libthread-db-search-path' is not
supported in 'gdbserver'.
'monitor exit'
Tell gdbserver to exit immediately. This command should be
followed by 'disconnect' to close the debugging session.
'gdbserver' will detach from any attached processes and kill any
processes it created. Use 'monitor exit' to terminate 'gdbserver'
at the end of a multi-process mode debug session.
20.3.4 Tracepoints support in 'gdbserver'
-----------------------------------------
On some targets, 'gdbserver' supports tracepoints, fast tracepoints and
static tracepoints.
For fast or static tracepoints to work, a special library called the
"in-process agent" (IPA), must be loaded in the inferior process. This
library is built and distributed as an integral part of 'gdbserver'. In
addition, support for static tracepoints requires building the
in-process agent library with static tracepoints support. At present,
the UST (LTTng Userspace Tracer, <http://lttng.org/ust>) tracing engine
is supported. This support is automatically available if UST
development headers are found in the standard include path when
'gdbserver' is built, or if 'gdbserver' was explicitly configured using
'--with-ust' to point at such headers. You can explicitly disable the
support using '--with-ust=no'.
There are several ways to load the in-process agent in your program:
'Specifying it as dependency at link time'
You can link your program dynamically with the in-process agent
library. On most systems, this is accomplished by adding
'-linproctrace' to the link command.
'Using the system's preloading mechanisms'
You can force loading the in-process agent at startup time by using
your system's support for preloading shared libraries. Many Unixes
support the concept of preloading user defined libraries. In most
cases, you do that by specifying 'LD_PRELOAD=libinproctrace.so' in
the environment. See also the description of 'gdbserver''s
'--wrapper' command line option.
'Using GDB to force loading the agent at run time'
On some systems, you can force the inferior to load a shared
library, by calling a dynamic loader function in the inferior that
takes care of dynamically looking up and loading a shared library.
On most Unix systems, the function is 'dlopen'. You'll use the
'call' command for that. For example:
(gdb) call dlopen ("libinproctrace.so", ...)
Note that on most Unix systems, for the 'dlopen' function to be
available, the program needs to be linked with '-ldl'.
On systems that have a userspace dynamic loader, like most Unix
systems, when you connect to 'gdbserver' using 'target remote', you'll
find that the program is stopped at the dynamic loader's entry point,
and no shared library has been loaded in the program's address space
yet, including the in-process agent. In that case, before being able to
use any of the fast or static tracepoints features, you need to let the
loader run and load the shared libraries. The simplest way to do that
is to run the program to the main procedure. E.g., if debugging a C or
C++ program, start 'gdbserver' like so:
$ gdbserver :9999 myprogram
Start GDB and connect to 'gdbserver' like so, and run to main:
$ gdb myprogram
(gdb) target remote myhost:9999
0x00007f215893ba60 in ?? () from /lib64/ld-linux-x86-64.so.2
(gdb) b main
(gdb) continue
The in-process tracing agent library should now be loaded into the
process; you can confirm it with the 'info sharedlibrary' command, which
will list 'libinproctrace.so' as loaded in the process. You are now
ready to install fast tracepoints, list static tracepoint markers, probe
static tracepoints markers, and start tracing.
---------- Footnotes ----------
(1) If you choose a port number that conflicts with another service,
'gdbserver' prints an error message and exits.

File: gdb.info, Node: Remote Configuration, Next: Remote Stub, Prev: Server, Up: Remote Debugging
20.4 Remote Configuration
=========================
This section documents the configuration options available when
debugging remote programs. For the options related to the File I/O
extensions of the remote protocol, see *note system-call-allowed:
system.
'set remoteaddresssize BITS'
Set the maximum size of address in a memory packet to the specified
number of bits. GDB will mask off the address bits above that
number, when it passes addresses to the remote target. The default
value is the number of bits in the target's address.
'show remoteaddresssize'
Show the current value of remote address size in bits.
'set serial baud N'
Set the baud rate for the remote serial I/O to N baud. The value
is used to set the speed of the serial port used for debugging
remote targets.
'show serial baud'
Show the current speed of the remote connection.
'set serial parity PARITY'
Set the parity for the remote serial I/O. Supported values of
PARITY are: 'even', 'none', and 'odd'. The default is 'none'.
'show serial parity'
Show the current parity of the serial port.
'set remotebreak'
If set to on, GDB sends a 'BREAK' signal to the remote when you
type 'Ctrl-c' to interrupt the program running on the remote. If
set to off, GDB sends the 'Ctrl-C' character instead. The default
is off, since most remote systems expect to see 'Ctrl-C' as the
interrupt signal.
'show remotebreak'
Show whether GDB sends 'BREAK' or 'Ctrl-C' to interrupt the remote
program.
'set remoteflow on'
'set remoteflow off'
Enable or disable hardware flow control ('RTS'/'CTS') on the serial
port used to communicate to the remote target.
'show remoteflow'
Show the current setting of hardware flow control.
'set remotelogbase BASE'
Set the base (a.k.a. radix) of logging serial protocol
communications to BASE. Supported values of BASE are: 'ascii',
'octal', and 'hex'. The default is 'ascii'.
'show remotelogbase'
Show the current setting of the radix for logging remote serial
protocol.
'set remotelogfile FILE'
Record remote serial communications on the named FILE. The default
is not to record at all.
'show remotelogfile'
Show the current setting of the file name on which to record the
serial communications.
'set remotetimeout NUM'
Set the timeout limit to wait for the remote target to respond to
NUM seconds. The default is 2 seconds.
'show remotetimeout'
Show the current number of seconds to wait for the remote target
responses.
'set remote hardware-watchpoint-limit LIMIT'
'set remote hardware-breakpoint-limit LIMIT'
Restrict GDB to using LIMIT remote hardware watchpoints or
breakpoints. The LIMIT can be set to 0 to disable hardware
watchpoints or breakpoints, and 'unlimited' for unlimited
watchpoints or breakpoints.
'show remote hardware-watchpoint-limit'
'show remote hardware-breakpoint-limit'
Show the current limit for the number of hardware watchpoints or
breakpoints that GDB can use.
'set remote hardware-watchpoint-length-limit LIMIT'
Restrict GDB to using LIMIT bytes for the maximum length of a
remote hardware watchpoint. A LIMIT of 0 disables hardware
watchpoints and 'unlimited' allows watchpoints of any length.
'show remote hardware-watchpoint-length-limit'
Show the current limit (in bytes) of the maximum length of a remote
hardware watchpoint.
'set remote exec-file FILENAME'
'show remote exec-file'
Select the file used for 'run' with 'target extended-remote'. This
should be set to a filename valid on the target system. If it is
not set, the target will use a default filename (e.g. the last
program run).
'set remote interrupt-sequence'
Allow the user to select one of 'Ctrl-C', a 'BREAK' or 'BREAK-g' as
the sequence to the remote target in order to interrupt the
execution. 'Ctrl-C' is a default. Some system prefers 'BREAK'
which is high level of serial line for some certain time. Linux
kernel prefers 'BREAK-g', a.k.a Magic SysRq g. It is 'BREAK'
signal followed by character 'g'.
'show interrupt-sequence'
Show which of 'Ctrl-C', 'BREAK' or 'BREAK-g' is sent by GDB to
interrupt the remote program. 'BREAK-g' is BREAK signal followed
by 'g' and also known as Magic SysRq g.
'set remote interrupt-on-connect'
Specify whether interrupt-sequence is sent to remote target when
GDB connects to it. This is mostly needed when you debug Linux
kernel. Linux kernel expects 'BREAK' followed by 'g' which is
known as Magic SysRq g in order to connect GDB.
'show interrupt-on-connect'
Show whether interrupt-sequence is sent to remote target when GDB
connects to it.
'set tcp auto-retry on'
Enable auto-retry for remote TCP connections. This is useful if
the remote debugging agent is launched in parallel with GDB; there
is a race condition because the agent may not become ready to
accept the connection before GDB attempts to connect. When
auto-retry is enabled, if the initial attempt to connect fails, GDB
reattempts to establish the connection using the timeout specified
by 'set tcp connect-timeout'.
'set tcp auto-retry off'
Do not auto-retry failed TCP connections.
'show tcp auto-retry'
Show the current auto-retry setting.
'set tcp connect-timeout SECONDS'
'set tcp connect-timeout unlimited'
Set the timeout for establishing a TCP connection to the remote
target to SECONDS. The timeout affects both polling to retry
failed connections (enabled by 'set tcp auto-retry on') and waiting
for connections that are merely slow to complete, and represents an
approximate cumulative value. If SECONDS is 'unlimited', there is
no timeout and GDB will keep attempting to establish a connection
forever, unless interrupted with 'Ctrl-c'. The default is 15
seconds.
'show tcp connect-timeout'
Show the current connection timeout setting.
The GDB remote protocol autodetects the packets supported by your
debugging stub. If you need to override the autodetection, you can use
these commands to enable or disable individual packets. Each packet can
be set to 'on' (the remote target supports this packet), 'off' (the
remote target does not support this packet), or 'auto' (detect remote
target support for this packet). They all default to 'auto'. For more
information about each packet, see *note Remote Protocol::.
During normal use, you should not have to use any of these commands.
If you do, that may be a bug in your remote debugging stub, or a bug in
GDB. You may want to report the problem to the GDB developers.
For each packet NAME, the command to enable or disable the packet is
'set remote NAME-packet'. The available settings are:
Command Name Remote Packet Related Features
'fetch-register' 'p' 'info registers'
'set-register' 'P' 'set'
'binary-download' 'X' 'load', 'set'
'read-aux-vector' 'qXfer:auxv:read' 'info auxv'
'symbol-lookup' 'qSymbol' Detecting
multiple threads
'attach' 'vAttach' 'attach'
'verbose-resume' 'vCont' Stepping or
resuming
multiple threads
'run' 'vRun' 'run'
'software-breakpoint''Z0' 'break'
'hardware-breakpoint''Z1' 'hbreak'
'write-watchpoint' 'Z2' 'watch'
'read-watchpoint' 'Z3' 'rwatch'
'access-watchpoint' 'Z4' 'awatch'
'pid-to-exec-file' 'qXfer:exec-file:read' 'attach', 'run'
'target-features' 'qXfer:features:read' 'set
architecture'
'library-info' 'qXfer:libraries:read' 'info
sharedlibrary'
'memory-map' 'qXfer:memory-map:read' 'info mem'
'read-sdata-object' 'qXfer:sdata:read' 'print $_sdata'
'read-siginfo-object''qXfer:siginfo:read' 'print
$_siginfo'
'write-siginfo-object''qXfer:siginfo:write' 'set $_siginfo'
'threads' 'qXfer:threads:read' 'info threads'
'get-thread-local- 'qGetTLSAddr' Displaying
storage-address' '__thread'
variables
'get-thread-information-block-address''qGetTIBAddr'Display
MS-Windows
Thread
Information
Block.
'search-memory' 'qSearch:memory' 'find'
'supported-packets' 'qSupported' Remote
communications
parameters
'catch-syscalls' 'QCatchSyscalls' 'catch syscall'
'pass-signals' 'QPassSignals' 'handle SIGNAL'
'program-signals' 'QProgramSignals' 'handle SIGNAL'
'hostio-close-packet''vFile:close' 'remote get',
'remote put'
'hostio-open-packet' 'vFile:open' 'remote get',
'remote put'
'hostio-pread-packet''vFile:pread' 'remote get',
'remote put'
'hostio-pwrite-packet''vFile:pwrite' 'remote get',
'remote put'
'hostio-unlink-packet''vFile:unlink' 'remote delete'
'hostio-readlink-packet''vFile:readlink' Host I/O
'hostio-fstat-packet''vFile:fstat' Host I/O
'hostio-setfs-packet''vFile:setfs' Host I/O
'noack-packet' 'QStartNoAckMode' Packet
acknowledgment
'osdata' 'qXfer:osdata:read' 'info os'
'query-attached' 'qAttached' Querying remote
process attach
state.
'trace-buffer-size' 'QTBuffer:size' 'set
trace-buffer-size'
'trace-status' 'qTStatus' 'tstatus'
'traceframe-info' 'qXfer:traceframe-info:read'Traceframe info
'install-in-trace' 'InstallInTrace' Install
tracepoint in
tracing
'disable-randomization''QDisableRandomization''set
disable-randomization'
'startup-with-shell' 'QStartupWithShell' 'set
startup-with-shell'
'environment-hex-encoded''QEnvironmentHexEncoded''set
environment'
'environment-unset' 'QEnvironmentUnset' 'unset
environment'
'environment-reset' 'QEnvironmentReset' 'Reset the
inferior
environment
(i.e., unset
user-set
variables)'
'set-working-dir' 'QSetWorkingDir' 'set cwd'
'conditional-breakpoints-packet''Z0 and Z1' 'Support for
target-side
breakpoint
condition
evaluation'
'multiprocess-extensions''multiprocess Debug multiple
extensions' processes and
remote process
PID awareness
'swbreak-feature' 'swbreak stop reason' 'break'
'hwbreak-feature' 'hwbreak stop reason' 'hbreak'
'fork-event-feature' 'fork stop reason' 'fork'
'vfork-event-feature''vfork stop reason' 'vfork'
'exec-event-feature' 'exec stop reason' 'exec'
'thread-events' 'QThreadEvents' Tracking thread
lifetime.
'no-resumed-stop-reply''no resumed thread Tracking thread
left stop reply' lifetime.

File: gdb.info, Node: Remote Stub, Prev: Remote Configuration, Up: Remote Debugging
20.5 Implementing a Remote Stub
===============================
The stub files provided with GDB implement the target side of the
communication protocol, and the GDB side is implemented in the GDB
source file 'remote.c'. Normally, you can simply allow these
subroutines to communicate, and ignore the details. (If you're
implementing your own stub file, you can still ignore the details: start
with one of the existing stub files. 'sparc-stub.c' is the best
organized, and therefore the easiest to read.)
To debug a program running on another machine (the debugging "target"
machine), you must first arrange for all the usual prerequisites for the
program to run by itself. For example, for a C program, you need:
1. A startup routine to set up the C runtime environment; these
usually have a name like 'crt0'. The startup routine may be
supplied by your hardware supplier, or you may have to write your
own.
2. A C subroutine library to support your program's subroutine calls,
notably managing input and output.
3. A way of getting your program to the other machine--for example, a
download program. These are often supplied by the hardware
manufacturer, but you may have to write your own from hardware
documentation.
The next step is to arrange for your program to use a serial port to
communicate with the machine where GDB is running (the "host" machine).
In general terms, the scheme looks like this:
_On the host,_
GDB already understands how to use this protocol; when everything
else is set up, you can simply use the 'target remote' command
(*note Specifying a Debugging Target: Targets.).
_On the target,_
you must link with your program a few special-purpose subroutines
that implement the GDB remote serial protocol. The file containing
these subroutines is called a "debugging stub".
On certain remote targets, you can use an auxiliary program
'gdbserver' instead of linking a stub into your program. *Note
Using the 'gdbserver' Program: Server, for details.
The debugging stub is specific to the architecture of the remote
machine; for example, use 'sparc-stub.c' to debug programs on SPARC
boards.
These working remote stubs are distributed with GDB:
'i386-stub.c'
For Intel 386 and compatible architectures.
'm68k-stub.c'
For Motorola 680x0 architectures.
'sh-stub.c'
For Renesas SH architectures.
'sparc-stub.c'
For SPARC architectures.
'sparcl-stub.c'
For Fujitsu SPARCLITE architectures.
The 'README' file in the GDB distribution may list other recently
added stubs.
* Menu:
* Stub Contents:: What the stub can do for you
* Bootstrapping:: What you must do for the stub
* Debug Session:: Putting it all together

File: gdb.info, Node: Stub Contents, Next: Bootstrapping, Up: Remote Stub
20.5.1 What the Stub Can Do for You
-----------------------------------
The debugging stub for your architecture supplies these three
subroutines:
'set_debug_traps'
This routine arranges for 'handle_exception' to run when your
program stops. You must call this subroutine explicitly in your
program's startup code.
'handle_exception'
This is the central workhorse, but your program never calls it
explicitly--the setup code arranges for 'handle_exception' to run
when a trap is triggered.
'handle_exception' takes control when your program stops during
execution (for example, on a breakpoint), and mediates
communications with GDB on the host machine. This is where the
communications protocol is implemented; 'handle_exception' acts as
the GDB representative on the target machine. It begins by sending
summary information on the state of your program, then continues to
execute, retrieving and transmitting any information GDB needs,
until you execute a GDB command that makes your program resume; at
that point, 'handle_exception' returns control to your own code on
the target machine.
'breakpoint'
Use this auxiliary subroutine to make your program contain a
breakpoint. Depending on the particular situation, this may be the
only way for GDB to get control. For instance, if your target
machine has some sort of interrupt button, you won't need to call
this; pressing the interrupt button transfers control to
'handle_exception'--in effect, to GDB. On some machines, simply
receiving characters on the serial port may also trigger a trap;
again, in that situation, you don't need to call 'breakpoint' from
your own program--simply running 'target remote' from the host GDB
session gets control.
Call 'breakpoint' if none of these is true, or if you simply want
to make certain your program stops at a predetermined point for the
start of your debugging session.

File: gdb.info, Node: Bootstrapping, Next: Debug Session, Prev: Stub Contents, Up: Remote Stub
20.5.2 What You Must Do for the Stub
------------------------------------
The debugging stubs that come with GDB are set up for a particular chip
architecture, but they have no information about the rest of your
debugging target machine.
First of all you need to tell the stub how to communicate with the
serial port.
'int getDebugChar()'
Write this subroutine to read a single character from the serial
port. It may be identical to 'getchar' for your target system; a
different name is used to allow you to distinguish the two if you
wish.
'void putDebugChar(int)'
Write this subroutine to write a single character to the serial
port. It may be identical to 'putchar' for your target system; a
different name is used to allow you to distinguish the two if you
wish.
If you want GDB to be able to stop your program while it is running,
you need to use an interrupt-driven serial driver, and arrange for it to
stop when it receives a '^C' ('\003', the control-C character). That is
the character which GDB uses to tell the remote system to stop.
Getting the debugging target to return the proper status to GDB
probably requires changes to the standard stub; one quick and dirty way
is to just execute a breakpoint instruction (the "dirty" part is that
GDB reports a 'SIGTRAP' instead of a 'SIGINT').
Other routines you need to supply are:
'void exceptionHandler (int EXCEPTION_NUMBER, void *EXCEPTION_ADDRESS)'
Write this function to install EXCEPTION_ADDRESS in the exception
handling tables. You need to do this because the stub does not
have any way of knowing what the exception handling tables on your
target system are like (for example, the processor's table might be
in ROM, containing entries which point to a table in RAM). The
EXCEPTION_NUMBER specifies the exception which should be changed;
its meaning is architecture-dependent (for example, different
numbers might represent divide by zero, misaligned access, etc).
When this exception occurs, control should be transferred directly
to EXCEPTION_ADDRESS, and the processor state (stack, registers,
and so on) should be just as it is when a processor exception
occurs. So if you want to use a jump instruction to reach
EXCEPTION_ADDRESS, it should be a simple jump, not a jump to
subroutine.
For the 386, EXCEPTION_ADDRESS should be installed as an interrupt
gate so that interrupts are masked while the handler runs. The
gate should be at privilege level 0 (the most privileged level).
The SPARC and 68k stubs are able to mask interrupts themselves
without help from 'exceptionHandler'.
'void flush_i_cache()'
On SPARC and SPARCLITE only, write this subroutine to flush the
instruction cache, if any, on your target machine. If there is no
instruction cache, this subroutine may be a no-op.
On target machines that have instruction caches, GDB requires this
function to make certain that the state of your program is stable.
You must also make sure this library routine is available:
'void *memset(void *, int, int)'
This is the standard library function 'memset' that sets an area of
memory to a known value. If you have one of the free versions of
'libc.a', 'memset' can be found there; otherwise, you must either
obtain it from your hardware manufacturer, or write your own.
If you do not use the GNU C compiler, you may need other standard
library subroutines as well; this varies from one stub to another, but
in general the stubs are likely to use any of the common library
subroutines which 'GCC' generates as inline code.

File: gdb.info, Node: Debug Session, Prev: Bootstrapping, Up: Remote Stub
20.5.3 Putting it All Together
------------------------------
In summary, when your program is ready to debug, you must follow these
steps.
1. Make sure you have defined the supporting low-level routines (*note
What You Must Do for the Stub: Bootstrapping.):
'getDebugChar', 'putDebugChar',
'flush_i_cache', 'memset', 'exceptionHandler'.
2. Insert these lines in your program's startup code, before the main
procedure is called:
set_debug_traps();
breakpoint();
On some machines, when a breakpoint trap is raised, the hardware
automatically makes the PC point to the instruction after the
breakpoint. If your machine doesn't do that, you may need to
adjust 'handle_exception' to arrange for it to return to the
instruction after the breakpoint on this first invocation, so that
your program doesn't keep hitting the initial breakpoint instead of
making progress.
3. For the 680x0 stub only, you need to provide a variable called
'exceptionHook'. Normally you just use:
void (*exceptionHook)() = 0;
but if before calling 'set_debug_traps', you set it to point to a
function in your program, that function is called when 'GDB'
continues after stopping on a trap (for example, bus error). The
function indicated by 'exceptionHook' is called with one parameter:
an 'int' which is the exception number.
4. Compile and link together: your program, the GDB debugging stub for
your target architecture, and the supporting subroutines.
5. Make sure you have a serial connection between your target machine
and the GDB host, and identify the serial port on the host.
6. Download your program to your target machine (or get it there by
whatever means the manufacturer provides), and start it.
7. Start GDB on the host, and connect to the target (*note Connecting
to a Remote Target: Connecting.).

File: gdb.info, Node: Configurations, Next: Controlling GDB, Prev: Remote Debugging, Up: Top
21 Configuration-Specific Information
*************************************
While nearly all GDB commands are available for all native and cross
versions of the debugger, there are some exceptions. This chapter
describes things that are only available in certain configurations.
There are three major categories of configurations: native
configurations, where the host and target are the same, embedded
operating system configurations, which are usually the same for several
different processor architectures, and bare embedded processors, which
are quite different from each other.
* Menu:
* Native::
* Embedded OS::
* Embedded Processors::
* Architectures::

File: gdb.info, Node: Native, Next: Embedded OS, Up: Configurations
21.1 Native
===========
This section describes details specific to particular native
configurations.
* Menu:
* BSD libkvm Interface:: Debugging BSD kernel memory images
* Process Information:: Process information
* DJGPP Native:: Features specific to the DJGPP port
* Cygwin Native:: Features specific to the Cygwin port
* Hurd Native:: Features specific to GNU Hurd
* Darwin:: Features specific to Darwin
* FreeBSD:: Features specific to FreeBSD

File: gdb.info, Node: BSD libkvm Interface, Next: Process Information, Up: Native
21.1.1 BSD libkvm Interface
---------------------------
BSD-derived systems (FreeBSD/NetBSD/OpenBSD) have a kernel memory
interface that provides a uniform interface for accessing kernel virtual
memory images, including live systems and crash dumps. GDB uses this
interface to allow you to debug live kernels and kernel crash dumps on
many native BSD configurations. This is implemented as a special 'kvm'
debugging target. For debugging a live system, load the currently
running kernel into GDB and connect to the 'kvm' target:
(gdb) target kvm
For debugging crash dumps, provide the file name of the crash dump as
an argument:
(gdb) target kvm /var/crash/bsd.0
Once connected to the 'kvm' target, the following commands are
available:
'kvm pcb'
Set current context from the "Process Control Block" (PCB) address.
'kvm proc'
Set current context from proc address. This command isn't
available on modern FreeBSD systems.

File: gdb.info, Node: Process Information, Next: DJGPP Native, Prev: BSD libkvm Interface, Up: Native
21.1.2 Process Information
--------------------------
Some operating systems provide interfaces to fetch additional
information about running processes beyond memory and per-thread
register state. If GDB is configured for an operating system with a
supported interface, the command 'info proc' is available to report
information about the process running your program, or about any process
running on your system.
One supported interface is a facility called '/proc' that can be used
to examine the image of a running process using file-system subroutines.
This facility is supported on GNU/Linux and Solaris systems.
On FreeBSD systems, system control nodes are used to query process
information.
In addition, some systems may provide additional process information
in core files. Note that a core file may include a subset of the
information available from a live process. Process information is
currently available from cores created on GNU/Linux and FreeBSD systems.
'info proc'
'info proc PROCESS-ID'
Summarize available information about a process. If a process ID
is specified by PROCESS-ID, display information about that process;
otherwise display information about the program being debugged.
The summary includes the debugged process ID, the command line used
to invoke it, its current working directory, and its executable
file's absolute file name.
On some systems, PROCESS-ID can be of the form '[PID]/TID' which
specifies a certain thread ID within a process. If the optional
PID part is missing, it means a thread from the process being
debugged (the leading '/' still needs to be present, or else GDB
will interpret the number as a process ID rather than a thread ID).
'info proc cmdline'
Show the original command line of the process. This command is
supported on GNU/Linux and FreeBSD.
'info proc cwd'
Show the current working directory of the process. This command is
supported on GNU/Linux and FreeBSD.
'info proc exe'
Show the name of executable of the process. This command is
supported on GNU/Linux and FreeBSD.
'info proc files'
Show the file descriptors open by the process. For each open file
descriptor, GDB shows its number, type (file, directory, character
device, socket), file pointer offset, and the name of the resource
open on the descriptor. The resource name can be a file name (for
files, directories, and devices) or a protocol followed by socket
address (for network connections). This command is supported on
FreeBSD.
This example shows the open file descriptors for a process using a
tty for standard input and output as well as two network sockets:
(gdb) info proc files 22136
process 22136
Open files:
FD Type Offset Flags Name
text file - r-------- /usr/bin/ssh
ctty chr - rw------- /dev/pts/20
cwd dir - r-------- /usr/home/john
root dir - r-------- /
0 chr 0x32933a4 rw------- /dev/pts/20
1 chr 0x32933a4 rw------- /dev/pts/20
2 chr 0x32933a4 rw------- /dev/pts/20
3 socket 0x0 rw----n-- tcp4 10.0.1.2:53014 -> 10.0.1.10:22
4 socket 0x0 rw------- unix stream:/tmp/ssh-FIt89oAzOn5f/agent.2456
'info proc mappings'
Report the memory address space ranges accessible in a process. On
Solaris and FreeBSD systems, each memory range includes information
on whether the process has read, write, or execute access rights to
each range. On GNU/Linux and FreeBSD systems, each memory range
includes the object file which is mapped to that range.
'info proc stat'
'info proc status'
Show additional process-related information, including the user ID
and group ID; virtual memory usage; the signals that are pending,
blocked, and ignored; its TTY; its consumption of system and user
time; its stack size; its 'nice' value; etc. These commands are
supported on GNU/Linux and FreeBSD.
For GNU/Linux systems, see the 'proc' man page for more information
(type 'man 5 proc' from your shell prompt).
For FreeBSD systems, 'info proc stat' is an alias for 'info proc
status'.
'info proc all'
Show all the information about the process described under all of
the above 'info proc' subcommands.
'set procfs-trace'
This command enables and disables tracing of 'procfs' API calls.
'show procfs-trace'
Show the current state of 'procfs' API call tracing.
'set procfs-file FILE'
Tell GDB to write 'procfs' API trace to the named FILE. GDB
appends the trace info to the previous contents of the file. The
default is to display the trace on the standard output.
'show procfs-file'
Show the file to which 'procfs' API trace is written.
'proc-trace-entry'
'proc-trace-exit'
'proc-untrace-entry'
'proc-untrace-exit'
These commands enable and disable tracing of entries into and exits
from the 'syscall' interface.
'info pidlist'
For QNX Neutrino only, this command displays the list of all the
processes and all the threads within each process.
'info meminfo'
For QNX Neutrino only, this command displays the list of all
mapinfos.

File: gdb.info, Node: DJGPP Native, Next: Cygwin Native, Prev: Process Information, Up: Native
21.1.3 Features for Debugging DJGPP Programs
--------------------------------------------
DJGPP is a port of the GNU development tools to MS-DOS and MS-Windows.
DJGPP programs are 32-bit protected-mode programs that use the "DPMI"
(DOS Protected-Mode Interface) API to run on top of real-mode DOS
systems and their emulations.
GDB supports native debugging of DJGPP programs, and defines a few
commands specific to the DJGPP port. This subsection describes those
commands.
'info dos'
This is a prefix of DJGPP-specific commands which print information
about the target system and important OS structures.
'info dos sysinfo'
This command displays assorted information about the underlying
platform: the CPU type and features, the OS version and flavor, the
DPMI version, and the available conventional and DPMI memory.
'info dos gdt'
'info dos ldt'
'info dos idt'
These 3 commands display entries from, respectively, Global, Local,
and Interrupt Descriptor Tables (GDT, LDT, and IDT). The descriptor
tables are data structures which store a descriptor for each
segment that is currently in use. The segment's selector is an
index into a descriptor table; the table entry for that index holds
the descriptor's base address and limit, and its attributes and
access rights.
A typical DJGPP program uses 3 segments: a code segment, a data
segment (used for both data and the stack), and a DOS segment
(which allows access to DOS/BIOS data structures and absolute
addresses in conventional memory). However, the DPMI host will
usually define additional segments in order to support the DPMI
environment.
These commands allow to display entries from the descriptor tables.
Without an argument, all entries from the specified table are
displayed. An argument, which should be an integer expression,
means display a single entry whose index is given by the argument.
For example, here's a convenient way to display information about
the debugged program's data segment:
(gdb) info dos ldt $ds
0x13f: base=0x11970000 limit=0x0009ffff 32-Bit Data (Read/Write, Exp-up)
This comes in handy when you want to see whether a pointer is
outside the data segment's limit (i.e. "garbled").
'info dos pde'
'info dos pte'
These two commands display entries from, respectively, the Page
Directory and the Page Tables. Page Directories and Page Tables
are data structures which control how virtual memory addresses are
mapped into physical addresses. A Page Table includes an entry for
every page of memory that is mapped into the program's address
space; there may be several Page Tables, each one holding up to
4096 entries. A Page Directory has up to 4096 entries, one each
for every Page Table that is currently in use.
Without an argument, 'info dos pde' displays the entire Page
Directory, and 'info dos pte' displays all the entries in all of
the Page Tables. An argument, an integer expression, given to the
'info dos pde' command means display only that entry from the Page
Directory table. An argument given to the 'info dos pte' command
means display entries from a single Page Table, the one pointed to
by the specified entry in the Page Directory.
These commands are useful when your program uses "DMA" (Direct
Memory Access), which needs physical addresses to program the DMA
controller.
These commands are supported only with some DPMI servers.
'info dos address-pte ADDR'
This command displays the Page Table entry for a specified linear
address. The argument ADDR is a linear address which should
already have the appropriate segment's base address added to it,
because this command accepts addresses which may belong to _any_
segment. For example, here's how to display the Page Table entry
for the page where a variable 'i' is stored:
(gdb) info dos address-pte __djgpp_base_address + (char *)&i
Page Table entry for address 0x11a00d30:
Base=0x02698000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0xd30
This says that 'i' is stored at offset '0xd30' from the page whose
physical base address is '0x02698000', and shows all the attributes
of that page.
Note that you must cast the addresses of variables to a 'char *',
since otherwise the value of '__djgpp_base_address', the base
address of all variables and functions in a DJGPP program, will be
added using the rules of C pointer arithmetics: if 'i' is declared
an 'int', GDB will add 4 times the value of '__djgpp_base_address'
to the address of 'i'.
Here's another example, it displays the Page Table entry for the
transfer buffer:
(gdb) info dos address-pte *((unsigned *)&_go32_info_block + 3)
Page Table entry for address 0x29110:
Base=0x00029000 Dirty Acc. Not-Cached Write-Back Usr Read-Write +0x110
(The '+ 3' offset is because the transfer buffer's address is the
3rd member of the '_go32_info_block' structure.) The output
clearly shows that this DPMI server maps the addresses in
conventional memory 1:1, i.e. the physical ('0x00029000' + '0x110')
and linear ('0x29110') addresses are identical.
This command is supported only with some DPMI servers.
In addition to native debugging, the DJGPP port supports remote
debugging via a serial data link. The following commands are specific
to remote serial debugging in the DJGPP port of GDB.
'set com1base ADDR'
This command sets the base I/O port address of the 'COM1' serial
port.
'set com1irq IRQ'
This command sets the "Interrupt Request" ('IRQ') line to use for
the 'COM1' serial port.
There are similar commands 'set com2base', 'set com3irq', etc. for
setting the port address and the 'IRQ' lines for the other 3 COM
ports.
The related commands 'show com1base', 'show com1irq' etc. display
the current settings of the base address and the 'IRQ' lines used
by the COM ports.
'info serial'
This command prints the status of the 4 DOS serial ports. For each
port, it prints whether it's active or not, its I/O base address
and IRQ number, whether it uses a 16550-style FIFO, its baudrate,
and the counts of various errors encountered so far.

File: gdb.info, Node: Cygwin Native, Next: Hurd Native, Prev: DJGPP Native, Up: Native
21.1.4 Features for Debugging MS Windows PE Executables
-------------------------------------------------------
GDB supports native debugging of MS Windows programs, including DLLs
with and without symbolic debugging information.
MS-Windows programs that call 'SetConsoleMode' to switch off the
special meaning of the 'Ctrl-C' keystroke cannot be interrupted by
typing 'C-c'. For this reason, GDB on MS-Windows supports 'C-<BREAK>'
as an alternative interrupt key sequence, which can be used to interrupt
the debuggee even if it ignores 'C-c'.
There are various additional Cygwin-specific commands, described in
this section. Working with DLLs that have no debugging symbols is
described in *note Non-debug DLL Symbols::.
'info w32'
This is a prefix of MS Windows-specific commands which print
information about the target system and important OS structures.
'info w32 selector'
This command displays information returned by the Win32 API
'GetThreadSelectorEntry' function. It takes an optional argument
that is evaluated to a long value to give the information about
this given selector. Without argument, this command displays
information about the six segment registers.
'info w32 thread-information-block'
This command displays thread specific information stored in the
Thread Information Block (readable on the X86 CPU family using
'$fs' selector for 32-bit programs and '$gs' for 64-bit programs).
'signal-event ID'
This command signals an event with user-provided ID. Used to
resume crashing process when attached to it using MS-Windows JIT
debugging (AeDebug).
To use it, create or edit the following keys in
'HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug' and/or
'HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows
NT\CurrentVersion\AeDebug' (for x86_64 versions):
- 'Debugger' (REG_SZ) -- a command to launch the debugger.
Suggested command is: 'FULLY-QUALIFIED-PATH-TO-GDB.EXE -ex
"attach %ld" -ex "signal-event %ld" -ex "continue"'.
The first '%ld' will be replaced by the process ID of the
crashing process, the second '%ld' will be replaced by the ID
of the event that blocks the crashing process, waiting for GDB
to attach.
- 'Auto' (REG_SZ) -- either '1' or '0'. '1' will make the
system run debugger specified by the Debugger key
automatically, '0' will cause a dialog box with "OK" and
"Cancel" buttons to appear, which allows the user to either
terminate the crashing process (OK) or debug it (Cancel).
'set cygwin-exceptions MODE'
If MODE is 'on', GDB will break on exceptions that happen inside
the Cygwin DLL. If MODE is 'off', GDB will delay recognition of
exceptions, and may ignore some exceptions which seem to be caused
by internal Cygwin DLL "bookkeeping". This option is meant
primarily for debugging the Cygwin DLL itself; the default value is
'off' to avoid annoying GDB users with false 'SIGSEGV' signals.
'show cygwin-exceptions'
Displays whether GDB will break on exceptions that happen inside
the Cygwin DLL itself.
'set new-console MODE'
If MODE is 'on' the debuggee will be started in a new console on
next start. If MODE is 'off', the debuggee will be started in the
same console as the debugger.
'show new-console'
Displays whether a new console is used when the debuggee is
started.
'set new-group MODE'
This boolean value controls whether the debuggee should start a new
group or stay in the same group as the debugger. This affects the
way the Windows OS handles 'Ctrl-C'.
'show new-group'
Displays current value of new-group boolean.
'set debugevents'
This boolean value adds debug output concerning kernel events
related to the debuggee seen by the debugger. This includes events
that signal thread and process creation and exit, DLL loading and
unloading, console interrupts, and debugging messages produced by
the Windows 'OutputDebugString' API call.
'set debugexec'
This boolean value adds debug output concerning execute events
(such as resume thread) seen by the debugger.
'set debugexceptions'
This boolean value adds debug output concerning exceptions in the
debuggee seen by the debugger.
'set debugmemory'
This boolean value adds debug output concerning debuggee memory
reads and writes by the debugger.
'set shell'
This boolean values specifies whether the debuggee is called via a
shell or directly (default value is on).
'show shell'
Displays if the debuggee will be started with a shell.
* Menu:
* Non-debug DLL Symbols:: Support for DLLs without debugging symbols

File: gdb.info, Node: Non-debug DLL Symbols, Up: Cygwin Native
21.1.4.1 Support for DLLs without Debugging Symbols
...................................................
Very often on windows, some of the DLLs that your program relies on do
not include symbolic debugging information (for example,
'kernel32.dll'). When GDB doesn't recognize any debugging symbols in a
DLL, it relies on the minimal amount of symbolic information contained
in the DLL's export table. This section describes working with such
symbols, known internally to GDB as "minimal symbols".
Note that before the debugged program has started execution, no DLLs
will have been loaded. The easiest way around this problem is simply to
start the program -- either by setting a breakpoint or letting the
program run once to completion.
21.1.4.2 DLL Name Prefixes
..........................
In keeping with the naming conventions used by the Microsoft debugging
tools, DLL export symbols are made available with a prefix based on the
DLL name, for instance 'KERNEL32!CreateFileA'. The plain name is also
entered into the symbol table, so 'CreateFileA' is often sufficient. In
some cases there will be name clashes within a program (particularly if
the executable itself includes full debugging symbols) necessitating the
use of the fully qualified name when referring to the contents of the
DLL. Use single-quotes around the name to avoid the exclamation mark
("!") being interpreted as a language operator.
Note that the internal name of the DLL may be all upper-case, even
though the file name of the DLL is lower-case, or vice-versa. Since
symbols within GDB are _case-sensitive_ this may cause some confusion.
If in doubt, try the 'info functions' and 'info variables' commands or
even 'maint print msymbols' (*note Symbols::). Here's an example:
(gdb) info function CreateFileA
All functions matching regular expression "CreateFileA":
Non-debugging symbols:
0x77e885f4 CreateFileA
0x77e885f4 KERNEL32!CreateFileA
(gdb) info function !
All functions matching regular expression "!":
Non-debugging symbols:
0x6100114c cygwin1!__assert
0x61004034 cygwin1!_dll_crt0@0
0x61004240 cygwin1!dll_crt0(per_process *)
[etc...]
21.1.4.3 Working with Minimal Symbols
.....................................
Symbols extracted from a DLL's export table do not contain very much
type information. All that GDB can do is guess whether a symbol refers
to a function or variable depending on the linker section that contains
the symbol. Also note that the actual contents of the memory contained
in a DLL are not available unless the program is running. This means
that you cannot examine the contents of a variable or disassemble a
function within a DLL without a running program.
Variables are generally treated as pointers and dereferenced
automatically. For this reason, it is often necessary to prefix a
variable name with the address-of operator ("&") and provide explicit
type information in the command. Here's an example of the type of
problem:
(gdb) print 'cygwin1!__argv'
'cygwin1!__argv' has unknown type; cast it to its declared type
(gdb) x 'cygwin1!__argv'
'cygwin1!__argv' has unknown type; cast it to its declared type
And two possible solutions:
(gdb) print ((char **)'cygwin1!__argv')[0]
$2 = 0x22fd98 "/cygdrive/c/mydirectory/myprogram"
(gdb) x/2x &'cygwin1!__argv'
0x610c0aa8 <cygwin1!__argv>: 0x10021608 0x00000000
(gdb) x/x 0x10021608
0x10021608: 0x0022fd98
(gdb) x/s 0x0022fd98
0x22fd98: "/cygdrive/c/mydirectory/myprogram"
Setting a break point within a DLL is possible even before the
program starts execution. However, under these circumstances, GDB can't
examine the initial instructions of the function in order to skip the
function's frame set-up code. You can work around this by using "*&" to
set the breakpoint at a raw memory address:
(gdb) break *&'python22!PyOS_Readline'
Breakpoint 1 at 0x1e04eff0
The author of these extensions is not entirely convinced that setting
a break point within a shared DLL like 'kernel32.dll' is completely
safe.

File: gdb.info, Node: Hurd Native, Next: Darwin, Prev: Cygwin Native, Up: Native
21.1.5 Commands Specific to GNU Hurd Systems
--------------------------------------------
This subsection describes GDB commands specific to the GNU Hurd native
debugging.
'set signals'
'set sigs'
This command toggles the state of inferior signal interception by
GDB. Mach exceptions, such as breakpoint traps, are not affected
by this command. 'sigs' is a shorthand alias for 'signals'.
'show signals'
'show sigs'
Show the current state of intercepting inferior's signals.
'set signal-thread'
'set sigthread'
This command tells GDB which thread is the 'libc' signal thread.
That thread is run when a signal is delivered to a running process.
'set sigthread' is the shorthand alias of 'set signal-thread'.
'show signal-thread'
'show sigthread'
These two commands show which thread will run when the inferior is
delivered a signal.
'set stopped'
This commands tells GDB that the inferior process is stopped, as
with the 'SIGSTOP' signal. The stopped process can be continued by
delivering a signal to it.
'show stopped'
This command shows whether GDB thinks the debuggee is stopped.
'set exceptions'
Use this command to turn off trapping of exceptions in the
inferior. When exception trapping is off, neither breakpoints nor
single-stepping will work. To restore the default, set exception
trapping on.
'show exceptions'
Show the current state of trapping exceptions in the inferior.
'set task pause'
This command toggles task suspension when GDB has control. Setting
it to on takes effect immediately, and the task is suspended
whenever GDB gets control. Setting it to off will take effect the
next time the inferior is continued. If this option is set to off,
you can use 'set thread default pause on' or 'set thread pause on'
(see below) to pause individual threads.
'show task pause'
Show the current state of task suspension.
'set task detach-suspend-count'
This command sets the suspend count the task will be left with when
GDB detaches from it.
'show task detach-suspend-count'
Show the suspend count the task will be left with when detaching.
'set task exception-port'
'set task excp'
This command sets the task exception port to which GDB will forward
exceptions. The argument should be the value of the "send rights"
of the task. 'set task excp' is a shorthand alias.
'set noninvasive'
This command switches GDB to a mode that is the least invasive as
far as interfering with the inferior is concerned. This is the
same as using 'set task pause', 'set exceptions', and 'set signals'
to values opposite to the defaults.
'info send-rights'
'info receive-rights'
'info port-rights'
'info port-sets'
'info dead-names'
'info ports'
'info psets'
These commands display information about, respectively, send
rights, receive rights, port rights, port sets, and dead names of a
task. There are also shorthand aliases: 'info ports' for 'info
port-rights' and 'info psets' for 'info port-sets'.
'set thread pause'
This command toggles current thread suspension when GDB has
control. Setting it to on takes effect immediately, and the
current thread is suspended whenever GDB gets control. Setting it
to off will take effect the next time the inferior is continued.
Normally, this command has no effect, since when GDB has control,
the whole task is suspended. However, if you used 'set task pause
off' (see above), this command comes in handy to suspend only the
current thread.
'show thread pause'
This command shows the state of current thread suspension.
'set thread run'
This command sets whether the current thread is allowed to run.
'show thread run'
Show whether the current thread is allowed to run.
'set thread detach-suspend-count'
This command sets the suspend count GDB will leave on a thread when
detaching. This number is relative to the suspend count found by
GDB when it notices the thread; use 'set thread
takeover-suspend-count' to force it to an absolute value.
'show thread detach-suspend-count'
Show the suspend count GDB will leave on the thread when detaching.
'set thread exception-port'
'set thread excp'
Set the thread exception port to which to forward exceptions. This
overrides the port set by 'set task exception-port' (see above).
'set thread excp' is the shorthand alias.
'set thread takeover-suspend-count'
Normally, GDB's thread suspend counts are relative to the value GDB
finds when it notices each thread. This command changes the
suspend counts to be absolute instead.
'set thread default'
'show thread default'
Each of the above 'set thread' commands has a 'set thread default'
counterpart (e.g., 'set thread default pause', 'set thread default
exception-port', etc.). The 'thread default' variety of commands
sets the default thread properties for all threads; you can then
change the properties of individual threads with the non-default
commands.

File: gdb.info, Node: Darwin, Next: FreeBSD, Prev: Hurd Native, Up: Native
21.1.6 Darwin
-------------
GDB provides the following commands specific to the Darwin target:
'set debug darwin NUM'
When set to a non zero value, enables debugging messages specific
to the Darwin support. Higher values produce more verbose output.
'show debug darwin'
Show the current state of Darwin messages.
'set debug mach-o NUM'
When set to a non zero value, enables debugging messages while GDB
is reading Darwin object files. ("Mach-O" is the file format used
on Darwin for object and executable files.) Higher values produce
more verbose output. This is a command to diagnose problems
internal to GDB and should not be needed in normal usage.
'show debug mach-o'
Show the current state of Mach-O file messages.
'set mach-exceptions on'
'set mach-exceptions off'
On Darwin, faults are first reported as a Mach exception and are
then mapped to a Posix signal. Use this command to turn on
trapping of Mach exceptions in the inferior. This might be
sometimes useful to better understand the cause of a fault. The
default is off.
'show mach-exceptions'
Show the current state of exceptions trapping.

File: gdb.info, Node: FreeBSD, Prev: Darwin, Up: Native
21.1.7 FreeBSD
--------------
When the ABI of a system call is changed in the FreeBSD kernel, this is
implemented by leaving a compatibility system call using the old ABI at
the existing number and allocating a new system call number for the
version using the new ABI. As a convenience, when a system call is
caught by name (*note catch syscall::), compatibility system calls are
also caught.
For example, FreeBSD 12 introduced a new variant of the 'kevent'
system call and catching the 'kevent' system call by name catches both
variants:
(gdb) catch syscall kevent
Catchpoint 1 (syscalls 'freebsd11_kevent' [363] 'kevent' [560])
(gdb)

File: gdb.info, Node: Embedded OS, Next: Embedded Processors, Prev: Native, Up: Configurations
21.2 Embedded Operating Systems
===============================
This section describes configurations involving the debugging of
embedded operating systems that are available for several different
architectures.
GDB includes the ability to debug programs running on various
real-time operating systems.

File: gdb.info, Node: Embedded Processors, Next: Architectures, Prev: Embedded OS, Up: Configurations
21.3 Embedded Processors
========================
This section goes into details specific to particular embedded
configurations.
Whenever a specific embedded processor has a simulator, GDB allows to
send an arbitrary command to the simulator.
'sim COMMAND'
Send an arbitrary COMMAND string to the simulator. Consult the
documentation for the specific simulator in use for information
about acceptable commands.
* Menu:
* ARC:: Synopsys ARC
* ARM:: ARM
* M68K:: Motorola M68K
* MicroBlaze:: Xilinx MicroBlaze
* MIPS Embedded:: MIPS Embedded
* OpenRISC 1000:: OpenRISC 1000 (or1k)
* PowerPC Embedded:: PowerPC Embedded
* AVR:: Atmel AVR
* CRIS:: CRIS
* Super-H:: Renesas Super-H

File: gdb.info, Node: ARC, Next: ARM, Up: Embedded Processors
21.3.1 Synopsys ARC
-------------------
GDB provides the following ARC-specific commands:
'set debug arc'
Control the level of ARC specific debug messages. Use 0 for no
messages (the default), 1 for debug messages, and 2 for even more
debug messages.
'show debug arc'
Show the level of ARC specific debugging in operation.
'maint print arc arc-instruction ADDRESS'
Print internal disassembler information about instruction at a
given address.

File: gdb.info, Node: ARM, Next: M68K, Prev: ARC, Up: Embedded Processors
21.3.2 ARM
----------
GDB provides the following ARM-specific commands:
'set arm disassembler'
This commands selects from a list of disassembly styles. The
'"std"' style is the standard style.
'show arm disassembler'
Show the current disassembly style.
'set arm apcs32'
This command toggles ARM operation mode between 32-bit and 26-bit.
'show arm apcs32'
Display the current usage of the ARM 32-bit mode.
'set arm fpu FPUTYPE'
This command sets the ARM floating-point unit (FPU) type. The
argument FPUTYPE can be one of these:
'auto'
Determine the FPU type by querying the OS ABI.
'softfpa'
Software FPU, with mixed-endian doubles on little-endian ARM
processors.
'fpa'
GCC-compiled FPA co-processor.
'softvfp'
Software FPU with pure-endian doubles.
'vfp'
VFP co-processor.
'show arm fpu'
Show the current type of the FPU.
'set arm abi'
This command forces GDB to use the specified ABI.
'show arm abi'
Show the currently used ABI.
'set arm fallback-mode (arm|thumb|auto)'
GDB uses the symbol table, when available, to determine whether
instructions are ARM or Thumb. This command controls GDB's default
behavior when the symbol table is not available. The default is
'auto', which causes GDB to use the current execution mode (from
the 'T' bit in the 'CPSR' register).
'show arm fallback-mode'
Show the current fallback instruction mode.
'set arm force-mode (arm|thumb|auto)'
This command overrides use of the symbol table to determine whether
instructions are ARM or Thumb. The default is 'auto', which causes
GDB to use the symbol table and then the setting of 'set arm
fallback-mode'.
'show arm force-mode'
Show the current forced instruction mode.
'set debug arm'
Toggle whether to display ARM-specific debugging messages from the
ARM target support subsystem.
'show debug arm'
Show whether ARM-specific debugging messages are enabled.
'target sim [SIMARGS] ...'
The GDB ARM simulator accepts the following optional arguments.
'--swi-support=TYPE'
Tell the simulator which SWI interfaces to support. The
argument TYPE may be a comma separated list of the following
values. The default value is 'all'.
'none'
'demon'
'angel'
'redboot'
'all'

File: gdb.info, Node: M68K, Next: MicroBlaze, Prev: ARM, Up: Embedded Processors
21.3.3 M68k
-----------
The Motorola m68k configuration includes ColdFire support.

File: gdb.info, Node: MicroBlaze, Next: MIPS Embedded, Prev: M68K, Up: Embedded Processors
21.3.4 MicroBlaze
-----------------
The MicroBlaze is a soft-core processor supported on various Xilinx
FPGAs, such as Spartan or Virtex series. Boards with these processors
usually have JTAG ports which connect to a host system running the
Xilinx Embedded Development Kit (EDK) or Software Development Kit (SDK).
This host system is used to download the configuration bitstream to the
target FPGA. The Xilinx Microprocessor Debugger (XMD) program
communicates with the target board using the JTAG interface and presents
a 'gdbserver' interface to the board. By default 'xmd' uses port
'1234'. (While it is possible to change this default port, it requires
the use of undocumented 'xmd' commands. Contact Xilinx support if you
need to do this.)
Use these GDB commands to connect to the MicroBlaze target processor.
'target remote :1234'
Use this command to connect to the target if you are running GDB on
the same system as 'xmd'.
'target remote XMD-HOST:1234'
Use this command to connect to the target if it is connected to
'xmd' running on a different system named XMD-HOST.
'load'
Use this command to download a program to the MicroBlaze target.
'set debug microblaze N'
Enable MicroBlaze-specific debugging messages if non-zero.
'show debug microblaze N'
Show MicroBlaze-specific debugging level.

File: gdb.info, Node: MIPS Embedded, Next: OpenRISC 1000, Prev: MicroBlaze, Up: Embedded Processors
21.3.5 MIPS Embedded
--------------------
GDB supports these special commands for MIPS targets:
'set mipsfpu double'
'set mipsfpu single'
'set mipsfpu none'
'set mipsfpu auto'
'show mipsfpu'
If your target board does not support the MIPS floating point
coprocessor, you should use the command 'set mipsfpu none' (if you
need this, you may wish to put the command in your GDB init file).
This tells GDB how to find the return value of functions which
return floating point values. It also allows GDB to avoid saving
the floating point registers when calling functions on the board.
If you are using a floating point coprocessor with only single
precision floating point support, as on the R4650 processor, use
the command 'set mipsfpu single'. The default double precision
floating point coprocessor may be selected using 'set mipsfpu
double'.
In previous versions the only choices were double precision or no
floating point, so 'set mipsfpu on' will select double precision
and 'set mipsfpu off' will select no floating point.
As usual, you can inquire about the 'mipsfpu' variable with 'show
mipsfpu'.

File: gdb.info, Node: OpenRISC 1000, Next: PowerPC Embedded, Prev: MIPS Embedded, Up: Embedded Processors
21.3.6 OpenRISC 1000
--------------------
The OpenRISC 1000 provides a free RISC instruction set architecture. It
is mainly provided as a soft-core which can run on Xilinx, Altera and
other FPGA's.
GDB for OpenRISC supports the below commands when connecting to a
target:
'target sim'
Runs the builtin CPU simulator which can run very basic programs
but does not support most hardware functions like MMU. For more
complex use cases the user is advised to run an external target,
and connect using 'target remote'.
Example: 'target sim'
'set debug or1k'
Toggle whether to display OpenRISC-specific debugging messages from
the OpenRISC target support subsystem.
'show debug or1k'
Show whether OpenRISC-specific debugging messages are enabled.

File: gdb.info, Node: PowerPC Embedded, Next: AVR, Prev: OpenRISC 1000, Up: Embedded Processors
21.3.7 PowerPC Embedded
-----------------------
GDB supports using the DVC (Data Value Compare) register to implement in
hardware simple hardware watchpoint conditions of the form:
(gdb) watch ADDRESS|VARIABLE \
if ADDRESS|VARIABLE == CONSTANT EXPRESSION
The DVC register will be automatically used when GDB detects such
pattern in a condition expression, and the created watchpoint uses one
debug register (either the 'exact-watchpoints' option is on and the
variable is scalar, or the variable has a length of one byte). This
feature is available in native GDB running on a Linux kernel version
2.6.34 or newer.
When running on PowerPC embedded processors, GDB automatically uses
ranged hardware watchpoints, unless the 'exact-watchpoints' option is
on, in which case watchpoints using only one debug register are created
when watching variables of scalar types.
You can create an artificial array to watch an arbitrary memory
region using one of the following commands (*note Expressions::):
(gdb) watch *((char *) ADDRESS)@LENGTH
(gdb) watch {char[LENGTH]} ADDRESS
PowerPC embedded processors support masked watchpoints. See the
discussion about the 'mask' argument in *note Set Watchpoints::.
PowerPC embedded processors support hardware accelerated "ranged
breakpoints". A ranged breakpoint stops execution of the inferior
whenever it executes an instruction at any address within the range it
specifies. To set a ranged breakpoint in GDB, use the 'break-range'
command.
GDB provides the following PowerPC-specific commands:
'break-range START-LOCATION, END-LOCATION'
Set a breakpoint for an address range given by START-LOCATION and
END-LOCATION, which can specify a function name, a line number, an
offset of lines from the current line or from the start location,
or an address of an instruction (see *note Specify Location::, for
a list of all the possible ways to specify a LOCATION.) The
breakpoint will stop execution of the inferior whenever it executes
an instruction at any address within the specified range,
(including START-LOCATION and END-LOCATION.)
'set powerpc soft-float'
'show powerpc soft-float'
Force GDB to use (or not use) a software floating point calling
convention. By default, GDB selects the calling convention based
on the selected architecture and the provided executable file.
'set powerpc vector-abi'
'show powerpc vector-abi'
Force GDB to use the specified calling convention for vector
arguments and return values. The valid options are 'auto';
'generic', to avoid vector registers even if they are present;
'altivec', to use AltiVec registers; and 'spe' to use SPE
registers. By default, GDB selects the calling convention based on
the selected architecture and the provided executable file.
'set powerpc exact-watchpoints'
'show powerpc exact-watchpoints'
Allow GDB to use only one debug register when watching a variable
of scalar type, thus assuming that the variable is accessed through
the address of its first byte.

File: gdb.info, Node: AVR, Next: CRIS, Prev: PowerPC Embedded, Up: Embedded Processors
21.3.8 Atmel AVR
----------------
When configured for debugging the Atmel AVR, GDB supports the following
AVR-specific commands:
'info io_registers'
This command displays information about the AVR I/O registers. For
each register, GDB prints its number and value.

File: gdb.info, Node: CRIS, Next: Super-H, Prev: AVR, Up: Embedded Processors
21.3.9 CRIS
-----------
When configured for debugging CRIS, GDB provides the following
CRIS-specific commands:
'set cris-version VER'
Set the current CRIS version to VER, either '10' or '32'. The CRIS
version affects register names and sizes. This command is useful
in case autodetection of the CRIS version fails.
'show cris-version'
Show the current CRIS version.
'set cris-dwarf2-cfi'
Set the usage of DWARF-2 CFI for CRIS debugging. The default is
'on'. Change to 'off' when using 'gcc-cris' whose version is below
'R59'.
'show cris-dwarf2-cfi'
Show the current state of using DWARF-2 CFI.
'set cris-mode MODE'
Set the current CRIS mode to MODE. It should only be changed when
debugging in guru mode, in which case it should be set to 'guru'
(the default is 'normal').
'show cris-mode'
Show the current CRIS mode.

File: gdb.info, Node: Super-H, Prev: CRIS, Up: Embedded Processors
21.3.10 Renesas Super-H
-----------------------
For the Renesas Super-H processor, GDB provides these commands:
'set sh calling-convention CONVENTION'
Set the calling-convention used when calling functions from GDB.
Allowed values are 'gcc', which is the default setting, and
'renesas'. With the 'gcc' setting, functions are called using the
GCC calling convention. If the DWARF-2 information of the called
function specifies that the function follows the Renesas calling
convention, the function is called using the Renesas calling
convention. If the calling convention is set to 'renesas', the
Renesas calling convention is always used, regardless of the
DWARF-2 information. This can be used to override the default of
'gcc' if debug information is missing, or the compiler does not
emit the DWARF-2 calling convention entry for a function.
'show sh calling-convention'
Show the current calling convention setting.

File: gdb.info, Node: Architectures, Prev: Embedded Processors, Up: Configurations
21.4 Architectures
==================
This section describes characteristics of architectures that affect all
uses of GDB with the architecture, both native and cross.
* Menu:
* AArch64::
* i386::
* Alpha::
* MIPS::
* HPPA:: HP PA architecture
* PowerPC::
* Nios II::
* Sparc64::
* S12Z::

File: gdb.info, Node: AArch64, Next: i386, Up: Architectures
21.4.1 AArch64
--------------
When GDB is debugging the AArch64 architecture, it provides the
following special commands:
'set debug aarch64'
This command determines whether AArch64 architecture-specific
debugging messages are to be displayed.
'show debug aarch64'
Show whether AArch64 debugging messages are displayed.
21.4.1.1 AArch64 SVE.
.....................
When GDB is debugging the AArch64 architecture, if the Scalable Vector
Extension (SVE) is present, then GDB will provide the vector registers
'$z0' through '$z31', vector predicate registers '$p0' through '$p15',
and the '$ffr' register. In addition, the pseudo register '$vg' will be
provided. This is the vector granule for the current thread and
represents the number of 64-bit chunks in an SVE 'z' register.
If the vector length changes, then the '$vg' register will be
updated, but the lengths of the 'z' and 'p' registers will not change.
This is a known limitation of GDB and does not affect the execution of
the target process.
21.4.1.2 AArch64 Pointer Authentication.
........................................
When GDB is debugging the AArch64 architecture, and the program is using
the v8.3-A feature Pointer Authentication (PAC), then whenever the link
register '$lr' is pointing to an PAC function its value will be masked.
When GDB prints a backtrace, any addresses that required unmasking will
be postfixed with the marker [PAC]. When using the MI, this is printed
as part of the 'addr_flags' field.

File: gdb.info, Node: i386, Next: Alpha, Prev: AArch64, Up: Architectures
21.4.2 x86 Architecture-specific Issues
---------------------------------------
'set struct-convention MODE'
Set the convention used by the inferior to return 'struct's and
'union's from functions to MODE. Possible values of MODE are
'"pcc"', '"reg"', and '"default"' (the default). '"default"' or
'"pcc"' means that 'struct's are returned on the stack, while
'"reg"' means that a 'struct' or a 'union' whose size is 1, 2, 4,
or 8 bytes will be returned in a register.
'show struct-convention'
Show the current setting of the convention to return 'struct's from
functions.
21.4.2.1 Intel "Memory Protection Extensions" (MPX).
....................................................
Memory Protection Extension (MPX) adds the bound registers 'BND0' (1)
through 'BND3'. Bound registers store a pair of 64-bit values which are
the lower bound and upper bound. Bounds are effective addresses or
memory locations. The upper bounds are architecturally represented in
1's complement form. A bound having lower bound = 0, and upper bound =
0 (1's complement of all bits set) will allow access to the entire
address space.
'BND0' through 'BND3' are represented in GDB as 'bnd0raw' through
'bnd3raw'. Pseudo registers 'bnd0' through 'bnd3' display the upper
bound performing the complement of one operation on the upper bound
value, i.e. when upper bound in 'bnd0raw' is 0 in the GDB 'bnd0' it will
be '0xfff...'. In this sense it can also be noted that the upper bounds
are inclusive.
As an example, assume that the register BND0 holds bounds for a
pointer having access allowed for the range between 0x32 and 0x71. The
values present on bnd0raw and bnd registers are presented as follows:
bnd0raw = {0x32, 0xffffffff8e}
bnd0 = {lbound = 0x32, ubound = 0x71} : size 64
This way the raw value can be accessed via bnd0raw...bnd3raw. Any
change on bnd0...bnd3 or bnd0raw...bnd3raw is reflect on its
counterpart. When the bnd0...bnd3 registers are displayed via Python,
the display includes the memory size, in bits, accessible to the
pointer.
Bounds can also be stored in bounds tables, which are stored in
application memory. These tables store bounds for pointers by
specifying the bounds pointer's value along with its bounds. Evaluating
and changing bounds located in bound tables is therefore interesting
while investigating bugs on MPX context. GDB provides commands for this
purpose:
'show mpx bound POINTER'
Display bounds of the given POINTER.
'set mpx bound POINTER, LBOUND, UBOUND'
Set the bounds of a pointer in the bound table. This command takes
three parameters: POINTER is the pointers whose bounds are to be
changed, LBOUND and UBOUND are new values for lower and upper
bounds respectively.
When you call an inferior function on an Intel MPX enabled program,
GDB sets the inferior's bound registers to the init (disabled) state
before calling the function. As a consequence, bounds checks for the
pointer arguments passed to the function will always pass.
This is necessary because when you call an inferior function, the
program is usually in the middle of the execution of other function.
Since at that point bound registers are in an arbitrary state, not
clearing them would lead to random bound violations in the called
function.
You can still examine the influence of the bound registers on the
execution of the called function by stopping the execution of the called
function at its prologue, setting bound registers, and continuing the
execution. For example:
$ break *upper
Breakpoint 2 at 0x4009de: file i386-mpx-call.c, line 47.
$ print upper (a, b, c, d, 1)
Breakpoint 2, upper (a=0x0, b=0x6e0000005b, c=0x0, d=0x0, len=48)....
$ print $bnd0
{lbound = 0x0, ubound = ffffffff} : size -1
At this last step the value of bnd0 can be changed for investigation
of bound violations caused along the execution of the call. In order to
know how to set the bound registers or bound table for the call consult
the ABI.
---------- Footnotes ----------
(1) The register named with capital letters represent the
architecture registers.

File: gdb.info, Node: Alpha, Next: MIPS, Prev: i386, Up: Architectures
21.4.3 Alpha
------------
See the following section.

File: gdb.info, Node: MIPS, Next: HPPA, Prev: Alpha, Up: Architectures
21.4.4 MIPS
-----------
Alpha- and MIPS-based computers use an unusual stack frame, which
sometimes requires GDB to search backward in the object code to find the
beginning of a function.
To improve response time (especially for embedded applications, where
GDB may be restricted to a slow serial line for this search) you may
want to limit the size of this search, using one of these commands:
'set heuristic-fence-post LIMIT'
Restrict GDB to examining at most LIMIT bytes in its search for the
beginning of a function. A value of 0 (the default) means there is
no limit. However, except for 0, the larger the limit the more
bytes 'heuristic-fence-post' must search and therefore the longer
it takes to run. You should only need to use this command when
debugging a stripped executable.
'show heuristic-fence-post'
Display the current limit.
These commands are available _only_ when GDB is configured for debugging
programs on Alpha or MIPS processors.
Several MIPS-specific commands are available when debugging MIPS
programs:
'set mips abi ARG'
Tell GDB which MIPS ABI is used by the inferior. Possible values
of ARG are:
'auto'
The default ABI associated with the current binary (this is
the default).
'o32'
'o64'
'n32'
'n64'
'eabi32'
'eabi64'
'show mips abi'
Show the MIPS ABI used by GDB to debug the inferior.
'set mips compression ARG'
Tell GDB which MIPS compressed ISA (Instruction Set Architecture)
encoding is used by the inferior. GDB uses this for code
disassembly and other internal interpretation purposes. This
setting is only referred to when no executable has been associated
with the debugging session or the executable does not provide
information about the encoding it uses. Otherwise this setting is
automatically updated from information provided by the executable.
Possible values of ARG are 'mips16' and 'micromips'. The default
compressed ISA encoding is 'mips16', as executables containing
MIPS16 code frequently are not identified as such.
This setting is "sticky"; that is, it retains its value across
debugging sessions until reset either explicitly with this command
or implicitly from an executable.
The compiler and/or assembler typically add symbol table
annotations to identify functions compiled for the MIPS16 or
microMIPS ISAs. If these function-scope annotations are present,
GDB uses them in preference to the global compressed ISA encoding
setting.
'show mips compression'
Show the MIPS compressed ISA encoding used by GDB to debug the
inferior.
'set mipsfpu'
'show mipsfpu'
*Note set mipsfpu: MIPS Embedded.
'set mips mask-address ARG'
This command determines whether the most-significant 32 bits of
64-bit MIPS addresses are masked off. The argument ARG can be
'on', 'off', or 'auto'. The latter is the default setting, which
lets GDB determine the correct value.
'show mips mask-address'
Show whether the upper 32 bits of MIPS addresses are masked off or
not.
'set remote-mips64-transfers-32bit-regs'
This command controls compatibility with 64-bit MIPS targets that
transfer data in 32-bit quantities. If you have an old MIPS 64
target that transfers 32 bits for some registers, like SR and FSR,
and 64 bits for other registers, set this option to 'on'.
'show remote-mips64-transfers-32bit-regs'
Show the current setting of compatibility with older MIPS 64
targets.
'set debug mips'
This command turns on and off debugging messages for the
MIPS-specific target code in GDB.
'show debug mips'
Show the current setting of MIPS debugging messages.

File: gdb.info, Node: HPPA, Next: PowerPC, Prev: MIPS, Up: Architectures
21.4.5 HPPA
-----------
When GDB is debugging the HP PA architecture, it provides the following
special commands:
'set debug hppa'
This command determines whether HPPA architecture-specific
debugging messages are to be displayed.
'show debug hppa'
Show whether HPPA debugging messages are displayed.
'maint print unwind ADDRESS'
This command displays the contents of the unwind table entry at the
given ADDRESS.

File: gdb.info, Node: PowerPC, Next: Nios II, Prev: HPPA, Up: Architectures
21.4.6 PowerPC
--------------
When GDB is debugging the PowerPC architecture, it provides a set of
pseudo-registers to enable inspection of 128-bit wide Decimal Floating
Point numbers stored in the floating point registers. These values must
be stored in two consecutive registers, always starting at an even
register like 'f0' or 'f2'.
The pseudo-registers go from '$dl0' through '$dl15', and are formed
by joining the even/odd register pairs 'f0' and 'f1' for '$dl0', 'f2'
and 'f3' for '$dl1' and so on.
For POWER7 processors, GDB provides a set of pseudo-registers, the
64-bit wide Extended Floating Point Registers ('f32' through 'f63').

File: gdb.info, Node: Nios II, Next: Sparc64, Prev: PowerPC, Up: Architectures
21.4.7 Nios II
--------------
When GDB is debugging the Nios II architecture, it provides the
following special commands:
'set debug nios2'
This command turns on and off debugging messages for the Nios II
target code in GDB.
'show debug nios2'
Show the current setting of Nios II debugging messages.

File: gdb.info, Node: Sparc64, Next: S12Z, Prev: Nios II, Up: Architectures
21.4.8 Sparc64
--------------
21.4.8.1 ADI Support
....................
The M7 processor supports an Application Data Integrity (ADI) feature
that detects invalid data accesses. When software allocates memory and
enables ADI on the allocated memory, it chooses a 4-bit version number,
sets the version in the upper 4 bits of the 64-bit pointer to that data,
and stores the 4-bit version in every cacheline of that data. Hardware
saves the latter in spare bits in the cache and memory hierarchy. On
each load and store, the processor compares the upper 4 VA (virtual
address) bits to the cacheline's version. If there is a mismatch, the
processor generates a version mismatch trap which can be either precise
or disrupting. The trap is an error condition which the kernel delivers
to the process as a SIGSEGV signal.
Note that only 64-bit applications can use ADI and need to be built
with ADI-enabled.
Values of the ADI version tags, which are in granularity of a
cacheline (64 bytes), can be viewed or modified.
'adi (examine | x) [ / N ] ADDR'
The 'adi examine' command displays the value of one ADI version tag
per cacheline.
N is a decimal integer specifying the number in bytes; the default
is 1. It specifies how much ADI version information, at the ratio
of 1:ADI block size, to display.
ADDR is the address in user address space where you want GDB to
begin displaying the ADI version tags.
Below is an example of displaying ADI versions of variable
"shmaddr".
(gdb) adi x/100 shmaddr
0xfff800010002c000: 0 0
'adi (assign | a) [ / N ] ADDR = TAG'
The 'adi assign' command is used to assign new ADI version tag to
an address.
N is a decimal integer specifying the number in bytes; the default
is 1. It specifies how much ADI version information, at the ratio
of 1:ADI block size, to modify.
ADDR is the address in user address space where you want GDB to
begin modifying the ADI version tags.
TAG is the new ADI version tag.
For example, do the following to modify then verify ADI versions of
variable "shmaddr":
(gdb) adi a/100 shmaddr = 7
(gdb) adi x/100 shmaddr
0xfff800010002c000: 7 7

File: gdb.info, Node: S12Z, Prev: Sparc64, Up: Architectures
21.4.9 S12Z
-----------
When GDB is debugging the S12Z architecture, it provides the following
special command:
'maint info bdccsr'
This command displays the current value of the microprocessor's
BDCCSR register.

File: gdb.info, Node: Controlling GDB, Next: Extending GDB, Prev: Configurations, Up: Top
22 Controlling GDB
******************
You can alter the way GDB interacts with you by using the 'set' command.
For commands controlling how GDB displays data, see *note Print
Settings: Print Settings. Other settings are described here.
* Menu:
* Prompt:: Prompt
* Editing:: Command editing
* Command History:: Command history
* Screen Size:: Screen size
* Output Styling:: Output styling
* Numbers:: Numbers
* ABI:: Configuring the current ABI
* Auto-loading:: Automatically loading associated files
* Messages/Warnings:: Optional warnings and messages
* Debugging Output:: Optional messages about internal happenings
* Other Misc Settings:: Other Miscellaneous Settings

File: gdb.info, Node: Prompt, Next: Editing, Up: Controlling GDB
22.1 Prompt
===========
GDB indicates its readiness to read a command by printing a string
called the "prompt". This string is normally '(gdb)'. You can change
the prompt string with the 'set prompt' command. For instance, when
debugging GDB with GDB, it is useful to change the prompt in one of the
GDB sessions so that you can always tell which one you are talking to.
_Note:_ 'set prompt' does not add a space for you after the prompt
you set. This allows you to set a prompt which ends in a space or a
prompt that does not.
'set prompt NEWPROMPT'
Directs GDB to use NEWPROMPT as its prompt string henceforth.
'show prompt'
Prints a line of the form: 'Gdb's prompt is: YOUR-PROMPT'
Versions of GDB that ship with Python scripting enabled have prompt
extensions. The commands for interacting with these extensions are:
'set extended-prompt PROMPT'
Set an extended prompt that allows for substitutions. *Note
gdb.prompt::, for a list of escape sequences that can be used for
substitution. Any escape sequences specified as part of the prompt
string are replaced with the corresponding strings each time the
prompt is displayed.
For example:
set extended-prompt Current working directory: \w (gdb)
Note that when an extended-prompt is set, it takes control of the
PROMPT_HOOK hook. *Note prompt_hook::, for further information.
'show extended-prompt'
Prints the extended prompt. Any escape sequences specified as part
of the prompt string with 'set extended-prompt', are replaced with
the corresponding strings each time the prompt is displayed.

File: gdb.info, Node: Editing, Next: Command History, Prev: Prompt, Up: Controlling GDB
22.2 Command Editing
====================
GDB reads its input commands via the "Readline" interface. This GNU
library provides consistent behavior for programs which provide a
command line interface to the user. Advantages are GNU Emacs-style or
"vi"-style inline editing of commands, 'csh'-like history substitution,
and a storage and recall of command history across debugging sessions.
You may control the behavior of command line editing in GDB with the
command 'set'.
'set editing'
'set editing on'
Enable command line editing (enabled by default).
'set editing off'
Disable command line editing.
'show editing'
Show whether command line editing is enabled.
*Note Command Line Editing::, for more details about the Readline
interface. Users unfamiliar with GNU Emacs or 'vi' are encouraged to
read that chapter.
GDB sets the Readline application name to 'gdb'. This is useful for
conditions in '.inputrc'.
GDB defines a bindable Readline command, 'operate-and-get-next'.
This is bound to 'C-o' by default. This command accepts the current
line for execution and fetches the next line relative to the current
line from the history for editing. Any argument is ignored.

File: gdb.info, Node: Command History, Next: Screen Size, Prev: Editing, Up: Controlling GDB
22.3 Command History
====================
GDB can keep track of the commands you type during your debugging
sessions, so that you can be certain of precisely what happened. Use
these commands to manage the GDB command history facility.
GDB uses the GNU History library, a part of the Readline package, to
provide the history facility. *Note Using History Interactively::, for
the detailed description of the History library.
To issue a command to GDB without affecting certain aspects of the
state which is seen by users, prefix it with 'server ' (*note Server
Prefix::). This means that this command will not affect the command
history, nor will it affect GDB's notion of which command to repeat if
<RET> is pressed on a line by itself.
The server prefix does not affect the recording of values into the
value history; to print a value without recording it into the value
history, use the 'output' command instead of the 'print' command.
Here is the description of GDB commands related to command history.
'set history filename FNAME'
Set the name of the GDB command history file to FNAME. This is the
file where GDB reads an initial command history list, and where it
writes the command history from this session when it exits. You
can access this list through history expansion or through the
history command editing characters listed below. This file
defaults to the value of the environment variable 'GDBHISTFILE', or
to './.gdb_history' ('./_gdb_history' on MS-DOS) if this variable
is not set.
'set history save'
'set history save on'
Record command history in a file, whose name may be specified with
the 'set history filename' command. By default, this option is
disabled.
'set history save off'
Stop recording command history in a file.
'set history size SIZE'
'set history size unlimited'
Set the number of commands which GDB keeps in its history list.
This defaults to the value of the environment variable
'GDBHISTSIZE', or to 256 if this variable is not set. Non-numeric
values of 'GDBHISTSIZE' are ignored. If SIZE is 'unlimited' or if
'GDBHISTSIZE' is either a negative number or the empty string, then
the number of commands GDB keeps in the history list is unlimited.
'set history remove-duplicates COUNT'
'set history remove-duplicates unlimited'
Control the removal of duplicate history entries in the command
history list. If COUNT is non-zero, GDB will look back at the last
COUNT history entries and remove the first entry that is a
duplicate of the current entry being added to the command history
list. If COUNT is 'unlimited' then this lookbehind is unbounded.
If COUNT is 0, then removal of duplicate history entries is
disabled.
Only history entries added during the current session are
considered for removal. This option is set to 0 by default.
History expansion assigns special meaning to the character '!'.
*Note Event Designators::, for more details.
Since '!' is also the logical not operator in C, history expansion is
off by default. If you decide to enable history expansion with the 'set
history expansion on' command, you may sometimes need to follow '!'
(when it is used as logical not, in an expression) with a space or a tab
to prevent it from being expanded. The readline history facilities do
not attempt substitution on the strings '!=' and '!(', even when history
expansion is enabled.
The commands to control history expansion are:
'set history expansion on'
'set history expansion'
Enable history expansion. History expansion is off by default.
'set history expansion off'
Disable history expansion.
'show history'
'show history filename'
'show history save'
'show history size'
'show history expansion'
These commands display the state of the GDB history parameters.
'show history' by itself displays all four states.
'show commands'
Display the last ten commands in the command history.
'show commands N'
Print ten commands centered on command number N.
'show commands +'
Print ten commands just after the commands last printed.