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This file documents the GNU debugger GDB.
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Permission is granted to copy, distribute and/or modify this document
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Documentation", with the Front-Cover Texts being "A GNU Manual," and
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File: gdb.info, Node: Expressions, Next: Ambiguous Expressions, Up: Data
10.1 Expressions
================
'print' and many other GDB commands accept an expression and compute its
value. Any kind of constant, variable or operator defined by the
programming language you are using is valid in an expression in GDB.
This includes conditional expressions, function calls, casts, and string
constants. It also includes preprocessor macros, if you compiled your
program to include this information; see *note Compilation::.
GDB supports array constants in expressions input by the user. The
syntax is {ELEMENT, ELEMENT...}. For example, you can use the command
'print {1, 2, 3}' to create an array of three integers. If you pass an
array to a function or assign it to a program variable, GDB copies the
array to memory that is 'malloc'ed in the target program.
Because C is so widespread, most of the expressions shown in examples
in this manual are in C. *Note Using GDB with Different Languages:
Languages, for information on how to use expressions in other languages.
In this section, we discuss operators that you can use in GDB
expressions regardless of your programming language.
Casts are supported in all languages, not just in C, because it is so
useful to cast a number into a pointer in order to examine a structure
at that address in memory.
GDB supports these operators, in addition to those common to
programming languages:
'@'
'@' is a binary operator for treating parts of memory as arrays.
*Note Artificial Arrays: Arrays, for more information.
'::'
'::' allows you to specify a variable in terms of the file or
function where it is defined. *Note Program Variables: Variables.
'{TYPE} ADDR'
Refers to an object of type TYPE stored at address ADDR in memory.
The address ADDR may be any expression whose value is an integer or
pointer (but parentheses are required around binary operators, just
as in a cast). This construct is allowed regardless of what kind
of data is normally supposed to reside at ADDR.

File: gdb.info, Node: Ambiguous Expressions, Next: Variables, Prev: Expressions, Up: Data
10.2 Ambiguous Expressions
==========================
Expressions can sometimes contain some ambiguous elements. For
instance, some programming languages (notably Ada, C++ and Objective-C)
permit a single function name to be defined several times, for
application in different contexts. This is called "overloading".
Another example involving Ada is generics. A "generic package" is
similar to C++ templates and is typically instantiated several times,
resulting in the same function name being defined in different contexts.
In some cases and depending on the language, it is possible to adjust
the expression to remove the ambiguity. For instance in C++, you can
specify the signature of the function you want to break on, as in 'break
FUNCTION(TYPES)'. In Ada, using the fully qualified name of your
function often makes the expression unambiguous as well.
When an ambiguity that needs to be resolved is detected, the debugger
has the capability to display a menu of numbered choices for each
possibility, and then waits for the selection with the prompt '>'. The
first option is always '[0] cancel', and typing '0 <RET>' aborts the
current command. If the command in which the expression was used allows
more than one choice to be selected, the next option in the menu is '[1]
all', and typing '1 <RET>' selects all possible choices.
For example, the following session excerpt shows an attempt to set a
breakpoint at the overloaded symbol 'String::after'. We choose three
particular definitions of that function name:
(gdb) b String::after
[0] cancel
[1] all
[2] file:String.cc; line number:867
[3] file:String.cc; line number:860
[4] file:String.cc; line number:875
[5] file:String.cc; line number:853
[6] file:String.cc; line number:846
[7] file:String.cc; line number:735
> 2 4 6
Breakpoint 1 at 0xb26c: file String.cc, line 867.
Breakpoint 2 at 0xb344: file String.cc, line 875.
Breakpoint 3 at 0xafcc: file String.cc, line 846.
Multiple breakpoints were set.
Use the "delete" command to delete unwanted
breakpoints.
(gdb)
'set multiple-symbols MODE'
This option allows you to adjust the debugger behavior when an
expression is ambiguous.
By default, MODE is set to 'all'. If the command with which the
expression is used allows more than one choice, then GDB
automatically selects all possible choices. For instance,
inserting a breakpoint on a function using an ambiguous name
results in a breakpoint inserted on each possible match. However,
if a unique choice must be made, then GDB uses the menu to help you
disambiguate the expression. For instance, printing the address of
an overloaded function will result in the use of the menu.
When MODE is set to 'ask', the debugger always uses the menu when
an ambiguity is detected.
Finally, when MODE is set to 'cancel', the debugger reports an
error due to the ambiguity and the command is aborted.
'show multiple-symbols'
Show the current value of the 'multiple-symbols' setting.

File: gdb.info, Node: Variables, Next: Arrays, Prev: Ambiguous Expressions, Up: Data
10.3 Program Variables
======================
The most common kind of expression to use is the name of a variable in
your program.
Variables in expressions are understood in the selected stack frame
(*note Selecting a Frame: Selection.); they must be either:
* global (or file-static)
or
* visible according to the scope rules of the programming language
from the point of execution in that frame
This means that in the function
foo (a)
int a;
{
bar (a);
{
int b = test ();
bar (b);
}
}
you can examine and use the variable 'a' whenever your program is
executing within the function 'foo', but you can only use or examine the
variable 'b' while your program is executing inside the block where 'b'
is declared.
There is an exception: you can refer to a variable or function whose
scope is a single source file even if the current execution point is not
in this file. But it is possible to have more than one such variable or
function with the same name (in different source files). If that
happens, referring to that name has unpredictable effects. If you wish,
you can specify a static variable in a particular function or file by
using the colon-colon ('::') notation:
FILE::VARIABLE
FUNCTION::VARIABLE
Here FILE or FUNCTION is the name of the context for the static
VARIABLE. In the case of file names, you can use quotes to make sure
GDB parses the file name as a single word--for example, to print a
global value of 'x' defined in 'f2.c':
(gdb) p 'f2.c'::x
The '::' notation is normally used for referring to static variables,
since you typically disambiguate uses of local variables in functions by
selecting the appropriate frame and using the simple name of the
variable. However, you may also use this notation to refer to local
variables in frames enclosing the selected frame:
void
foo (int a)
{
if (a < 10)
bar (a);
else
process (a); /* Stop here */
}
int
bar (int a)
{
foo (a + 5);
}
For example, if there is a breakpoint at the commented line, here is
what you might see when the program stops after executing the call
'bar(0)':
(gdb) p a
$1 = 10
(gdb) p bar::a
$2 = 5
(gdb) up 2
#2 0x080483d0 in foo (a=5) at foobar.c:12
(gdb) p a
$3 = 5
(gdb) p bar::a
$4 = 0
These uses of '::' are very rarely in conflict with the very similar
use of the same notation in C++. When they are in conflict, the C++
meaning takes precedence; however, this can be overridden by quoting the
file or function name with single quotes.
For example, suppose the program is stopped in a method of a class
that has a field named 'includefile', and there is also an include file
named 'includefile' that defines a variable, 'some_global'.
(gdb) p includefile
$1 = 23
(gdb) p includefile::some_global
A syntax error in expression, near `'.
(gdb) p 'includefile'::some_global
$2 = 27
_Warning:_ Occasionally, a local variable may appear to have the
wrong value at certain points in a function--just after entry to a
new scope, and just before exit.
You may see this problem when you are stepping by machine
instructions. This is because, on most machines, it takes more than one
instruction to set up a stack frame (including local variable
definitions); if you are stepping by machine instructions, variables may
appear to have the wrong values until the stack frame is completely
built. On exit, it usually also takes more than one machine instruction
to destroy a stack frame; after you begin stepping through that group of
instructions, local variable definitions may be gone.
This may also happen when the compiler does significant
optimizations. To be sure of always seeing accurate values, turn off
all optimization when compiling.
Another possible effect of compiler optimizations is to optimize
unused variables out of existence, or assign variables to registers (as
opposed to memory addresses). Depending on the support for such cases
offered by the debug info format used by the compiler, GDB might not be
able to display values for such local variables. If that happens, GDB
will print a message like this:
No symbol "foo" in current context.
To solve such problems, either recompile without optimizations, or
use a different debug info format, if the compiler supports several such
formats. *Note Compilation::, for more information on choosing compiler
options. *Note C and C++: C, for more information about debug info
formats that are best suited to C++ programs.
If you ask to print an object whose contents are unknown to GDB,
e.g., because its data type is not completely specified by the debug
information, GDB will say '<incomplete type>'. *Note incomplete type:
Symbols, for more about this.
If you append '@entry' string to a function parameter name you get
its value at the time the function got called. If the value is not
available an error message is printed. Entry values are available only
with some compilers. Entry values are normally also printed at the
function parameter list according to *note set print entry-values::.
Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29
29 i++;
(gdb) next
30 e (i);
(gdb) print i
$1 = 31
(gdb) print i@entry
$2 = 30
Strings are identified as arrays of 'char' values without specified
signedness. Arrays of either 'signed char' or 'unsigned char' get
printed as arrays of 1 byte sized integers. '-fsigned-char' or
'-funsigned-char' GCC options have no effect as GDB defines literal
string type '"char"' as 'char' without a sign. For program code
char var0[] = "A";
signed char var1[] = "A";
You get during debugging
(gdb) print var0
$1 = "A"
(gdb) print var1
$2 = {65 'A', 0 '\0'}

File: gdb.info, Node: Arrays, Next: Output Formats, Prev: Variables, Up: Data
10.4 Artificial Arrays
======================
It is often useful to print out several successive objects of the same
type in memory; a section of an array, or an array of dynamically
determined size for which only a pointer exists in the program.
You can do this by referring to a contiguous span of memory as an
"artificial array", using the binary operator '@'. The left operand of
'@' should be the first element of the desired array and be an
individual object. The right operand should be the desired length of
the array. The result is an array value whose elements are all of the
type of the left argument. The first element is actually the left
argument; the second element comes from bytes of memory immediately
following those that hold the first element, and so on. Here is an
example. If a program says
int *array = (int *) malloc (len * sizeof (int));
you can print the contents of 'array' with
p *array@len
The left operand of '@' must reside in memory. Array values made
with '@' in this way behave just like other arrays in terms of
subscripting, and are coerced to pointers when used in expressions.
Artificial arrays most often appear in expressions via the value history
(*note Value History: Value History.), after printing one out.
Another way to create an artificial array is to use a cast. This
re-interprets a value as if it were an array. The value need not be in
memory:
(gdb) p/x (short[2])0x12345678
$1 = {0x1234, 0x5678}
As a convenience, if you leave the array length out (as in
'(TYPE[])VALUE') GDB calculates the size to fill the value (as
'sizeof(VALUE)/sizeof(TYPE)':
(gdb) p/x (short[])0x12345678
$2 = {0x1234, 0x5678}
Sometimes the artificial array mechanism is not quite enough; in
moderately complex data structures, the elements of interest may not
actually be adjacent--for example, if you are interested in the values
of pointers in an array. One useful work-around in this situation is to
use a convenience variable (*note Convenience Variables: Convenience
Vars.) as a counter in an expression that prints the first interesting
value, and then repeat that expression via <RET>. For instance, suppose
you have an array 'dtab' of pointers to structures, and you are
interested in the values of a field 'fv' in each structure. Here is an
example of what you might type:
set $i = 0
p dtab[$i++]->fv
<RET>
<RET>
...

File: gdb.info, Node: Output Formats, Next: Memory, Prev: Arrays, Up: Data
10.5 Output Formats
===================
By default, GDB prints a value according to its data type. Sometimes
this is not what you want. For example, you might want to print a
number in hex, or a pointer in decimal. Or you might want to view data
in memory at a certain address as a character string or as an
instruction. To do these things, specify an "output format" when you
print a value.
The simplest use of output formats is to say how to print a value
already computed. This is done by starting the arguments of the 'print'
command with a slash and a format letter. The format letters supported
are:
'x'
Regard the bits of the value as an integer, and print the integer
in hexadecimal.
'd'
Print as integer in signed decimal.
'u'
Print as integer in unsigned decimal.
'o'
Print as integer in octal.
't'
Print as integer in binary. The letter 't' stands for "two". (1)
'a'
Print as an address, both absolute in hexadecimal and as an offset
from the nearest preceding symbol. You can use this format used to
discover where (in what function) an unknown address is located:
(gdb) p/a 0x54320
$3 = 0x54320 <_initialize_vx+396>
The command 'info symbol 0x54320' yields similar results. *Note
info symbol: Symbols.
'c'
Regard as an integer and print it as a character constant. This
prints both the numerical value and its character representation.
The character representation is replaced with the octal escape
'\nnn' for characters outside the 7-bit ASCII range.
Without this format, GDB displays 'char', 'unsigned char', and 'signed char'
data as character constants. Single-byte members of vectors are
displayed as integer data.
'f'
Regard the bits of the value as a floating point number and print
using typical floating point syntax.
's'
Regard as a string, if possible. With this format, pointers to
single-byte data are displayed as null-terminated strings and
arrays of single-byte data are displayed as fixed-length strings.
Other values are displayed in their natural types.
Without this format, GDB displays pointers to and arrays of 'char',
'unsigned char', and 'signed char' as strings. Single-byte members
of a vector are displayed as an integer array.
'z'
Like 'x' formatting, the value is treated as an integer and printed
as hexadecimal, but leading zeros are printed to pad the value to
the size of the integer type.
'r'
Print using the 'raw' formatting. By default, GDB will use a
Python-based pretty-printer, if one is available (*note Pretty
Printing::). This typically results in a higher-level display of
the value's contents. The 'r' format bypasses any Python
pretty-printer which might exist.
For example, to print the program counter in hex (*note Registers::),
type
p/x $pc
Note that no space is required before the slash; this is because command
names in GDB cannot contain a slash.
To reprint the last value in the value history with a different
format, you can use the 'print' command with just a format and no
expression. For example, 'p/x' reprints the last value in hex.
---------- Footnotes ----------
(1) 'b' cannot be used because these format letters are also used
with the 'x' command, where 'b' stands for "byte"; see *note Examining
Memory: Memory.

File: gdb.info, Node: Memory, Next: Auto Display, Prev: Output Formats, Up: Data
10.6 Examining Memory
=====================
You can use the command 'x' (for "examine") to examine memory in any of
several formats, independently of your program's data types.
'x/NFU ADDR'
'x ADDR'
'x'
Use the 'x' command to examine memory.
N, F, and U are all optional parameters that specify how much memory
to display and how to format it; ADDR is an expression giving the
address where you want to start displaying memory. If you use defaults
for NFU, you need not type the slash '/'. Several commands set
convenient defaults for ADDR.
N, the repeat count
The repeat count is a decimal integer; the default is 1. It
specifies how much memory (counting by units U) to display.
F, the display format
The display format is one of the formats used by 'print' ('x', 'd',
'u', 'o', 't', 'a', 'c', 'f', 's'), and in addition 'i' (for
machine instructions). The default is 'x' (hexadecimal) initially.
The default changes each time you use either 'x' or 'print'.
U, the unit size
The unit size is any of
'b'
Bytes.
'h'
Halfwords (two bytes).
'w'
Words (four bytes). This is the initial default.
'g'
Giant words (eight bytes).
Each time you specify a unit size with 'x', that size becomes the
default unit the next time you use 'x'. For the 'i' format, the
unit size is ignored and is normally not written. For the 's'
format, the unit size defaults to 'b', unless it is explicitly
given. Use 'x /hs' to display 16-bit char strings and 'x /ws' to
display 32-bit strings. The next use of 'x /s' will again display
8-bit strings. Note that the results depend on the programming
language of the current compilation unit. If the language is C,
the 's' modifier will use the UTF-16 encoding while 'w' will use
UTF-32. The encoding is set by the programming language and cannot
be altered.
ADDR, starting display address
ADDR is the address where you want GDB to begin displaying memory.
The expression need not have a pointer value (though it may); it is
always interpreted as an integer address of a byte of memory.
*Note Expressions: Expressions, for more information on
expressions. The default for ADDR is usually just after the last
address examined--but several other commands also set the default
address: 'info breakpoints' (to the address of the last breakpoint
listed), 'info line' (to the starting address of a line), and
'print' (if you use it to display a value from memory).
For example, 'x/3uh 0x54320' is a request to display three halfwords
('h') of memory, formatted as unsigned decimal integers ('u'), starting
at address '0x54320'. 'x/4xw $sp' prints the four words ('w') of memory
above the stack pointer (here, '$sp'; *note Registers: Registers.) in
hexadecimal ('x').
Since the letters indicating unit sizes are all distinct from the
letters specifying output formats, you do not have to remember whether
unit size or format comes first; either order works. The output
specifications '4xw' and '4wx' mean exactly the same thing. (However,
the count N must come first; 'wx4' does not work.)
Even though the unit size U is ignored for the formats 's' and 'i',
you might still want to use a count N; for example, '3i' specifies that
you want to see three machine instructions, including any operands. For
convenience, especially when used with the 'display' command, the 'i'
format also prints branch delay slot instructions, if any, beyond the
count specified, which immediately follow the last instruction that is
within the count. The command 'disassemble' gives an alternative way of
inspecting machine instructions; see *note Source and Machine Code:
Machine Code.
All the defaults for the arguments to 'x' are designed to make it
easy to continue scanning memory with minimal specifications each time
you use 'x'. For example, after you have inspected three machine
instructions with 'x/3i ADDR', you can inspect the next seven with just
'x/7'. If you use <RET> to repeat the 'x' command, the repeat count N
is used again; the other arguments default as for successive uses of
'x'.
When examining machine instructions, the instruction at current
program counter is shown with a '=>' marker. For example:
(gdb) x/5i $pc-6
0x804837f <main+11>: mov %esp,%ebp
0x8048381 <main+13>: push %ecx
0x8048382 <main+14>: sub $0x4,%esp
=> 0x8048385 <main+17>: movl $0x8048460,(%esp)
0x804838c <main+24>: call 0x80482d4 <puts@plt>
The addresses and contents printed by the 'x' command are not saved
in the value history because there is often too much of them and they
would get in the way. Instead, GDB makes these values available for
subsequent use in expressions as values of the convenience variables
'$_' and '$__'. After an 'x' command, the last address examined is
available for use in expressions in the convenience variable '$_'. The
contents of that address, as examined, are available in the convenience
variable '$__'.
If the 'x' command has a repeat count, the address and contents saved
are from the last memory unit printed; this is not the same as the last
address printed if several units were printed on the last line of
output.
Most targets have an addressable memory unit size of 8 bits. This
means that to each memory address are associated 8 bits of data. Some
targets, however, have other addressable memory unit sizes. Within GDB
and this document, the term "addressable memory unit" (or "memory unit"
for short) is used when explicitly referring to a chunk of data of that
size. The word "byte" is used to refer to a chunk of data of 8 bits,
regardless of the addressable memory unit size of the target. For most
systems, addressable memory unit is a synonym of byte.
When you are debugging a program running on a remote target machine
(*note Remote Debugging::), you may wish to verify the program's image
in the remote machine's memory against the executable file you
downloaded to the target. Or, on any target, you may want to check
whether the program has corrupted its own read-only sections. The
'compare-sections' command is provided for such situations.
'compare-sections [SECTION-NAME|-r]'
Compare the data of a loadable section SECTION-NAME in the
executable file of the program being debugged with the same section
in the target machine's memory, and report any mismatches. With no
arguments, compares all loadable sections. With an argument of
'-r', compares all loadable read-only sections.
Note: for remote targets, this command can be accelerated if the
target supports computing the CRC checksum of a block of memory
(*note qCRC packet::).

File: gdb.info, Node: Auto Display, Next: Print Settings, Prev: Memory, Up: Data
10.7 Automatic Display
======================
If you find that you want to print the value of an expression frequently
(to see how it changes), you might want to add it to the "automatic
display list" so that GDB prints its value each time your program stops.
Each expression added to the list is given a number to identify it; to
remove an expression from the list, you specify that number. The
automatic display looks like this:
2: foo = 38
3: bar[5] = (struct hack *) 0x3804
This display shows item numbers, expressions and their current values.
As with displays you request manually using 'x' or 'print', you can
specify the output format you prefer; in fact, 'display' decides whether
to use 'print' or 'x' depending your format specification--it uses 'x'
if you specify either the 'i' or 's' format, or a unit size; otherwise
it uses 'print'.
'display EXPR'
Add the expression EXPR to the list of expressions to display each
time your program stops. *Note Expressions: Expressions.
'display' does not repeat if you press <RET> again after using it.
'display/FMT EXPR'
For FMT specifying only a display format and not a size or count,
add the expression EXPR to the auto-display list but arrange to
display it each time in the specified format FMT. *Note Output
Formats: Output Formats.
'display/FMT ADDR'
For FMT 'i' or 's', or including a unit-size or a number of units,
add the expression ADDR as a memory address to be examined each
time your program stops. Examining means in effect doing 'x/FMT
ADDR'. *Note Examining Memory: Memory.
For example, 'display/i $pc' can be helpful, to see the machine
instruction about to be executed each time execution stops ('$pc' is a
common name for the program counter; *note Registers: Registers.).
'undisplay DNUMS...'
'delete display DNUMS...'
Remove items from the list of expressions to display. Specify the
numbers of the displays that you want affected with the command
argument DNUMS. It can be a single display number, one of the
numbers shown in the first field of the 'info display' display; or
it could be a range of display numbers, as in '2-4'.
'undisplay' does not repeat if you press <RET> after using it.
(Otherwise you would just get the error 'No display number ...'.)
'disable display DNUMS...'
Disable the display of item numbers DNUMS. A disabled display item
is not printed automatically, but is not forgotten. It may be
enabled again later. Specify the numbers of the displays that you
want affected with the command argument DNUMS. It can be a single
display number, one of the numbers shown in the first field of the
'info display' display; or it could be a range of display numbers,
as in '2-4'.
'enable display DNUMS...'
Enable display of item numbers DNUMS. It becomes effective once
again in auto display of its expression, until you specify
otherwise. Specify the numbers of the displays that you want
affected with the command argument DNUMS. It can be a single
display number, one of the numbers shown in the first field of the
'info display' display; or it could be a range of display numbers,
as in '2-4'.
'display'
Display the current values of the expressions on the list, just as
is done when your program stops.
'info display'
Print the list of expressions previously set up to display
automatically, each one with its item number, but without showing
the values. This includes disabled expressions, which are marked
as such. It also includes expressions which would not be displayed
right now because they refer to automatic variables not currently
available.
If a display expression refers to local variables, then it does not
make sense outside the lexical context for which it was set up. Such an
expression is disabled when execution enters a context where one of its
variables is not defined. For example, if you give the command 'display
last_char' while inside a function with an argument 'last_char', GDB
displays this argument while your program continues to stop inside that
function. When it stops elsewhere--where there is no variable
'last_char'--the display is disabled automatically. The next time your
program stops where 'last_char' is meaningful, you can enable the
display expression once again.

File: gdb.info, Node: Print Settings, Next: Pretty Printing, Prev: Auto Display, Up: Data
10.8 Print Settings
===================
GDB provides the following ways to control how arrays, structures, and
symbols are printed.
These settings are useful for debugging programs in any language:
'set print address'
'set print address on'
GDB prints memory addresses showing the location of stack traces,
structure values, pointer values, breakpoints, and so forth, even
when it also displays the contents of those addresses. The default
is 'on'. For example, this is what a stack frame display looks
like with 'set print address on':
(gdb) f
#0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>")
at input.c:530
530 if (lquote != def_lquote)
'set print address off'
Do not print addresses when displaying their contents. For
example, this is the same stack frame displayed with 'set print
address off':
(gdb) set print addr off
(gdb) f
#0 set_quotes (lq="<<", rq=">>") at input.c:530
530 if (lquote != def_lquote)
You can use 'set print address off' to eliminate all machine
dependent displays from the GDB interface. For example, with
'print address off', you should get the same text for backtraces on
all machines--whether or not they involve pointer arguments.
'show print address'
Show whether or not addresses are to be printed.
When GDB prints a symbolic address, it normally prints the closest
earlier symbol plus an offset. If that symbol does not uniquely
identify the address (for example, it is a name whose scope is a single
source file), you may need to clarify. One way to do this is with 'info
line', for example 'info line *0x4537'. Alternately, you can set GDB to
print the source file and line number when it prints a symbolic address:
'set print symbol-filename on'
Tell GDB to print the source file name and line number of a symbol
in the symbolic form of an address.
'set print symbol-filename off'
Do not print source file name and line number of a symbol. This is
the default.
'show print symbol-filename'
Show whether or not GDB will print the source file name and line
number of a symbol in the symbolic form of an address.
Another situation where it is helpful to show symbol filenames and
line numbers is when disassembling code; GDB shows you the line number
and source file that corresponds to each instruction.
Also, you may wish to see the symbolic form only if the address being
printed is reasonably close to the closest earlier symbol:
'set print max-symbolic-offset MAX-OFFSET'
'set print max-symbolic-offset unlimited'
Tell GDB to only display the symbolic form of an address if the
offset between the closest earlier symbol and the address is less
than MAX-OFFSET. The default is 'unlimited', which tells GDB to
always print the symbolic form of an address if any symbol precedes
it. Zero is equivalent to 'unlimited'.
'show print max-symbolic-offset'
Ask how large the maximum offset is that GDB prints in a symbolic
address.
If you have a pointer and you are not sure where it points, try 'set
print symbol-filename on'. Then you can determine the name and source
file location of the variable where it points, using 'p/a POINTER'.
This interprets the address in symbolic form. For example, here GDB
shows that a variable 'ptt' points at another variable 't', defined in
'hi2.c':
(gdb) set print symbol-filename on
(gdb) p/a ptt
$4 = 0xe008 <t in hi2.c>
_Warning:_ For pointers that point to a local variable, 'p/a' does
not show the symbol name and filename of the referent, even with
the appropriate 'set print' options turned on.
You can also enable '/a'-like formatting all the time using 'set
print symbol on':
'set print symbol on'
Tell GDB to print the symbol corresponding to an address, if one
exists.
'set print symbol off'
Tell GDB not to print the symbol corresponding to an address. In
this mode, GDB will still print the symbol corresponding to
pointers to functions. This is the default.
'show print symbol'
Show whether GDB will display the symbol corresponding to an
address.
Other settings control how different kinds of objects are printed:
'set print array'
'set print array on'
Pretty print arrays. This format is more convenient to read, but
uses more space. The default is off.
'set print array off'
Return to compressed format for arrays.
'show print array'
Show whether compressed or pretty format is selected for displaying
arrays.
'set print array-indexes'
'set print array-indexes on'
Print the index of each element when displaying arrays. May be
more convenient to locate a given element in the array or quickly
find the index of a given element in that printed array. The
default is off.
'set print array-indexes off'
Stop printing element indexes when displaying arrays.
'show print array-indexes'
Show whether the index of each element is printed when displaying
arrays.
'set print elements NUMBER-OF-ELEMENTS'
'set print elements unlimited'
Set a limit on how many elements of an array GDB will print. If
GDB is printing a large array, it stops printing after it has
printed the number of elements set by the 'set print elements'
command. This limit also applies to the display of strings. When
GDB starts, this limit is set to 200. Setting NUMBER-OF-ELEMENTS
to 'unlimited' or zero means that the number of elements to print
is unlimited.
'show print elements'
Display the number of elements of a large array that GDB will
print. If the number is 0, then the printing is unlimited.
'set print frame-arguments VALUE'
This command allows to control how the values of arguments are
printed when the debugger prints a frame (*note Frames::). The
possible values are:
'all'
The values of all arguments are printed.
'scalars'
Print the value of an argument only if it is a scalar. The
value of more complex arguments such as arrays, structures,
unions, etc, is replaced by '...'. This is the default. Here
is an example where only scalar arguments are shown:
#1 0x08048361 in call_me (i=3, s=..., ss=0xbf8d508c, u=..., e=green)
at frame-args.c:23
'none'
None of the argument values are printed. Instead, the value
of each argument is replaced by '...'. In this case, the
example above now becomes:
#1 0x08048361 in call_me (i=..., s=..., ss=..., u=..., e=...)
at frame-args.c:23
By default, only scalar arguments are printed. This command can be
used to configure the debugger to print the value of all arguments,
regardless of their type. However, it is often advantageous to not
print the value of more complex parameters. For instance, it
reduces the amount of information printed in each frame, making the
backtrace more readable. Also, it improves performance when
displaying Ada frames, because the computation of large arguments
can sometimes be CPU-intensive, especially in large applications.
Setting 'print frame-arguments' to 'scalars' (the default) or
'none' avoids this computation, thus speeding up the display of
each Ada frame.
'show print frame-arguments'
Show how the value of arguments should be displayed when printing a
frame.
'set print raw frame-arguments on'
Print frame arguments in raw, non pretty-printed, form.
'set print raw frame-arguments off'
Print frame arguments in pretty-printed form, if there is a
pretty-printer for the value (*note Pretty Printing::), otherwise
print the value in raw form. This is the default.
'show print raw frame-arguments'
Show whether to print frame arguments in raw form.
'set print entry-values VALUE'
Set printing of frame argument values at function entry. In some
cases GDB can determine the value of function argument which was
passed by the function caller, even if the value was modified
inside the called function and therefore is different. With
optimized code, the current value could be unavailable, but the
entry value may still be known.
The default value is 'default' (see below for its description).
Older GDB behaved as with the setting 'no'. Compilers not
supporting this feature will behave in the 'default' setting the
same way as with the 'no' setting.
This functionality is currently supported only by DWARF 2 debugging
format and the compiler has to produce 'DW_TAG_GNU_call_site' tags.
With GCC, you need to specify '-O -g' during compilation, to get
this information.
The VALUE parameter can be one of the following:
'no'
Print only actual parameter values, never print values from
function entry point.
#0 equal (val=5)
#0 different (val=6)
#0 lost (val=<optimized out>)
#0 born (val=10)
#0 invalid (val=<optimized out>)
'only'
Print only parameter values from function entry point. The
actual parameter values are never printed.
#0 equal (val@entry=5)
#0 different (val@entry=5)
#0 lost (val@entry=5)
#0 born (val@entry=<optimized out>)
#0 invalid (val@entry=<optimized out>)
'preferred'
Print only parameter values from function entry point. If
value from function entry point is not known while the actual
value is known, print the actual value for such parameter.
#0 equal (val@entry=5)
#0 different (val@entry=5)
#0 lost (val@entry=5)
#0 born (val=10)
#0 invalid (val@entry=<optimized out>)
'if-needed'
Print actual parameter values. If actual parameter value is
not known while value from function entry point is known,
print the entry point value for such parameter.
#0 equal (val=5)
#0 different (val=6)
#0 lost (val@entry=5)
#0 born (val=10)
#0 invalid (val=<optimized out>)
'both'
Always print both the actual parameter value and its value
from function entry point, even if values of one or both are
not available due to compiler optimizations.
#0 equal (val=5, val@entry=5)
#0 different (val=6, val@entry=5)
#0 lost (val=<optimized out>, val@entry=5)
#0 born (val=10, val@entry=<optimized out>)
#0 invalid (val=<optimized out>, val@entry=<optimized out>)
'compact'
Print the actual parameter value if it is known and also its
value from function entry point if it is known. If neither is
known, print for the actual value '<optimized out>'. If not
in MI mode (*note GDB/MI::) and if both values are known and
identical, print the shortened 'param=param@entry=VALUE'
notation.
#0 equal (val=val@entry=5)
#0 different (val=6, val@entry=5)
#0 lost (val@entry=5)
#0 born (val=10)
#0 invalid (val=<optimized out>)
'default'
Always print the actual parameter value. Print also its value
from function entry point, but only if it is known. If not in
MI mode (*note GDB/MI::) and if both values are known and
identical, print the shortened 'param=param@entry=VALUE'
notation.
#0 equal (val=val@entry=5)
#0 different (val=6, val@entry=5)
#0 lost (val=<optimized out>, val@entry=5)
#0 born (val=10)
#0 invalid (val=<optimized out>)
For analysis messages on possible failures of frame argument values
at function entry resolution see *note set debug entry-values::.
'show print entry-values'
Show the method being used for printing of frame argument values at
function entry.
'set print repeats NUMBER-OF-REPEATS'
'set print repeats unlimited'
Set the threshold for suppressing display of repeated array
elements. When the number of consecutive identical elements of an
array exceeds the threshold, GDB prints the string '"<repeats N
times>"', where N is the number of identical repetitions, instead
of displaying the identical elements themselves. Setting the
threshold to 'unlimited' or zero will cause all elements to be
individually printed. The default threshold is 10.
'show print repeats'
Display the current threshold for printing repeated identical
elements.
'set print null-stop'
Cause GDB to stop printing the characters of an array when the
first NULL is encountered. This is useful when large arrays
actually contain only short strings. The default is off.
'show print null-stop'
Show whether GDB stops printing an array on the first NULL
character.
'set print pretty on'
Cause GDB to print structures in an indented format with one member
per line, like this:
$1 = {
next = 0x0,
flags = {
sweet = 1,
sour = 1
},
meat = 0x54 "Pork"
}
'set print pretty off'
Cause GDB to print structures in a compact format, like this:
$1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \
meat = 0x54 "Pork"}
This is the default format.
'show print pretty'
Show which format GDB is using to print structures.
'set print sevenbit-strings on'
Print using only seven-bit characters; if this option is set, GDB
displays any eight-bit characters (in strings or character values)
using the notation '\'NNN. This setting is best if you are working
in English (ASCII) and you use the high-order bit of characters as
a marker or "meta" bit.
'set print sevenbit-strings off'
Print full eight-bit characters. This allows the use of more
international character sets, and is the default.
'show print sevenbit-strings'
Show whether or not GDB is printing only seven-bit characters.
'set print union on'
Tell GDB to print unions which are contained in structures and
other unions. This is the default setting.
'set print union off'
Tell GDB not to print unions which are contained in structures and
other unions. GDB will print '"{...}"' instead.
'show print union'
Ask GDB whether or not it will print unions which are contained in
structures and other unions.
For example, given the declarations
typedef enum {Tree, Bug} Species;
typedef enum {Big_tree, Acorn, Seedling} Tree_forms;
typedef enum {Caterpillar, Cocoon, Butterfly}
Bug_forms;
struct thing {
Species it;
union {
Tree_forms tree;
Bug_forms bug;
} form;
};
struct thing foo = {Tree, {Acorn}};
with 'set print union on' in effect 'p foo' would print
$1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}}
and with 'set print union off' in effect it would print
$1 = {it = Tree, form = {...}}
'set print union' affects programs written in C-like languages and
in Pascal.
These settings are of interest when debugging C++ programs:
'set print demangle'
'set print demangle on'
Print C++ names in their source form rather than in the encoded
("mangled") form passed to the assembler and linker for type-safe
linkage. The default is on.
'show print demangle'
Show whether C++ names are printed in mangled or demangled form.
'set print asm-demangle'
'set print asm-demangle on'
Print C++ names in their source form rather than their mangled
form, even in assembler code printouts such as instruction
disassemblies. The default is off.
'show print asm-demangle'
Show whether C++ names in assembly listings are printed in mangled
or demangled form.
'set demangle-style STYLE'
Choose among several encoding schemes used by different compilers
to represent C++ names. The choices for STYLE are currently:
'auto'
Allow GDB to choose a decoding style by inspecting your
program. This is the default.
'gnu'
Decode based on the GNU C++ compiler ('g++') encoding
algorithm.
'hp'
Decode based on the HP ANSI C++ ('aCC') encoding algorithm.
'lucid'
Decode based on the Lucid C++ compiler ('lcc') encoding
algorithm.
'arm'
Decode using the algorithm in the 'C++ Annotated Reference
Manual'. *Warning:* this setting alone is not sufficient to
allow debugging 'cfront'-generated executables. GDB would
require further enhancement to permit that.
If you omit STYLE, you will see a list of possible formats.
'show demangle-style'
Display the encoding style currently in use for decoding C++
symbols.
'set print object'
'set print object on'
When displaying a pointer to an object, identify the _actual_
(derived) type of the object rather than the _declared_ type, using
the virtual function table. Note that the virtual function table
is required--this feature can only work for objects that have
run-time type identification; a single virtual method in the
object's declared type is sufficient. Note that this setting is
also taken into account when working with variable objects via MI
(*note GDB/MI::).
'set print object off'
Display only the declared type of objects, without reference to the
virtual function table. This is the default setting.
'show print object'
Show whether actual, or declared, object types are displayed.
'set print static-members'
'set print static-members on'
Print static members when displaying a C++ object. The default is
on.
'set print static-members off'
Do not print static members when displaying a C++ object.
'show print static-members'
Show whether C++ static members are printed or not.
'set print pascal_static-members'
'set print pascal_static-members on'
Print static members when displaying a Pascal object. The default
is on.
'set print pascal_static-members off'
Do not print static members when displaying a Pascal object.
'show print pascal_static-members'
Show whether Pascal static members are printed or not.
'set print vtbl'
'set print vtbl on'
Pretty print C++ virtual function tables. The default is off.
(The 'vtbl' commands do not work on programs compiled with the HP
ANSI C++ compiler ('aCC').)
'set print vtbl off'
Do not pretty print C++ virtual function tables.
'show print vtbl'
Show whether C++ virtual function tables are pretty printed, or
not.

File: gdb.info, Node: Pretty Printing, Next: Value History, Prev: Print Settings, Up: Data
10.9 Pretty Printing
====================
GDB provides a mechanism to allow pretty-printing of values using Python
code. It greatly simplifies the display of complex objects. This
mechanism works for both MI and the CLI.
* Menu:
* Pretty-Printer Introduction:: Introduction to pretty-printers
* Pretty-Printer Example:: An example pretty-printer
* Pretty-Printer Commands:: Pretty-printer commands

File: gdb.info, Node: Pretty-Printer Introduction, Next: Pretty-Printer Example, Up: Pretty Printing
10.9.1 Pretty-Printer Introduction
----------------------------------
When GDB prints a value, it first sees if there is a pretty-printer
registered for the value. If there is then GDB invokes the
pretty-printer to print the value. Otherwise the value is printed
normally.
Pretty-printers are normally named. This makes them easy to manage.
The 'info pretty-printer' command will list all the installed
pretty-printers with their names. If a pretty-printer can handle
multiple data types, then its "subprinters" are the printers for the
individual data types. Each such subprinter has its own name. The
format of the name is PRINTER-NAME;SUBPRINTER-NAME.
Pretty-printers are installed by "registering" them with GDB.
Typically they are automatically loaded and registered when the
corresponding debug information is loaded, thus making them available
without having to do anything special.
There are three places where a pretty-printer can be registered.
* Pretty-printers registered globally are available when debugging
all inferiors.
* Pretty-printers registered with a program space are available only
when debugging that program. *Note Progspaces In Python::, for
more details on program spaces in Python.
* Pretty-printers registered with an objfile are loaded and unloaded
with the corresponding objfile (e.g., shared library). *Note
Objfiles In Python::, for more details on objfiles in Python.
*Note Selecting Pretty-Printers::, for further information on how
pretty-printers are selected,
*Note Writing a Pretty-Printer::, for implementing pretty printers
for new types.

File: gdb.info, Node: Pretty-Printer Example, Next: Pretty-Printer Commands, Prev: Pretty-Printer Introduction, Up: Pretty Printing
10.9.2 Pretty-Printer Example
-----------------------------
Here is how a C++ 'std::string' looks without a pretty-printer:
(gdb) print s
$1 = {
static npos = 4294967295,
_M_dataplus = {
<std::allocator<char>> = {
<__gnu_cxx::new_allocator<char>> = {
<No data fields>}, <No data fields>
},
members of std::basic_string<char, std::char_traits<char>,
std::allocator<char> >::_Alloc_hider:
_M_p = 0x804a014 "abcd"
}
}
With a pretty-printer for 'std::string' only the contents are
printed:
(gdb) print s
$2 = "abcd"

File: gdb.info, Node: Pretty-Printer Commands, Prev: Pretty-Printer Example, Up: Pretty Printing
10.9.3 Pretty-Printer Commands
------------------------------
'info pretty-printer [OBJECT-REGEXP [NAME-REGEXP]]'
Print the list of installed pretty-printers. This includes
disabled pretty-printers, which are marked as such.
OBJECT-REGEXP is a regular expression matching the objects whose
pretty-printers to list. Objects can be 'global', the program
space's file (*note Progspaces In Python::), and the object files
within that program space (*note Objfiles In Python::). *Note
Selecting Pretty-Printers::, for details on how GDB looks up a
printer from these three objects.
NAME-REGEXP is a regular expression matching the name of the
printers to list.
'disable pretty-printer [OBJECT-REGEXP [NAME-REGEXP]]'
Disable pretty-printers matching OBJECT-REGEXP and NAME-REGEXP. A
disabled pretty-printer is not forgotten, it may be enabled again
later.
'enable pretty-printer [OBJECT-REGEXP [NAME-REGEXP]]'
Enable pretty-printers matching OBJECT-REGEXP and NAME-REGEXP.
Example:
Suppose we have three pretty-printers installed: one from library1.so
named 'foo' that prints objects of type 'foo', and another from
library2.so named 'bar' that prints two types of objects, 'bar1' and
'bar2'.
(gdb) info pretty-printer
library1.so:
foo
library2.so:
bar
bar1
bar2
(gdb) info pretty-printer library2
library2.so:
bar
bar1
bar2
(gdb) disable pretty-printer library1
1 printer disabled
2 of 3 printers enabled
(gdb) info pretty-printer
library1.so:
foo [disabled]
library2.so:
bar
bar1
bar2
(gdb) disable pretty-printer library2 bar:bar1
1 printer disabled
1 of 3 printers enabled
(gdb) info pretty-printer library2
library1.so:
foo [disabled]
library2.so:
bar
bar1 [disabled]
bar2
(gdb) disable pretty-printer library2 bar
1 printer disabled
0 of 3 printers enabled
(gdb) info pretty-printer library2
library1.so:
foo [disabled]
library2.so:
bar [disabled]
bar1 [disabled]
bar2
Note that for 'bar' the entire printer can be disabled, as can each
individual subprinter.

File: gdb.info, Node: Value History, Next: Convenience Vars, Prev: Pretty Printing, Up: Data
10.10 Value History
===================
Values printed by the 'print' command are saved in the GDB "value
history". This allows you to refer to them in other expressions.
Values are kept until the symbol table is re-read or discarded (for
example with the 'file' or 'symbol-file' commands). When the symbol
table changes, the value history is discarded, since the values may
contain pointers back to the types defined in the symbol table.
The values printed are given "history numbers" by which you can refer
to them. These are successive integers starting with one. 'print'
shows you the history number assigned to a value by printing '$NUM = '
before the value; here NUM is the history number.
To refer to any previous value, use '$' followed by the value's
history number. The way 'print' labels its output is designed to remind
you of this. Just '$' refers to the most recent value in the history,
and '$$' refers to the value before that. '$$N' refers to the Nth value
from the end; '$$2' is the value just prior to '$$', '$$1' is equivalent
to '$$', and '$$0' is equivalent to '$'.
For example, suppose you have just printed a pointer to a structure
and want to see the contents of the structure. It suffices to type
p *$
If you have a chain of structures where the component 'next' points
to the next one, you can print the contents of the next one with this:
p *$.next
You can print successive links in the chain by repeating this
command--which you can do by just typing <RET>.
Note that the history records values, not expressions. If the value
of 'x' is 4 and you type these commands:
print x
set x=5
then the value recorded in the value history by the 'print' command
remains 4 even though the value of 'x' has changed.
'show values'
Print the last ten values in the value history, with their item
numbers. This is like 'p $$9' repeated ten times, except that
'show values' does not change the history.
'show values N'
Print ten history values centered on history item number N.
'show values +'
Print ten history values just after the values last printed. If no
more values are available, 'show values +' produces no display.
Pressing <RET> to repeat 'show values N' has exactly the same effect
as 'show values +'.

File: gdb.info, Node: Convenience Vars, Next: Convenience Funs, Prev: Value History, Up: Data
10.11 Convenience Variables
===========================
GDB provides "convenience variables" that you can use within GDB to hold
on to a value and refer to it later. These variables exist entirely
within GDB; they are not part of your program, and setting a convenience
variable has no direct effect on further execution of your program.
That is why you can use them freely.
Convenience variables are prefixed with '$'. Any name preceded by
'$' can be used for a convenience variable, unless it is one of the
predefined machine-specific register names (*note Registers:
Registers.). (Value history references, in contrast, are _numbers_
preceded by '$'. *Note Value History: Value History.)
You can save a value in a convenience variable with an assignment
expression, just as you would set a variable in your program. For
example:
set $foo = *object_ptr
would save in '$foo' the value contained in the object pointed to by
'object_ptr'.
Using a convenience variable for the first time creates it, but its
value is 'void' until you assign a new value. You can alter the value
with another assignment at any time.
Convenience variables have no fixed types. You can assign a
convenience variable any type of value, including structures and arrays,
even if that variable already has a value of a different type. The
convenience variable, when used as an expression, has the type of its
current value.
'show convenience'
Print a list of convenience variables used so far, and their
values, as well as a list of the convenience functions.
Abbreviated 'show conv'.
'init-if-undefined $VARIABLE = EXPRESSION'
Set a convenience variable if it has not already been set. This is
useful for user-defined commands that keep some state. It is
similar, in concept, to using local static variables with
initializers in C (except that convenience variables are global).
It can also be used to allow users to override default values used
in a command script.
If the variable is already defined then the expression is not
evaluated so any side-effects do not occur.
One of the ways to use a convenience variable is as a counter to be
incremented or a pointer to be advanced. For example, to print a field
from successive elements of an array of structures:
set $i = 0
print bar[$i++]->contents
Repeat that command by typing <RET>.
Some convenience variables are created automatically by GDB and given
values likely to be useful.
'$_'
The variable '$_' is automatically set by the 'x' command to the
last address examined (*note Examining Memory: Memory.). Other
commands which provide a default address for 'x' to examine also
set '$_' to that address; these commands include 'info line' and
'info breakpoint'. The type of '$_' is 'void *' except when set by
the 'x' command, in which case it is a pointer to the type of
'$__'.
'$__'
The variable '$__' is automatically set by the 'x' command to the
value found in the last address examined. Its type is chosen to
match the format in which the data was printed.
'$_exitcode'
When the program being debugged terminates normally, GDB
automatically sets this variable to the exit code of the program,
and resets '$_exitsignal' to 'void'.
'$_exitsignal'
When the program being debugged dies due to an uncaught signal, GDB
automatically sets this variable to that signal's number, and
resets '$_exitcode' to 'void'.
To distinguish between whether the program being debugged has
exited (i.e., '$_exitcode' is not 'void') or signalled (i.e.,
'$_exitsignal' is not 'void'), the convenience function '$_isvoid'
can be used (*note Convenience Functions: Convenience Funs.). For
example, considering the following source code:
#include <signal.h>
int
main (int argc, char *argv[])
{
raise (SIGALRM);
return 0;
}
A valid way of telling whether the program being debugged has
exited or signalled would be:
(gdb) define has_exited_or_signalled
Type commands for definition of ``has_exited_or_signalled''.
End with a line saying just ``end''.
>if $_isvoid ($_exitsignal)
>echo The program has exited\n
>else
>echo The program has signalled\n
>end
>end
(gdb) run
Starting program:
Program terminated with signal SIGALRM, Alarm clock.
The program no longer exists.
(gdb) has_exited_or_signalled
The program has signalled
As can be seen, GDB correctly informs that the program being
debugged has signalled, since it calls 'raise' and raises a
'SIGALRM' signal. If the program being debugged had not called
'raise', then GDB would report a normal exit:
(gdb) has_exited_or_signalled
The program has exited
'$_exception'
The variable '$_exception' is set to the exception object being
thrown at an exception-related catchpoint. *Note Set
Catchpoints::.
'$_probe_argc'
'$_probe_arg0...$_probe_arg11'
Arguments to a static probe. *Note Static Probe Points::.
'$_sdata'
The variable '$_sdata' contains extra collected static tracepoint
data. *Note Tracepoint Action Lists: Tracepoint Actions. Note
that '$_sdata' could be empty, if not inspecting a trace buffer, or
if extra static tracepoint data has not been collected.
'$_siginfo'
The variable '$_siginfo' contains extra signal information (*note
extra signal information::). Note that '$_siginfo' could be empty,
if the application has not yet received any signals. For example,
it will be empty before you execute the 'run' command.
'$_tlb'
The variable '$_tlb' is automatically set when debugging
applications running on MS-Windows in native mode or connected to
gdbserver that supports the 'qGetTIBAddr' request. *Note General
Query Packets::. This variable contains the address of the thread
information block.
On HP-UX systems, if you refer to a function or variable name that
begins with a dollar sign, GDB searches for a user or system name first,
before it searches for a convenience variable.

File: gdb.info, Node: Convenience Funs, Next: Registers, Prev: Convenience Vars, Up: Data
10.12 Convenience Functions
===========================
GDB also supplies some "convenience functions". These have a syntax
similar to convenience variables. A convenience function can be used in
an expression just like an ordinary function; however, a convenience
function is implemented internally to GDB.
These functions do not require GDB to be configured with 'Python'
support, which means that they are always available.
'$_isvoid (EXPR)'
Return one if the expression EXPR is 'void'. Otherwise it returns
zero.
A 'void' expression is an expression where the type of the result
is 'void'. For example, you can examine a convenience variable
(see *note Convenience Variables: Convenience Vars.) to check
whether it is 'void':
(gdb) print $_exitcode
$1 = void
(gdb) print $_isvoid ($_exitcode)
$2 = 1
(gdb) run
Starting program: ./a.out
[Inferior 1 (process 29572) exited normally]
(gdb) print $_exitcode
$3 = 0
(gdb) print $_isvoid ($_exitcode)
$4 = 0
In the example above, we used '$_isvoid' to check whether
'$_exitcode' is 'void' before and after the execution of the
program being debugged. Before the execution there is no exit code
to be examined, therefore '$_exitcode' is 'void'. After the
execution the program being debugged returned zero, therefore
'$_exitcode' is zero, which means that it is not 'void' anymore.
The 'void' expression can also be a call of a function from the
program being debugged. For example, given the following function:
void
foo (void)
{
}
The result of calling it inside GDB is 'void':
(gdb) print foo ()
$1 = void
(gdb) print $_isvoid (foo ())
$2 = 1
(gdb) set $v = foo ()
(gdb) print $v
$3 = void
(gdb) print $_isvoid ($v)
$4 = 1
These functions require GDB to be configured with 'Python' support.
'$_memeq(BUF1, BUF2, LENGTH)'
Returns one if the LENGTH bytes at the addresses given by BUF1 and
BUF2 are equal. Otherwise it returns zero.
'$_regex(STR, REGEX)'
Returns one if the string STR matches the regular expression REGEX.
Otherwise it returns zero. The syntax of the regular expression is
that specified by 'Python''s regular expression support.
'$_streq(STR1, STR2)'
Returns one if the strings STR1 and STR2 are equal. Otherwise it
returns zero.
'$_strlen(STR)'
Returns the length of string STR.
'$_caller_is(NAME[, NUMBER_OF_FRAMES])'
Returns one if the calling function's name is equal to NAME.
Otherwise it returns zero.
If the optional argument NUMBER_OF_FRAMES is provided, it is the
number of frames up in the stack to look. The default is 1.
Example:
(gdb) backtrace
#0 bottom_func ()
at testsuite/gdb.python/py-caller-is.c:21
#1 0x00000000004005a0 in middle_func ()
at testsuite/gdb.python/py-caller-is.c:27
#2 0x00000000004005ab in top_func ()
at testsuite/gdb.python/py-caller-is.c:33
#3 0x00000000004005b6 in main ()
at testsuite/gdb.python/py-caller-is.c:39
(gdb) print $_caller_is ("middle_func")
$1 = 1
(gdb) print $_caller_is ("top_func", 2)
$1 = 1
'$_caller_matches(REGEXP[, NUMBER_OF_FRAMES])'
Returns one if the calling function's name matches the regular
expression REGEXP. Otherwise it returns zero.
If the optional argument NUMBER_OF_FRAMES is provided, it is the
number of frames up in the stack to look. The default is 1.
'$_any_caller_is(NAME[, NUMBER_OF_FRAMES])'
Returns one if any calling function's name is equal to NAME.
Otherwise it returns zero.
If the optional argument NUMBER_OF_FRAMES is provided, it is the
number of frames up in the stack to look. The default is 1.
This function differs from '$_caller_is' in that this function
checks all stack frames from the immediate caller to the frame
specified by NUMBER_OF_FRAMES, whereas '$_caller_is' only checks
the frame specified by NUMBER_OF_FRAMES.
'$_any_caller_matches(REGEXP[, NUMBER_OF_FRAMES])'
Returns one if any calling function's name matches the regular
expression REGEXP. Otherwise it returns zero.
If the optional argument NUMBER_OF_FRAMES is provided, it is the
number of frames up in the stack to look. The default is 1.
This function differs from '$_caller_matches' in that this function
checks all stack frames from the immediate caller to the frame
specified by NUMBER_OF_FRAMES, whereas '$_caller_matches' only
checks the frame specified by NUMBER_OF_FRAMES.
GDB provides the ability to list and get help on convenience
functions.
'help function'
Print a list of all convenience functions.

File: gdb.info, Node: Registers, Next: Floating Point Hardware, Prev: Convenience Funs, Up: Data
10.13 Registers
===============
You can refer to machine register contents, in expressions, as variables
with names starting with '$'. The names of registers are different for
each machine; use 'info registers' to see the names used on your
machine.
'info registers'
Print the names and values of all registers except floating-point
and vector registers (in the selected stack frame).
'info all-registers'
Print the names and values of all registers, including
floating-point and vector registers (in the selected stack frame).
'info registers REGNAME ...'
Print the "relativized" value of each specified register REGNAME.
As discussed in detail below, register values are normally relative
to the selected stack frame. The REGNAME may be any register name
valid on the machine you are using, with or without the initial
'$'.
GDB has four "standard" register names that are available (in
expressions) on most machines--whenever they do not conflict with an
architecture's canonical mnemonics for registers. The register names
'$pc' and '$sp' are used for the program counter register and the stack
pointer. '$fp' is used for a register that contains a pointer to the
current stack frame, and '$ps' is used for a register that contains the
processor status. For example, you could print the program counter in
hex with
p/x $pc
or print the instruction to be executed next with
x/i $pc
or add four to the stack pointer(1) with
set $sp += 4
Whenever possible, these four standard register names are available
on your machine even though the machine has different canonical
mnemonics, so long as there is no conflict. The 'info registers'
command shows the canonical names. For example, on the SPARC, 'info
registers' displays the processor status register as '$psr' but you can
also refer to it as '$ps'; and on x86-based machines '$ps' is an alias
for the EFLAGS register.
GDB always considers the contents of an ordinary register as an
integer when the register is examined in this way. Some machines have
special registers which can hold nothing but floating point; these
registers are considered to have floating point values. There is no way
to refer to the contents of an ordinary register as floating point value
(although you can _print_ it as a floating point value with 'print/f
$REGNAME').
Some registers have distinct "raw" and "virtual" data formats. This
means that the data format in which the register contents are saved by
the operating system is not the same one that your program normally
sees. For example, the registers of the 68881 floating point
coprocessor are always saved in "extended" (raw) format, but all C
programs expect to work with "double" (virtual) format. In such cases,
GDB normally works with the virtual format only (the format that makes
sense for your program), but the 'info registers' command prints the
data in both formats.
Some machines have special registers whose contents can be
interpreted in several different ways. For example, modern x86-based
machines have SSE and MMX registers that can hold several values packed
together in several different formats. GDB refers to such registers in
'struct' notation:
(gdb) print $xmm1
$1 = {
v4_float = {0, 3.43859137e-038, 1.54142831e-044, 1.821688e-044},
v2_double = {9.92129282474342e-303, 2.7585945287983262e-313},
v16_int8 = "\000\000\000\000\3706;\001\v\000\000\000\r\000\000",
v8_int16 = {0, 0, 14072, 315, 11, 0, 13, 0},
v4_int32 = {0, 20657912, 11, 13},
v2_int64 = {88725056443645952, 55834574859},
uint128 = 0x0000000d0000000b013b36f800000000
}
To set values of such registers, you need to tell GDB which view of the
register you wish to change, as if you were assigning value to a
'struct' member:
(gdb) set $xmm1.uint128 = 0x000000000000000000000000FFFFFFFF
Normally, register values are relative to the selected stack frame
(*note Selecting a Frame: Selection.). This means that you get the
value that the register would contain if all stack frames farther in
were exited and their saved registers restored. In order to see the
true contents of hardware registers, you must select the innermost frame
(with 'frame 0').
Usually ABIs reserve some registers as not needed to be saved by the
callee (a.k.a.: "caller-saved", "call-clobbered" or "volatile"
registers). It may therefore not be possible for GDB to know the value
a register had before the call (in other words, in the outer frame), if
the register value has since been changed by the callee. GDB tries to
deduce where the inner frame saved ("callee-saved") registers, from the
debug info, unwind info, or the machine code generated by your compiler.
If some register is not saved, and GDB knows the register is
"caller-saved" (via its own knowledge of the ABI, or because the
debug/unwind info explicitly says the register's value is undefined),
GDB displays '<not saved>' as the register's value. With targets that
GDB has no knowledge of the register saving convention, if a register
was not saved by the callee, then its value and location in the outer
frame are assumed to be the same of the inner frame. This is usually
harmless, because if the register is call-clobbered, the caller either
does not care what is in the register after the call, or has code to
restore the value that it does care about. Note, however, that if you
change such a register in the outer frame, you may also be affecting the
inner frame. Also, the more "outer" the frame is you're looking at, the
more likely a call-clobbered register's value is to be wrong, in the
sense that it doesn't actually represent the value the register had just
before the call.
---------- Footnotes ----------
(1) This is a way of removing one word from the stack, on machines
where stacks grow downward in memory (most machines, nowadays). This
assumes that the innermost stack frame is selected; setting '$sp' is not
allowed when other stack frames are selected. To pop entire frames off
the stack, regardless of machine architecture, use 'return'; see *note
Returning from a Function: Returning.

File: gdb.info, Node: Floating Point Hardware, Next: Vector Unit, Prev: Registers, Up: Data
10.14 Floating Point Hardware
=============================
Depending on the configuration, GDB may be able to give you more
information about the status of the floating point hardware.
'info float'
Display hardware-dependent information about the floating point
unit. The exact contents and layout vary depending on the floating
point chip. Currently, 'info float' is supported on the ARM and
x86 machines.

File: gdb.info, Node: Vector Unit, Next: OS Information, Prev: Floating Point Hardware, Up: Data
10.15 Vector Unit
=================
Depending on the configuration, GDB may be able to give you more
information about the status of the vector unit.
'info vector'
Display information about the vector unit. The exact contents and
layout vary depending on the hardware.

File: gdb.info, Node: OS Information, Next: Memory Region Attributes, Prev: Vector Unit, Up: Data
10.16 Operating System Auxiliary Information
============================================
GDB provides interfaces to useful OS facilities that can help you debug
your program.
Some operating systems supply an "auxiliary vector" to programs at
startup. This is akin to the arguments and environment that you specify
for a program, but contains a system-dependent variety of binary values
that tell system libraries important details about the hardware,
operating system, and process. Each value's purpose is identified by an
integer tag; the meanings are well-known but system-specific. Depending
on the configuration and operating system facilities, GDB may be able to
show you this information. For remote targets, this functionality may
further depend on the remote stub's support of the 'qXfer:auxv:read'
packet, see *note qXfer auxiliary vector read::.
'info auxv'
Display the auxiliary vector of the inferior, which can be either a
live process or a core dump file. GDB prints each tag value
numerically, and also shows names and text descriptions for
recognized tags. Some values in the vector are numbers, some bit
masks, and some pointers to strings or other data. GDB displays
each value in the most appropriate form for a recognized tag, and
in hexadecimal for an unrecognized tag.
On some targets, GDB can access operating system-specific information
and show it to you. The types of information available will differ
depending on the type of operating system running on the target. The
mechanism used to fetch the data is described in *note Operating System
Information::. For remote targets, this functionality depends on the
remote stub's support of the 'qXfer:osdata:read' packet, see *note qXfer
osdata read::.
'info os INFOTYPE'
Display OS information of the requested type.
On GNU/Linux, the following values of INFOTYPE are valid:
'cpus'
Display the list of all CPUs/cores. For each CPU/core, GDB
prints the available fields from /proc/cpuinfo. For each
supported architecture different fields are available. Two
common entries are processor which gives CPU number and
bogomips; a system constant that is calculated during kernel
initialization.
'files'
Display the list of open file descriptors on the target. For
each file descriptor, GDB prints the identifier of the process
owning the descriptor, the command of the owning process, the
value of the descriptor, and the target of the descriptor.
'modules'
Display the list of all loaded kernel modules on the target.
For each module, GDB prints the module name, the size of the
module in bytes, the number of times the module is used, the
dependencies of the module, the status of the module, and the
address of the loaded module in memory.
'msg'
Display the list of all System V message queues on the target.
For each message queue, GDB prints the message queue key, the
message queue identifier, the access permissions, the current
number of bytes on the queue, the current number of messages
on the queue, the processes that last sent and received a
message on the queue, the user and group of the owner and
creator of the message queue, the times at which a message was
last sent and received on the queue, and the time at which the
message queue was last changed.
'processes'
Display the list of processes on the target. For each
process, GDB prints the process identifier, the name of the
user, the command corresponding to the process, and the list
of processor cores that the process is currently running on.
(To understand what these properties mean, for this and the
following info types, please consult the general GNU/Linux
documentation.)
'procgroups'
Display the list of process groups on the target. For each
process, GDB prints the identifier of the process group that
it belongs to, the command corresponding to the process group
leader, the process identifier, and the command line of the
process. The list is sorted first by the process group
identifier, then by the process identifier, so that processes
belonging to the same process group are grouped together and
the process group leader is listed first.
'semaphores'
Display the list of all System V semaphore sets on the target.
For each semaphore set, GDB prints the semaphore set key, the
semaphore set identifier, the access permissions, the number
of semaphores in the set, the user and group of the owner and
creator of the semaphore set, and the times at which the
semaphore set was operated upon and changed.
'shm'
Display the list of all System V shared-memory regions on the
target. For each shared-memory region, GDB prints the region
key, the shared-memory identifier, the access permissions, the
size of the region, the process that created the region, the
process that last attached to or detached from the region, the
current number of live attaches to the region, and the times
at which the region was last attached to, detach from, and
changed.
'sockets'
Display the list of Internet-domain sockets on the target.
For each socket, GDB prints the address and port of the local
and remote endpoints, the current state of the connection, the
creator of the socket, the IP address family of the socket,
and the type of the connection.
'threads'
Display the list of threads running on the target. For each
thread, GDB prints the identifier of the process that the
thread belongs to, the command of the process, the thread
identifier, and the processor core that it is currently
running on. The main thread of a process is not listed.
'info os'
If INFOTYPE is omitted, then list the possible values for INFOTYPE
and the kind of OS information available for each INFOTYPE. If the
target does not return a list of possible types, this command will
report an error.

File: gdb.info, Node: Memory Region Attributes, Next: Dump/Restore Files, Prev: OS Information, Up: Data
10.17 Memory Region Attributes
==============================
"Memory region attributes" allow you to describe special handling
required by regions of your target's memory. GDB uses attributes to
determine whether to allow certain types of memory accesses; whether to
use specific width accesses; and whether to cache target memory. By
default the description of memory regions is fetched from the target (if
the current target supports this), but the user can override the fetched
regions.
Defined memory regions can be individually enabled and disabled.
When a memory region is disabled, GDB uses the default attributes when
accessing memory in that region. Similarly, if no memory regions have
been defined, GDB uses the default attributes when accessing all memory.
When a memory region is defined, it is given a number to identify it;
to enable, disable, or remove a memory region, you specify that number.
'mem LOWER UPPER ATTRIBUTES...'
Define a memory region bounded by LOWER and UPPER with attributes
ATTRIBUTES..., and add it to the list of regions monitored by GDB.
Note that UPPER == 0 is a special case: it is treated as the
target's maximum memory address. (0xffff on 16 bit targets,
0xffffffff on 32 bit targets, etc.)
'mem auto'
Discard any user changes to the memory regions and use
target-supplied regions, if available, or no regions if the target
does not support.
'delete mem NUMS...'
Remove memory regions NUMS... from the list of regions monitored by
GDB.
'disable mem NUMS...'
Disable monitoring of memory regions NUMS.... A disabled memory
region is not forgotten. It may be enabled again later.
'enable mem NUMS...'
Enable monitoring of memory regions NUMS....
'info mem'
Print a table of all defined memory regions, with the following
columns for each region:
_Memory Region Number_
_Enabled or Disabled._
Enabled memory regions are marked with 'y'. Disabled memory
regions are marked with 'n'.
_Lo Address_
The address defining the inclusive lower bound of the memory
region.
_Hi Address_
The address defining the exclusive upper bound of the memory
region.
_Attributes_
The list of attributes set for this memory region.
10.17.1 Attributes
------------------
10.17.1.1 Memory Access Mode
............................
The access mode attributes set whether GDB may make read or write
accesses to a memory region.
While these attributes prevent GDB from performing invalid memory
accesses, they do nothing to prevent the target system, I/O DMA, etc.
from accessing memory.
'ro'
Memory is read only.
'wo'
Memory is write only.
'rw'
Memory is read/write. This is the default.
10.17.1.2 Memory Access Size
............................
The access size attribute tells GDB to use specific sized accesses in
the memory region. Often memory mapped device registers require
specific sized accesses. If no access size attribute is specified, GDB
may use accesses of any size.
'8'
Use 8 bit memory accesses.
'16'
Use 16 bit memory accesses.
'32'
Use 32 bit memory accesses.
'64'
Use 64 bit memory accesses.
10.17.1.3 Data Cache
....................
The data cache attributes set whether GDB will cache target memory.
While this generally improves performance by reducing debug protocol
overhead, it can lead to incorrect results because GDB does not know
about volatile variables or memory mapped device registers.
'cache'
Enable GDB to cache target memory.
'nocache'
Disable GDB from caching target memory. This is the default.
10.17.2 Memory Access Checking
------------------------------
GDB can be instructed to refuse accesses to memory that is not
explicitly described. This can be useful if accessing such regions has
undesired effects for a specific target, or to provide better error
checking. The following commands control this behaviour.
'set mem inaccessible-by-default [on|off]'
If 'on' is specified, make GDB treat memory not explicitly
described by the memory ranges as non-existent and refuse accesses
to such memory. The checks are only performed if there's at least
one memory range defined. If 'off' is specified, make GDB treat
the memory not explicitly described by the memory ranges as RAM.
The default value is 'on'.
'show mem inaccessible-by-default'
Show the current handling of accesses to unknown memory.

File: gdb.info, Node: Dump/Restore Files, Next: Core File Generation, Prev: Memory Region Attributes, Up: Data
10.18 Copy Between Memory and a File
====================================
You can use the commands 'dump', 'append', and 'restore' to copy data
between target memory and a file. The 'dump' and 'append' commands
write data to a file, and the 'restore' command reads data from a file
back into the inferior's memory. Files may be in binary, Motorola
S-record, Intel hex, Tektronix Hex, or Verilog Hex format; however, GDB
can only append to binary files, and cannot read from Verilog Hex files.
'dump [FORMAT] memory FILENAME START_ADDR END_ADDR'
'dump [FORMAT] value FILENAME EXPR'
Dump the contents of memory from START_ADDR to END_ADDR, or the
value of EXPR, to FILENAME in the given format.
The FORMAT parameter may be any one of:
'binary'
Raw binary form.
'ihex'
Intel hex format.
'srec'
Motorola S-record format.
'tekhex'
Tektronix Hex format.
'verilog'
Verilog Hex format.
GDB uses the same definitions of these formats as the GNU binary
utilities, like 'objdump' and 'objcopy'. If FORMAT is omitted, GDB
dumps the data in raw binary form.
'append [binary] memory FILENAME START_ADDR END_ADDR'
'append [binary] value FILENAME EXPR'
Append the contents of memory from START_ADDR to END_ADDR, or the
value of EXPR, to the file FILENAME, in raw binary form. (GDB can
only append data to files in raw binary form.)
'restore FILENAME [binary] BIAS START END'
Restore the contents of file FILENAME into memory. The 'restore'
command can automatically recognize any known BFD file format,
except for raw binary. To restore a raw binary file you must
specify the optional keyword 'binary' after the filename.
If BIAS is non-zero, its value will be added to the addresses
contained in the file. Binary files always start at address zero,
so they will be restored at address BIAS. Other bfd files have a
built-in location; they will be restored at offset BIAS from that
location.
If START and/or END are non-zero, then only data between file
offset START and file offset END will be restored. These offsets
are relative to the addresses in the file, before the BIAS argument
is applied.

File: gdb.info, Node: Core File Generation, Next: Character Sets, Prev: Dump/Restore Files, Up: Data
10.19 How to Produce a Core File from Your Program
==================================================
A "core file" or "core dump" is a file that records the memory image of
a running process and its process status (register values etc.). Its
primary use is post-mortem debugging of a program that crashed while it
ran outside a debugger. A program that crashes automatically produces a
core file, unless this feature is disabled by the user. *Note Files::,
for information on invoking GDB in the post-mortem debugging mode.
Occasionally, you may wish to produce a core file of the program you
are debugging in order to preserve a snapshot of its state. GDB has a
special command for that.
'generate-core-file [FILE]'
'gcore [FILE]'
Produce a core dump of the inferior process. The optional argument
FILE specifies the file name where to put the core dump. If not
specified, the file name defaults to 'core.PID', where PID is the
inferior process ID.
Note that this command is implemented only for some systems (as of
this writing, GNU/Linux, FreeBSD, Solaris, and S390).
On GNU/Linux, this command can take into account the value of the
file '/proc/PID/coredump_filter' when generating the core dump
(*note set use-coredump-filter::).
'set use-coredump-filter on'
'set use-coredump-filter off'
Enable or disable the use of the file '/proc/PID/coredump_filter'
when generating core dump files. This file is used by the Linux
kernel to decide what types of memory mappings will be dumped or
ignored when generating a core dump file. PID is the process ID of
a currently running process.
To make use of this feature, you have to write in the
'/proc/PID/coredump_filter' file a value, in hexadecimal, which is
a bit mask representing the memory mapping types. If a bit is set
in the bit mask, then the memory mappings of the corresponding
types will be dumped; otherwise, they will be ignored. This
configuration is inherited by child processes. For more
information about the bits that can be set in the
'/proc/PID/coredump_filter' file, please refer to the manpage of
'core(5)'.
By default, this option is 'on'. If this option is turned 'off',
GDB does not read the 'coredump_filter' file and instead uses the
same default value as the Linux kernel in order to decide which
pages will be dumped in the core dump file. This value is
currently '0x33', which means that bits '0' (anonymous private
mappings), '1' (anonymous shared mappings), '4' (ELF headers) and
'5' (private huge pages) are active. This will cause these memory
mappings to be dumped automatically.

File: gdb.info, Node: Character Sets, Next: Caching Target Data, Prev: Core File Generation, Up: Data
10.20 Character Sets
====================
If the program you are debugging uses a different character set to
represent characters and strings than the one GDB uses itself, GDB can
automatically translate between the character sets for you. The
character set GDB uses we call the "host character set"; the one the
inferior program uses we call the "target character set".
For example, if you are running GDB on a GNU/Linux system, which uses
the ISO Latin 1 character set, but you are using GDB's remote protocol
(*note Remote Debugging::) to debug a program running on an IBM
mainframe, which uses the EBCDIC character set, then the host character
set is Latin-1, and the target character set is EBCDIC. If you give GDB
the command 'set target-charset EBCDIC-US', then GDB translates between
EBCDIC and Latin 1 as you print character or string values, or use
character and string literals in expressions.
GDB has no way to automatically recognize which character set the
inferior program uses; you must tell it, using the 'set target-charset'
command, described below.
Here are the commands for controlling GDB's character set support:
'set target-charset CHARSET'
Set the current target character set to CHARSET. To display the
list of supported target character sets, type
'set target-charset <TAB><TAB>'.
'set host-charset CHARSET'
Set the current host character set to CHARSET.
By default, GDB uses a host character set appropriate to the system
it is running on; you can override that default using the 'set
host-charset' command. On some systems, GDB cannot automatically
determine the appropriate host character set. In this case, GDB
uses 'UTF-8'.
GDB can only use certain character sets as its host character set.
If you type 'set host-charset <TAB><TAB>', GDB will list the host
character sets it supports.
'set charset CHARSET'
Set the current host and target character sets to CHARSET. As
above, if you type 'set charset <TAB><TAB>', GDB will list the
names of the character sets that can be used for both host and
target.
'show charset'
Show the names of the current host and target character sets.
'show host-charset'
Show the name of the current host character set.
'show target-charset'
Show the name of the current target character set.
'set target-wide-charset CHARSET'
Set the current target's wide character set to CHARSET. This is
the character set used by the target's 'wchar_t' type. To display
the list of supported wide character sets, type
'set target-wide-charset <TAB><TAB>'.
'show target-wide-charset'
Show the name of the current target's wide character set.
Here is an example of GDB's character set support in action. Assume
that the following source code has been placed in the file
'charset-test.c':
#include <stdio.h>
char ascii_hello[]
= {72, 101, 108, 108, 111, 44, 32, 119,
111, 114, 108, 100, 33, 10, 0};
char ibm1047_hello[]
= {200, 133, 147, 147, 150, 107, 64, 166,
150, 153, 147, 132, 90, 37, 0};
main ()
{
printf ("Hello, world!\n");
}
In this program, 'ascii_hello' and 'ibm1047_hello' are arrays
containing the string 'Hello, world!' followed by a newline, encoded in
the ASCII and IBM1047 character sets.
We compile the program, and invoke the debugger on it:
$ gcc -g charset-test.c -o charset-test
$ gdb -nw charset-test
GNU gdb 2001-12-19-cvs
Copyright 2001 Free Software Foundation, Inc.
...
(gdb)
We can use the 'show charset' command to see what character sets GDB
is currently using to interpret and display characters and strings:
(gdb) show charset
The current host and target character set is `ISO-8859-1'.
(gdb)
For the sake of printing this manual, let's use ASCII as our initial
character set:
(gdb) set charset ASCII
(gdb) show charset
The current host and target character set is `ASCII'.
(gdb)
Let's assume that ASCII is indeed the correct character set for our
host system -- in other words, let's assume that if GDB prints
characters using the ASCII character set, our terminal will display them
properly. Since our current target character set is also ASCII, the
contents of 'ascii_hello' print legibly:
(gdb) print ascii_hello
$1 = 0x401698 "Hello, world!\n"
(gdb) print ascii_hello[0]
$2 = 72 'H'
(gdb)
GDB uses the target character set for character and string literals
you use in expressions:
(gdb) print '+'
$3 = 43 '+'
(gdb)
The ASCII character set uses the number 43 to encode the '+'
character.
GDB relies on the user to tell it which character set the target
program uses. If we print 'ibm1047_hello' while our target character
set is still ASCII, we get jibberish:
(gdb) print ibm1047_hello
$4 = 0x4016a8 "\310\205\223\223\226k@\246\226\231\223\204Z%"
(gdb) print ibm1047_hello[0]
$5 = 200 '\310'
(gdb)
If we invoke the 'set target-charset' followed by <TAB><TAB>, GDB
tells us the character sets it supports:
(gdb) set target-charset
ASCII EBCDIC-US IBM1047 ISO-8859-1
(gdb) set target-charset
We can select IBM1047 as our target character set, and examine the
program's strings again. Now the ASCII string is wrong, but GDB
translates the contents of 'ibm1047_hello' from the target character
set, IBM1047, to the host character set, ASCII, and they display
correctly:
(gdb) set target-charset IBM1047
(gdb) show charset
The current host character set is `ASCII'.
The current target character set is `IBM1047'.
(gdb) print ascii_hello
$6 = 0x401698 "\110\145%%?\054\040\167?\162%\144\041\012"
(gdb) print ascii_hello[0]
$7 = 72 '\110'
(gdb) print ibm1047_hello
$8 = 0x4016a8 "Hello, world!\n"
(gdb) print ibm1047_hello[0]
$9 = 200 'H'
(gdb)
As above, GDB uses the target character set for character and string
literals you use in expressions:
(gdb) print '+'
$10 = 78 '+'
(gdb)
The IBM1047 character set uses the number 78 to encode the '+'
character.

File: gdb.info, Node: Caching Target Data, Next: Searching Memory, Prev: Character Sets, Up: Data
10.21 Caching Data of Targets
=============================
GDB caches data exchanged between the debugger and a target. Each cache
is associated with the address space of the inferior. *Note Inferiors
and Programs::, about inferior and address space. Such caching
generally improves performance in remote debugging (*note Remote
Debugging::), because it reduces the overhead of the remote protocol by
bundling memory reads and writes into large chunks. Unfortunately,
simply caching everything would lead to incorrect results, since GDB
does not necessarily know anything about volatile values, memory-mapped
I/O addresses, etc. Furthermore, in non-stop mode (*note Non-Stop
Mode::) memory can be changed _while_ a gdb command is executing.
Therefore, by default, GDB only caches data known to be on the stack(1)
or in the code segment. Other regions of memory can be explicitly
marked as cacheable; *note Memory Region Attributes::.
'set remotecache on'
'set remotecache off'
This option no longer does anything; it exists for compatibility
with old scripts.
'show remotecache'
Show the current state of the obsolete remotecache flag.
'set stack-cache on'
'set stack-cache off'
Enable or disable caching of stack accesses. When 'on', use
caching. By default, this option is 'on'.
'show stack-cache'
Show the current state of data caching for memory accesses.
'set code-cache on'
'set code-cache off'
Enable or disable caching of code segment accesses. When 'on', use
caching. By default, this option is 'on'. This improves
performance of disassembly in remote debugging.
'show code-cache'
Show the current state of target memory cache for code segment
accesses.
'info dcache [line]'
Print the information about the performance of data cache of the
current inferior's address space. The information displayed
includes the dcache width and depth, and for each cache line, its
number, address, and how many times it was referenced. This
command is useful for debugging the data cache operation.
If a line number is specified, the contents of that line will be
printed in hex.
'set dcache size SIZE'
Set maximum number of entries in dcache (dcache depth above).
'set dcache line-size LINE-SIZE'
Set number of bytes each dcache entry caches (dcache width above).
Must be a power of 2.
'show dcache size'
Show maximum number of dcache entries. *Note info dcache: Caching
Target Data.
'show dcache line-size'
Show default size of dcache lines.
---------- Footnotes ----------
(1) In non-stop mode, it is moderately rare for a running thread to
modify the stack of a stopped thread in a way that would interfere with
a backtrace, and caching of stack reads provides a significant speed up
of remote backtraces.

File: gdb.info, Node: Searching Memory, Prev: Caching Target Data, Up: Data
10.22 Search Memory
===================
Memory can be searched for a particular sequence of bytes with the
'find' command.
'find [/SN] START_ADDR, +LEN, VAL1 [, VAL2, ...]'
'find [/SN] START_ADDR, END_ADDR, VAL1 [, VAL2, ...]'
Search memory for the sequence of bytes specified by VAL1, VAL2,
etc. The search begins at address START_ADDR and continues for
either LEN bytes or through to END_ADDR inclusive.
S and N are optional parameters. They may be specified in either
order, apart or together.
S, search query size
The size of each search query value.
'b'
bytes
'h'
halfwords (two bytes)
'w'
words (four bytes)
'g'
giant words (eight bytes)
All values are interpreted in the current language. This means,
for example, that if the current source language is C/C++ then
searching for the string "hello" includes the trailing '\0'.
If the value size is not specified, it is taken from the value's
type in the current language. This is useful when one wants to
specify the search pattern as a mixture of types. Note that this
means, for example, that in the case of C-like languages a search
for an untyped 0x42 will search for '(int) 0x42' which is typically
four bytes.
N, maximum number of finds
The maximum number of matches to print. The default is to print
all finds.
You can use strings as search values. Quote them with double-quotes
('"'). The string value is copied into the search pattern byte by byte,
regardless of the endianness of the target and the size specification.
The address of each match found is printed as well as a count of the
number of matches found.
The address of the last value found is stored in convenience variable
'$_'. A count of the number of matches is stored in '$numfound'.
For example, if stopped at the 'printf' in this function:
void
hello ()
{
static char hello[] = "hello-hello";
static struct { char c; short s; int i; }
__attribute__ ((packed)) mixed
= { 'c', 0x1234, 0x87654321 };
printf ("%s\n", hello);
}
you get during debugging:
(gdb) find &hello[0], +sizeof(hello), "hello"
0x804956d <hello.1620+6>
1 pattern found
(gdb) find &hello[0], +sizeof(hello), 'h', 'e', 'l', 'l', 'o'
0x8049567 <hello.1620>
0x804956d <hello.1620+6>
2 patterns found
(gdb) find /b1 &hello[0], +sizeof(hello), 'h', 0x65, 'l'
0x8049567 <hello.1620>
1 pattern found
(gdb) find &mixed, +sizeof(mixed), (char) 'c', (short) 0x1234, (int) 0x87654321
0x8049560 <mixed.1625>
1 pattern found
(gdb) print $numfound
$1 = 1
(gdb) print $_
$2 = (void *) 0x8049560

File: gdb.info, Node: Optimized Code, Next: Macros, Prev: Data, Up: Top
11 Debugging Optimized Code
***************************
Almost all compilers support optimization. With optimization disabled,
the compiler generates assembly code that corresponds directly to your
source code, in a simplistic way. As the compiler applies more powerful
optimizations, the generated assembly code diverges from your original
source code. With help from debugging information generated by the
compiler, GDB can map from the running program back to constructs from
your original source.
GDB is more accurate with optimization disabled. If you can
recompile without optimization, it is easier to follow the progress of
your program during debugging. But, there are many cases where you may
need to debug an optimized version.
When you debug a program compiled with '-g -O', remember that the
optimizer has rearranged your code; the debugger shows you what is
really there. Do not be too surprised when the execution path does not
exactly match your source file! An extreme example: if you define a
variable, but never use it, GDB never sees that variable--because the
compiler optimizes it out of existence.
Some things do not work as well with '-g -O' as with just '-g',
particularly on machines with instruction scheduling. If in doubt,
recompile with '-g' alone, and if this fixes the problem, please report
it to us as a bug (including a test case!). *Note Variables::, for more
information about debugging optimized code.
* Menu:
* Inline Functions:: How GDB presents inlining
* Tail Call Frames:: GDB analysis of jumps to functions

File: gdb.info, Node: Inline Functions, Next: Tail Call Frames, Up: Optimized Code
11.1 Inline Functions
=====================
"Inlining" is an optimization that inserts a copy of the function body
directly at each call site, instead of jumping to a shared routine. GDB
displays inlined functions just like non-inlined functions. They appear
in backtraces. You can view their arguments and local variables, step
into them with 'step', skip them with 'next', and escape from them with
'finish'. You can check whether a function was inlined by using the
'info frame' command.
For GDB to support inlined functions, the compiler must record
information about inlining in the debug information -- GCC using the
DWARF 2 format does this, and several other compilers do also. GDB only
supports inlined functions when using DWARF 2. Versions of GCC before
4.1 do not emit two required attributes ('DW_AT_call_file' and
'DW_AT_call_line'); GDB does not display inlined function calls with
earlier versions of GCC. It instead displays the arguments and local
variables of inlined functions as local variables in the caller.
The body of an inlined function is directly included at its call
site; unlike a non-inlined function, there are no instructions devoted
to the call. GDB still pretends that the call site and the start of the
inlined function are different instructions. Stepping to the call site
shows the call site, and then stepping again shows the first line of the
inlined function, even though no additional instructions are executed.
This makes source-level debugging much clearer; you can see both the
context of the call and then the effect of the call. Only stepping by a
single instruction using 'stepi' or 'nexti' does not do this; single
instruction steps always show the inlined body.
There are some ways that GDB does not pretend that inlined function
calls are the same as normal calls:
* Setting breakpoints at the call site of an inlined function may not
work, because the call site does not contain any code. GDB may
incorrectly move the breakpoint to the next line of the enclosing
function, after the call. This limitation will be removed in a
future version of GDB; until then, set a breakpoint on an earlier
line or inside the inlined function instead.
* GDB cannot locate the return value of inlined calls after using the
'finish' command. This is a limitation of compiler-generated
debugging information; after 'finish', you can step to the next
line and print a variable where your program stored the return
value.

File: gdb.info, Node: Tail Call Frames, Prev: Inline Functions, Up: Optimized Code
11.2 Tail Call Frames
=====================
Function 'B' can call function 'C' in its very last statement. In
unoptimized compilation the call of 'C' is immediately followed by
return instruction at the end of 'B' code. Optimizing compiler may
replace the call and return in function 'B' into one jump to function
'C' instead. Such use of a jump instruction is called "tail call".
During execution of function 'C', there will be no indication in the
function call stack frames that it was tail-called from 'B'. If
function 'A' regularly calls function 'B' which tail-calls function 'C',
then GDB will see 'A' as the caller of 'C'. However, in some cases GDB
can determine that 'C' was tail-called from 'B', and it will then create
fictitious call frame for that, with the return address set up as if 'B'
called 'C' normally.
This functionality is currently supported only by DWARF 2 debugging
format and the compiler has to produce 'DW_TAG_GNU_call_site' tags.
With GCC, you need to specify '-O -g' during compilation, to get this
information.
'info frame' command (*note Frame Info::) will indicate the tail call
frame kind by text 'tail call frame' such as in this sample GDB output:
(gdb) x/i $pc - 2
0x40066b <b(int, double)+11>: jmp 0x400640 <c(int, double)>
(gdb) info frame
Stack level 1, frame at 0x7fffffffda30:
rip = 0x40066d in b (amd64-entry-value.cc:59); saved rip 0x4004c5
tail call frame, caller of frame at 0x7fffffffda30
source language c++.
Arglist at unknown address.
Locals at unknown address, Previous frame's sp is 0x7fffffffda30
The detection of all the possible code path executions can find them
ambiguous. There is no execution history stored (possible *note Reverse
Execution:: is never used for this purpose) and the last known caller
could have reached the known callee by multiple different jump
sequences. In such case GDB still tries to show at least all the
unambiguous top tail callers and all the unambiguous bottom tail calees,
if any.
'set debug entry-values'
When set to on, enables printing of analysis messages for both
frame argument values at function entry and tail calls. It will
show all the possible valid tail calls code paths it has
considered. It will also print the intersection of them with the
final unambiguous (possibly partial or even empty) code path
result.
'show debug entry-values'
Show the current state of analysis messages printing for both frame
argument values at function entry and tail calls.
The analysis messages for tail calls can for example show why the
virtual tail call frame for function 'c' has not been recognized (due to
the indirect reference by variable 'x'):
static void __attribute__((noinline, noclone)) c (void);
void (*x) (void) = c;
static void __attribute__((noinline, noclone)) a (void) { x++; }
static void __attribute__((noinline, noclone)) c (void) { a (); }
int main (void) { x (); return 0; }
Breakpoint 1, DW_OP_GNU_entry_value resolving cannot find
DW_TAG_GNU_call_site 0x40039a in main
a () at t.c:3
3 static void __attribute__((noinline, noclone)) a (void) { x++; }
(gdb) bt
#0 a () at t.c:3
#1 0x000000000040039a in main () at t.c:5
Another possibility is an ambiguous virtual tail call frames
resolution:
int i;
static void __attribute__((noinline, noclone)) f (void) { i++; }
static void __attribute__((noinline, noclone)) e (void) { f (); }
static void __attribute__((noinline, noclone)) d (void) { f (); }
static void __attribute__((noinline, noclone)) c (void) { d (); }
static void __attribute__((noinline, noclone)) b (void)
{ if (i) c (); else e (); }
static void __attribute__((noinline, noclone)) a (void) { b (); }
int main (void) { a (); return 0; }
tailcall: initial: 0x4004d2(a) 0x4004ce(b) 0x4004b2(c) 0x4004a2(d)
tailcall: compare: 0x4004d2(a) 0x4004cc(b) 0x400492(e)
tailcall: reduced: 0x4004d2(a) |
(gdb) bt
#0 f () at t.c:2
#1 0x00000000004004d2 in a () at t.c:8
#2 0x0000000000400395 in main () at t.c:9
Frames #0 and #2 are real, #1 is a virtual tail call frame. The code
can have possible execution paths 'main->a->b->c->d->f' or
'main->a->b->e->f', GDB cannot find which one from the inferior state.
'initial:' state shows some random possible calling sequence GDB has
found. It then finds another possible calling sequcen - that one is
prefixed by 'compare:'. The non-ambiguous intersection of these two is
printed as the 'reduced:' calling sequence. That one could have many
futher 'compare:' and 'reduced:' statements as long as there remain any
non-ambiguous sequence entries.
For the frame of function 'b' in both cases there are different
possible '$pc' values ('0x4004cc' or '0x4004ce'), therefore this frame
is also ambigous. The only non-ambiguous frame is the one for function
'a', therefore this one is displayed to the user while the ambiguous
frames are omitted.
There can be also reasons why printing of frame argument values at
function entry may fail:
int v;
static void __attribute__((noinline, noclone)) c (int i) { v++; }
static void __attribute__((noinline, noclone)) a (int i);
static void __attribute__((noinline, noclone)) b (int i) { a (i); }
static void __attribute__((noinline, noclone)) a (int i)
{ if (i) b (i - 1); else c (0); }
int main (void) { a (5); return 0; }
(gdb) bt
#0 c (i=i@entry=0) at t.c:2
#1 0x0000000000400428 in a (DW_OP_GNU_entry_value resolving has found
function "a" at 0x400420 can call itself via tail calls
i=<optimized out>) at t.c:6
#2 0x000000000040036e in main () at t.c:7
GDB cannot find out from the inferior state if and how many times did
function 'a' call itself (via function 'b') as these calls would be tail
calls. Such tail calls would modify thue 'i' variable, therefore GDB
cannot be sure the value it knows would be right - GDB prints
'<optimized out>' instead.

File: gdb.info, Node: Macros, Next: Tracepoints, Prev: Optimized Code, Up: Top
12 C Preprocessor Macros
************************
Some languages, such as C and C++, provide a way to define and invoke
"preprocessor macros" which expand into strings of tokens. GDB can
evaluate expressions containing macro invocations, show the result of
macro expansion, and show a macro's definition, including where it was
defined.
You may need to compile your program specially to provide GDB with
information about preprocessor macros. Most compilers do not include
macros in their debugging information, even when you compile with the
'-g' flag. *Note Compilation::.
A program may define a macro at one point, remove that definition
later, and then provide a different definition after that. Thus, at
different points in the program, a macro may have different definitions,
or have no definition at all. If there is a current stack frame, GDB
uses the macros in scope at that frame's source code line. Otherwise,
GDB uses the macros in scope at the current listing location; see *note
List::.
Whenever GDB evaluates an expression, it always expands any macro
invocations present in the expression. GDB also provides the following
commands for working with macros explicitly.
'macro expand EXPRESSION'
'macro exp EXPRESSION'
Show the results of expanding all preprocessor macro invocations in
EXPRESSION. Since GDB simply expands macros, but does not parse
the result, EXPRESSION need not be a valid expression; it can be
any string of tokens.
'macro expand-once EXPRESSION'
'macro exp1 EXPRESSION'
(This command is not yet implemented.) Show the results of
expanding those preprocessor macro invocations that appear
explicitly in EXPRESSION. Macro invocations appearing in that
expansion are left unchanged. This command allows you to see the
effect of a particular macro more clearly, without being confused
by further expansions. Since GDB simply expands macros, but does
not parse the result, EXPRESSION need not be a valid expression; it
can be any string of tokens.
'info macro [-a|-all] [--] MACRO'
Show the current definition or all definitions of the named MACRO,
and describe the source location or compiler command-line where
that definition was established. The optional double dash is to
signify the end of argument processing and the beginning of MACRO
for non C-like macros where the macro may begin with a hyphen.
'info macros LINESPEC'
Show all macro definitions that are in effect at the location
specified by LINESPEC, and describe the source location or compiler
command-line where those definitions were established.
'macro define MACRO REPLACEMENT-LIST'
'macro define MACRO(ARGLIST) REPLACEMENT-LIST'
Introduce a definition for a preprocessor macro named MACRO,
invocations of which are replaced by the tokens given in
REPLACEMENT-LIST. The first form of this command defines an
"object-like" macro, which takes no arguments; the second form
defines a "function-like" macro, which takes the arguments given in
ARGLIST.
A definition introduced by this command is in scope in every
expression evaluated in GDB, until it is removed with the 'macro
undef' command, described below. The definition overrides all
definitions for MACRO present in the program being debugged, as
well as any previous user-supplied definition.
'macro undef MACRO'
Remove any user-supplied definition for the macro named MACRO.
This command only affects definitions provided with the 'macro
define' command, described above; it cannot remove definitions
present in the program being debugged.
'macro list'
List all the macros defined using the 'macro define' command.
Here is a transcript showing the above commands in action. First, we
show our source files:
$ cat sample.c
#include <stdio.h>
#include "sample.h"
#define M 42
#define ADD(x) (M + x)
main ()
{
#define N 28
printf ("Hello, world!\n");
#undef N
printf ("We're so creative.\n");
#define N 1729
printf ("Goodbye, world!\n");
}
$ cat sample.h
#define Q <
$
Now, we compile the program using the GNU C compiler, GCC. We pass
the '-gdwarf-2'(1) _and_ '-g3' flags to ensure the compiler includes
information about preprocessor macros in the debugging information.
$ gcc -gdwarf-2 -g3 sample.c -o sample
$
Now, we start GDB on our sample program:
$ gdb -nw sample
GNU gdb 2002-05-06-cvs
Copyright 2002 Free Software Foundation, Inc.
GDB is free software, ...
(gdb)
We can expand macros and examine their definitions, even when the
program is not running. GDB uses the current listing position to decide
which macro definitions are in scope:
(gdb) list main
3
4 #define M 42
5 #define ADD(x) (M + x)
6
7 main ()
8 {
9 #define N 28
10 printf ("Hello, world!\n");
11 #undef N
12 printf ("We're so creative.\n");
(gdb) info macro ADD
Defined at /home/jimb/gdb/macros/play/sample.c:5
#define ADD(x) (M + x)
(gdb) info macro Q
Defined at /home/jimb/gdb/macros/play/sample.h:1
included at /home/jimb/gdb/macros/play/sample.c:2
#define Q <
(gdb) macro expand ADD(1)
expands to: (42 + 1)
(gdb) macro expand-once ADD(1)
expands to: once (M + 1)
(gdb)
In the example above, note that 'macro expand-once' expands only the
macro invocation explicit in the original text -- the invocation of
'ADD' -- but does not expand the invocation of the macro 'M', which was
introduced by 'ADD'.
Once the program is running, GDB uses the macro definitions in force
at the source line of the current stack frame:
(gdb) break main
Breakpoint 1 at 0x8048370: file sample.c, line 10.
(gdb) run
Starting program: /home/jimb/gdb/macros/play/sample
Breakpoint 1, main () at sample.c:10
10 printf ("Hello, world!\n");
(gdb)
At line 10, the definition of the macro 'N' at line 9 is in force:
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:9
#define N 28
(gdb) macro expand N Q M
expands to: 28 < 42
(gdb) print N Q M
$1 = 1
(gdb)
As we step over directives that remove 'N''s definition, and then
give it a new definition, GDB finds the definition (or lack thereof) in
force at each point:
(gdb) next
Hello, world!
12 printf ("We're so creative.\n");
(gdb) info macro N
The symbol `N' has no definition as a C/C++ preprocessor macro
at /home/jimb/gdb/macros/play/sample.c:12
(gdb) next
We're so creative.
14 printf ("Goodbye, world!\n");
(gdb) info macro N
Defined at /home/jimb/gdb/macros/play/sample.c:13
#define N 1729
(gdb) macro expand N Q M
expands to: 1729 < 42
(gdb) print N Q M
$2 = 0
(gdb)
In addition to source files, macros can be defined on the compilation
command line using the '-DNAME=VALUE' syntax. For macros defined in
such a way, GDB displays the location of their definition as line zero
of the source file submitted to the compiler.
(gdb) info macro __STDC__
Defined at /home/jimb/gdb/macros/play/sample.c:0
-D__STDC__=1
(gdb)
---------- Footnotes ----------
(1) This is the minimum. Recent versions of GCC support '-gdwarf-3'
and '-gdwarf-4'; we recommend always choosing the most recent version of
DWARF.

File: gdb.info, Node: Tracepoints, Next: Overlays, Prev: Macros, Up: Top
13 Tracepoints
**************
In some applications, it is not feasible for the debugger to interrupt
the program's execution long enough for the developer to learn anything
helpful about its behavior. If the program's correctness depends on its
real-time behavior, delays introduced by a debugger might cause the
program to change its behavior drastically, or perhaps fail, even when
the code itself is correct. It is useful to be able to observe the
program's behavior without interrupting it.
Using GDB's 'trace' and 'collect' commands, you can specify locations
in the program, called "tracepoints", and arbitrary expressions to
evaluate when those tracepoints are reached. Later, using the 'tfind'
command, you can examine the values those expressions had when the
program hit the tracepoints. The expressions may also denote objects in
memory--structures or arrays, for example--whose values GDB should
record; while visiting a particular tracepoint, you may inspect those
objects as if they were in memory at that moment. However, because GDB
records these values without interacting with you, it can do so quickly
and unobtrusively, hopefully not disturbing the program's behavior.
The tracepoint facility is currently available only for remote
targets. *Note Targets::. In addition, your remote target must know
how to collect trace data. This functionality is implemented in the
remote stub; however, none of the stubs distributed with GDB support
tracepoints as of this writing. The format of the remote packets used
to implement tracepoints are described in *note Tracepoint Packets::.
It is also possible to get trace data from a file, in a manner
reminiscent of corefiles; you specify the filename, and use 'tfind' to
search through the file. *Note Trace Files::, for more details.
This chapter describes the tracepoint commands and features.
* Menu:
* Set Tracepoints::
* Analyze Collected Data::
* Tracepoint Variables::
* Trace Files::

File: gdb.info, Node: Set Tracepoints, Next: Analyze Collected Data, Up: Tracepoints
13.1 Commands to Set Tracepoints
================================
Before running such a "trace experiment", an arbitrary number of
tracepoints can be set. A tracepoint is actually a special type of
breakpoint (*note Set Breaks::), so you can manipulate it using standard
breakpoint commands. For instance, as with breakpoints, tracepoint
numbers are successive integers starting from one, and many of the
commands associated with tracepoints take the tracepoint number as their
argument, to identify which tracepoint to work on.
For each tracepoint, you can specify, in advance, some arbitrary set
of data that you want the target to collect in the trace buffer when it
hits that tracepoint. The collected data can include registers, local
variables, or global data. Later, you can use GDB commands to examine
the values these data had at the time the tracepoint was hit.
Tracepoints do not support every breakpoint feature. Ignore counts
on tracepoints have no effect, and tracepoints cannot run GDB commands
when they are hit. Tracepoints may not be thread-specific either.
Some targets may support "fast tracepoints", which are inserted in a
different way (such as with a jump instead of a trap), that is faster
but possibly restricted in where they may be installed.
Regular and fast tracepoints are dynamic tracing facilities, meaning
that they can be used to insert tracepoints at (almost) any location in
the target. Some targets may also support controlling "static
tracepoints" from GDB. With static tracing, a set of instrumentation
points, also known as "markers", are embedded in the target program, and
can be activated or deactivated by name or address. These are usually
placed at locations which facilitate investigating what the target is
actually doing. GDB's support for static tracing includes being able to
list instrumentation points, and attach them with GDB defined high level
tracepoints that expose the whole range of convenience of GDB's
tracepoints support. Namely, support for collecting registers values
and values of global or local (to the instrumentation point) variables;
tracepoint conditions and trace state variables. The act of installing
a GDB static tracepoint on an instrumentation point, or marker, is
referred to as "probing" a static tracepoint marker.
'gdbserver' supports tracepoints on some target systems. *Note
Tracepoints support in 'gdbserver': Server.
This section describes commands to set tracepoints and associated
conditions and actions.
* Menu:
* Create and Delete Tracepoints::
* Enable and Disable Tracepoints::
* Tracepoint Passcounts::
* Tracepoint Conditions::
* Trace State Variables::
* Tracepoint Actions::
* Listing Tracepoints::
* Listing Static Tracepoint Markers::
* Starting and Stopping Trace Experiments::
* Tracepoint Restrictions::

File: gdb.info, Node: Create and Delete Tracepoints, Next: Enable and Disable Tracepoints, Up: Set Tracepoints
13.1.1 Create and Delete Tracepoints
------------------------------------
'trace LOCATION'
The 'trace' command is very similar to the 'break' command. Its
argument LOCATION can be a source line, a function name, or an
address in the target program. *Note Specify Location::. The
'trace' command defines a tracepoint, which is a point in the
target program where the debugger will briefly stop, collect some
data, and then allow the program to continue. Setting a tracepoint
or changing its actions takes effect immediately if the remote stub
supports the 'InstallInTrace' feature (*note install tracepoint in
tracing::). If remote stub doesn't support the 'InstallInTrace'
feature, all these changes don't take effect until the next
'tstart' command, and once a trace experiment is running, further
changes will not have any effect until the next trace experiment
starts. In addition, GDB supports "pending
tracepoints"--tracepoints whose address is not yet resolved. (This
is similar to pending breakpoints.) Pending tracepoints are not
downloaded to the target and not installed until they are resolved.
The resolution of pending tracepoints requires GDB support--when
debugging with the remote target, and GDB disconnects from the
remote stub (*note disconnected tracing::), pending tracepoints can
not be resolved (and downloaded to the remote stub) while GDB is
disconnected.
Here are some examples of using the 'trace' command:
(gdb) trace foo.c:121 // a source file and line number
(gdb) trace +2 // 2 lines forward
(gdb) trace my_function // first source line of function
(gdb) trace *my_function // EXACT start address of function
(gdb) trace *0x2117c4 // an address
You can abbreviate 'trace' as 'tr'.
'trace LOCATION if COND'
Set a tracepoint with condition COND; evaluate the expression COND
each time the tracepoint is reached, and collect data only if the
value is nonzero--that is, if COND evaluates as true. *Note
Tracepoint Conditions: Tracepoint Conditions, for more information
on tracepoint conditions.
'ftrace LOCATION [ if COND ]'
The 'ftrace' command sets a fast tracepoint. For targets that
support them, fast tracepoints will use a more efficient but
possibly less general technique to trigger data collection, such as
a jump instruction instead of a trap, or some sort of hardware
support. It may not be possible to create a fast tracepoint at the
desired location, in which case the command will exit with an
explanatory message.
GDB handles arguments to 'ftrace' exactly as for 'trace'.
On 32-bit x86-architecture systems, fast tracepoints normally need
to be placed at an instruction that is 5 bytes or longer, but can
be placed at 4-byte instructions if the low 64K of memory of the
target program is available to install trampolines. Some Unix-type
systems, such as GNU/Linux, exclude low addresses from the
program's address space; but for instance with the Linux kernel it
is possible to let GDB use this area by doing a 'sysctl' command to
set the 'mmap_min_addr' kernel parameter, as in
sudo sysctl -w vm.mmap_min_addr=32768
which sets the low address to 32K, which leaves plenty of room for
trampolines. The minimum address should be set to a page boundary.
'strace LOCATION [ if COND ]'
The 'strace' command sets a static tracepoint. For targets that
support it, setting a static tracepoint probes a static
instrumentation point, or marker, found at LOCATION. It may not be
possible to set a static tracepoint at the desired location, in
which case the command will exit with an explanatory message.
GDB handles arguments to 'strace' exactly as for 'trace', with the
addition that the user can also specify '-m MARKER' as LOCATION.
This probes the marker identified by the MARKER string identifier.
This identifier depends on the static tracepoint backend library
your program is using. You can find all the marker identifiers in
the 'ID' field of the 'info static-tracepoint-markers' command
output. *Note Listing Static Tracepoint Markers: Listing Static
Tracepoint Markers. For example, in the following small program
using the UST tracing engine:
main ()
{
trace_mark(ust, bar33, "str %s", "FOOBAZ");
}
the marker id is composed of joining the first two arguments to the
'trace_mark' call with a slash, which translates to:
(gdb) info static-tracepoint-markers
Cnt Enb ID Address What
1 n ust/bar33 0x0000000000400ddc in main at stexample.c:22
Data: "str %s"
[etc...]
so you may probe the marker above with:
(gdb) strace -m ust/bar33
Static tracepoints accept an extra collect action -- 'collect
$_sdata'. This collects arbitrary user data passed in the probe
point call to the tracing library. In the UST example above,
you'll see that the third argument to 'trace_mark' is a printf-like
format string. The user data is then the result of running that
formating string against the following arguments. Note that 'info
static-tracepoint-markers' command output lists that format string
in the 'Data:' field.
You can inspect this data when analyzing the trace buffer, by
printing the $_sdata variable like any other variable available to
GDB. *Note Tracepoint Action Lists: Tracepoint Actions.
The convenience variable '$tpnum' records the tracepoint number of
the most recently set tracepoint.
'delete tracepoint [NUM]'
Permanently delete one or more tracepoints. With no argument, the
default is to delete all tracepoints. Note that the regular
'delete' command can remove tracepoints also.
Examples:
(gdb) delete trace 1 2 3 // remove three tracepoints
(gdb) delete trace // remove all tracepoints
You can abbreviate this command as 'del tr'.

File: gdb.info, Node: Enable and Disable Tracepoints, Next: Tracepoint Passcounts, Prev: Create and Delete Tracepoints, Up: Set Tracepoints
13.1.2 Enable and Disable Tracepoints
-------------------------------------
These commands are deprecated; they are equivalent to plain 'disable'
and 'enable'.
'disable tracepoint [NUM]'
Disable tracepoint NUM, or all tracepoints if no argument NUM is
given. A disabled tracepoint will have no effect during a trace
experiment, but it is not forgotten. You can re-enable a disabled
tracepoint using the 'enable tracepoint' command. If the command
is issued during a trace experiment and the debug target has
support for disabling tracepoints during a trace experiment, then
the change will be effective immediately. Otherwise, it will be
applied to the next trace experiment.
'enable tracepoint [NUM]'
Enable tracepoint NUM, or all tracepoints. If this command is
issued during a trace experiment and the debug target supports
enabling tracepoints during a trace experiment, then the enabled
tracepoints will become effective immediately. Otherwise, they
will become effective the next time a trace experiment is run.

File: gdb.info, Node: Tracepoint Passcounts, Next: Tracepoint Conditions, Prev: Enable and Disable Tracepoints, Up: Set Tracepoints
13.1.3 Tracepoint Passcounts
----------------------------
'passcount [N [NUM]]'
Set the "passcount" of a tracepoint. The passcount is a way to
automatically stop a trace experiment. If a tracepoint's passcount
is N, then the trace experiment will be automatically stopped on
the N'th time that tracepoint is hit. If the tracepoint number NUM
is not specified, the 'passcount' command sets the passcount of the
most recently defined tracepoint. If no passcount is given, the
trace experiment will run until stopped explicitly by the user.
Examples:
(gdb) passcount 5 2 // Stop on the 5th execution of
// tracepoint 2
(gdb) passcount 12 // Stop on the 12th execution of the
// most recently defined tracepoint.
(gdb) trace foo
(gdb) pass 3
(gdb) trace bar
(gdb) pass 2
(gdb) trace baz
(gdb) pass 1 // Stop tracing when foo has been
// executed 3 times OR when bar has
// been executed 2 times
// OR when baz has been executed 1 time.

File: gdb.info, Node: Tracepoint Conditions, Next: Trace State Variables, Prev: Tracepoint Passcounts, Up: Set Tracepoints
13.1.4 Tracepoint Conditions
----------------------------
The simplest sort of tracepoint collects data every time your program
reaches a specified place. You can also specify a "condition" for a
tracepoint. A condition is just a Boolean expression in your
programming language (*note Expressions: Expressions.). A tracepoint
with a condition evaluates the expression each time your program reaches
it, and data collection happens only if the condition is true.
Tracepoint conditions can be specified when a tracepoint is set, by
using 'if' in the arguments to the 'trace' command. *Note Setting
Tracepoints: Create and Delete Tracepoints. They can also be set or
changed at any time with the 'condition' command, just as with
breakpoints.
Unlike breakpoint conditions, GDB does not actually evaluate the
conditional expression itself. Instead, GDB encodes the expression into
an agent expression (*note Agent Expressions::) suitable for execution
on the target, independently of GDB. Global variables become raw memory
locations, locals become stack accesses, and so forth.
For instance, suppose you have a function that is usually called
frequently, but should not be called after an error has occurred. You
could use the following tracepoint command to collect data about calls
of that function that happen while the error code is propagating through
the program; an unconditional tracepoint could end up collecting
thousands of useless trace frames that you would have to search through.
(gdb) trace normal_operation if errcode > 0

File: gdb.info, Node: Trace State Variables, Next: Tracepoint Actions, Prev: Tracepoint Conditions, Up: Set Tracepoints
13.1.5 Trace State Variables
----------------------------
A "trace state variable" is a special type of variable that is created
and managed by target-side code. The syntax is the same as that for
GDB's convenience variables (a string prefixed with "$"), but they are
stored on the target. They must be created explicitly, using a
'tvariable' command. They are always 64-bit signed integers.
Trace state variables are remembered by GDB, and downloaded to the
target along with tracepoint information when the trace experiment
starts. There are no intrinsic limits on the number of trace state
variables, beyond memory limitations of the target.
Although trace state variables are managed by the target, you can use
them in print commands and expressions as if they were convenience
variables; GDB will get the current value from the target while the
trace experiment is running. Trace state variables share the same
namespace as other "$" variables, which means that you cannot have trace
state variables with names like '$23' or '$pc', nor can you have a trace
state variable and a convenience variable with the same name.
'tvariable $NAME [ = EXPRESSION ]'
The 'tvariable' command creates a new trace state variable named
'$NAME', and optionally gives it an initial value of EXPRESSION.
The EXPRESSION is evaluated when this command is entered; the
result will be converted to an integer if possible, otherwise GDB
will report an error. A subsequent 'tvariable' command specifying
the same name does not create a variable, but instead assigns the
supplied initial value to the existing variable of that name,
overwriting any previous initial value. The default initial value
is 0.
'info tvariables'
List all the trace state variables along with their initial values.
Their current values may also be displayed, if the trace experiment
is currently running.
'delete tvariable [ $NAME ... ]'
Delete the given trace state variables, or all of them if no
arguments are specified.

File: gdb.info, Node: Tracepoint Actions, Next: Listing Tracepoints, Prev: Trace State Variables, Up: Set Tracepoints
13.1.6 Tracepoint Action Lists
------------------------------
'actions [NUM]'
This command will prompt for a list of actions to be taken when the
tracepoint is hit. If the tracepoint number NUM is not specified,
this command sets the actions for the one that was most recently
defined (so that you can define a tracepoint and then say 'actions'
without bothering about its number). You specify the actions
themselves on the following lines, one action at a time, and
terminate the actions list with a line containing just 'end'. So
far, the only defined actions are 'collect', 'teval', and
'while-stepping'.
'actions' is actually equivalent to 'commands' (*note Breakpoint
Command Lists: Break Commands.), except that only the defined
actions are allowed; any other GDB command is rejected.
To remove all actions from a tracepoint, type 'actions NUM' and
follow it immediately with 'end'.
(gdb) collect DATA // collect some data
(gdb) while-stepping 5 // single-step 5 times, collect data
(gdb) end // signals the end of actions.
In the following example, the action list begins with 'collect'
commands indicating the things to be collected when the tracepoint
is hit. Then, in order to single-step and collect additional data
following the tracepoint, a 'while-stepping' command is used,
followed by the list of things to be collected after each step in a
sequence of single steps. The 'while-stepping' command is
terminated by its own separate 'end' command. Lastly, the action
list is terminated by an 'end' command.
(gdb) trace foo
(gdb) actions
Enter actions for tracepoint 1, one per line:
> collect bar,baz
> collect $regs
> while-stepping 12
> collect $pc, arr[i]
> end
end
'collect[/MODS] EXPR1, EXPR2, ...'
Collect values of the given expressions when the tracepoint is hit.
This command accepts a comma-separated list of any valid
expressions. In addition to global, static, or local variables,
the following special arguments are supported:
'$regs'
Collect all registers.
'$args'
Collect all function arguments.
'$locals'
Collect all local variables.
'$_ret'
Collect the return address. This is helpful if you want to
see more of a backtrace.
'$_probe_argc'
Collects the number of arguments from the static probe at
which the tracepoint is located. *Note Static Probe Points::.
'$_probe_argN'
N is an integer between 0 and 11. Collects the Nth argument
from the static probe at which the tracepoint is located.
*Note Static Probe Points::.
'$_sdata'
Collect static tracepoint marker specific data. Only
available for static tracepoints. *Note Tracepoint Action
Lists: Tracepoint Actions. On the UST static tracepoints
library backend, an instrumentation point resembles a 'printf'
function call. The tracing library is able to collect user
specified data formatted to a character string using the
format provided by the programmer that instrumented the
program. Other backends have similar mechanisms. Here's an
example of a UST marker call:
const char master_name[] = "$your_name";
trace_mark(channel1, marker1, "hello %s", master_name)
In this case, collecting '$_sdata' collects the string 'hello
$yourname'. When analyzing the trace buffer, you can inspect
'$_sdata' like any other variable available to GDB.
You can give several consecutive 'collect' commands, each one with
a single argument, or one 'collect' command with several arguments
separated by commas; the effect is the same.
The optional MODS changes the usual handling of the arguments. 's'
requests that pointers to chars be handled as strings, in
particular collecting the contents of the memory being pointed at,
up to the first zero. The upper bound is by default the value of
the 'print elements' variable; if 's' is followed by a decimal
number, that is the upper bound instead. So for instance
'collect/s25 mystr' collects as many as 25 characters at 'mystr'.
The command 'info scope' (*note info scope: Symbols.) is
particularly useful for figuring out what data to collect.
'teval EXPR1, EXPR2, ...'
Evaluate the given expressions when the tracepoint is hit. This
command accepts a comma-separated list of expressions. The results
are discarded, so this is mainly useful for assigning values to
trace state variables (*note Trace State Variables::) without
adding those values to the trace buffer, as would be the case if
the 'collect' action were used.
'while-stepping N'
Perform N single-step instruction traces after the tracepoint,
collecting new data after each step. The 'while-stepping' command
is followed by the list of what to collect while stepping (followed
by its own 'end' command):
> while-stepping 12
> collect $regs, myglobal
> end
>
Note that '$pc' is not automatically collected by 'while-stepping';
you need to explicitly collect that register if you need it. You
may abbreviate 'while-stepping' as 'ws' or 'stepping'.
'set default-collect EXPR1, EXPR2, ...'
This variable is a list of expressions to collect at each
tracepoint hit. It is effectively an additional 'collect' action
prepended to every tracepoint action list. The expressions are
parsed individually for each tracepoint, so for instance a variable
named 'xyz' may be interpreted as a global for one tracepoint, and
a local for another, as appropriate to the tracepoint's location.
'show default-collect'
Show the list of expressions that are collected by default at each
tracepoint hit.

File: gdb.info, Node: Listing Tracepoints, Next: Listing Static Tracepoint Markers, Prev: Tracepoint Actions, Up: Set Tracepoints
13.1.7 Listing Tracepoints
--------------------------
'info tracepoints [NUM...]'
Display information about the tracepoint NUM. If you don't specify
a tracepoint number, displays information about all the tracepoints
defined so far. The format is similar to that used for 'info
breakpoints'; in fact, 'info tracepoints' is the same command,
simply restricting itself to tracepoints.
A tracepoint's listing may include additional information specific
to tracing:
* its passcount as given by the 'passcount N' command
* the state about installed on target of each location
(gdb) info trace
Num Type Disp Enb Address What
1 tracepoint keep y 0x0804ab57 in foo() at main.cxx:7
while-stepping 20
collect globfoo, $regs
end
collect globfoo2
end
pass count 1200
2 tracepoint keep y <MULTIPLE>
collect $eip
2.1 y 0x0804859c in func4 at change-loc.h:35
installed on target
2.2 y 0xb7ffc480 in func4 at change-loc.h:35
installed on target
2.3 y <PENDING> set_tracepoint
3 tracepoint keep y 0x080485b1 in foo at change-loc.c:29
not installed on target
(gdb)
This command can be abbreviated 'info tp'.

File: gdb.info, Node: Listing Static Tracepoint Markers, Next: Starting and Stopping Trace Experiments, Prev: Listing Tracepoints, Up: Set Tracepoints
13.1.8 Listing Static Tracepoint Markers
----------------------------------------
'info static-tracepoint-markers'
Display information about all static tracepoint markers defined in
the program.
For each marker, the following columns are printed:
_Count_
An incrementing counter, output to help readability. This is
not a stable identifier.
_ID_
The marker ID, as reported by the target.
_Enabled or Disabled_
Probed markers are tagged with 'y'. 'n' identifies marks that
are not enabled.
_Address_
Where the marker is in your program, as a memory address.
_What_
Where the marker is in the source for your program, as a file
and line number. If the debug information included in the
program does not allow GDB to locate the source of the marker,
this column will be left blank.
In addition, the following information may be printed for each
marker:
_Data_
User data passed to the tracing library by the marker call.
In the UST backend, this is the format string passed as
argument to the marker call.
_Static tracepoints probing the marker_
The list of static tracepoints attached to the marker.
(gdb) info static-tracepoint-markers
Cnt ID Enb Address What
1 ust/bar2 y 0x0000000000400e1a in main at stexample.c:25
Data: number1 %d number2 %d
Probed by static tracepoints: #2
2 ust/bar33 n 0x0000000000400c87 in main at stexample.c:24
Data: str %s
(gdb)

File: gdb.info, Node: Starting and Stopping Trace Experiments, Next: Tracepoint Restrictions, Prev: Listing Static Tracepoint Markers, Up: Set Tracepoints
13.1.9 Starting and Stopping Trace Experiments
----------------------------------------------
'tstart'
This command starts the trace experiment, and begins collecting
data. It has the side effect of discarding all the data collected
in the trace buffer during the previous trace experiment. If any
arguments are supplied, they are taken as a note and stored with
the trace experiment's state. The notes may be arbitrary text, and
are especially useful with disconnected tracing in a multi-user
context; the notes can explain what the trace is doing, supply user
contact information, and so forth.
'tstop'
This command stops the trace experiment. If any arguments are
supplied, they are recorded with the experiment as a note. This is
useful if you are stopping a trace started by someone else, for
instance if the trace is interfering with the system's behavior and
needs to be stopped quickly.
*Note*: a trace experiment and data collection may stop
automatically if any tracepoint's passcount is reached (*note
Tracepoint Passcounts::), or if the trace buffer becomes full.
'tstatus'
This command displays the status of the current trace data
collection.
Here is an example of the commands we described so far:
(gdb) trace gdb_c_test
(gdb) actions
Enter actions for tracepoint #1, one per line.
> collect $regs,$locals,$args
> while-stepping 11
> collect $regs
> end
> end
(gdb) tstart
[time passes ...]
(gdb) tstop
You can choose to continue running the trace experiment even if GDB
disconnects from the target, voluntarily or involuntarily. For commands
such as 'detach', the debugger will ask what you want to do with the
trace. But for unexpected terminations (GDB crash, network outage), it
would be unfortunate to lose hard-won trace data, so the variable
'disconnected-tracing' lets you decide whether the trace should continue
running without GDB.
'set disconnected-tracing on'
'set disconnected-tracing off'
Choose whether a tracing run should continue to run if GDB has
disconnected from the target. Note that 'detach' or 'quit' will
ask you directly what to do about a running trace no matter what
this variable's setting, so the variable is mainly useful for
handling unexpected situations, such as loss of the network.
'show disconnected-tracing'
Show the current choice for disconnected tracing.
When you reconnect to the target, the trace experiment may or may not
still be running; it might have filled the trace buffer in the meantime,
or stopped for one of the other reasons. If it is running, it will
continue after reconnection.
Upon reconnection, the target will upload information about the
tracepoints in effect. GDB will then compare that information to the
set of tracepoints currently defined, and attempt to match them up,
allowing for the possibility that the numbers may have changed due to
creation and deletion in the meantime. If one of the target's
tracepoints does not match any in GDB, the debugger will create a new
tracepoint, so that you have a number with which to specify that
tracepoint. This matching-up process is necessarily heuristic, and it
may result in useless tracepoints being created; you may simply delete
them if they are of no use.
If your target agent supports a "circular trace buffer", then you can
run a trace experiment indefinitely without filling the trace buffer;
when space runs out, the agent deletes already-collected trace frames,
oldest first, until there is enough room to continue collecting. This
is especially useful if your tracepoints are being hit too often, and
your trace gets terminated prematurely because the buffer is full. To
ask for a circular trace buffer, simply set 'circular-trace-buffer' to
on. You can set this at any time, including during tracing; if the
agent can do it, it will change buffer handling on the fly, otherwise it
will not take effect until the next run.
'set circular-trace-buffer on'
'set circular-trace-buffer off'
Choose whether a tracing run should use a linear or circular buffer
for trace data. A linear buffer will not lose any trace data, but
may fill up prematurely, while a circular buffer will discard old
trace data, but it will have always room for the latest tracepoint
hits.
'show circular-trace-buffer'
Show the current choice for the trace buffer. Note that this may
not match the agent's current buffer handling, nor is it guaranteed
to match the setting that might have been in effect during a past
run, for instance if you are looking at frames from a trace file.
'set trace-buffer-size N'
'set trace-buffer-size unlimited'
Request that the target use a trace buffer of N bytes. Not all
targets will honor the request; they may have a compiled-in size
for the trace buffer, or some other limitation. Set to a value of
'unlimited' or '-1' to let the target use whatever size it likes.
This is also the default.
'show trace-buffer-size'
Show the current requested size for the trace buffer. Note that
this will only match the actual size if the target supports
size-setting, and was able to handle the requested size. For
instance, if the target can only change buffer size between runs,
this variable will not reflect the change until the next run
starts. Use 'tstatus' to get a report of the actual buffer size.
'set trace-user TEXT'
'show trace-user'
'set trace-notes TEXT'
Set the trace run's notes.
'show trace-notes'
Show the trace run's notes.
'set trace-stop-notes TEXT'
Set the trace run's stop notes. The handling of the note is as for
'tstop' arguments; the set command is convenient way to fix a stop
note that is mistaken or incomplete.
'show trace-stop-notes'
Show the trace run's stop notes.

File: gdb.info, Node: Tracepoint Restrictions, Prev: Starting and Stopping Trace Experiments, Up: Set Tracepoints
13.1.10 Tracepoint Restrictions
-------------------------------
There are a number of restrictions on the use of tracepoints. As
described above, tracepoint data gathering occurs on the target without
interaction from GDB. Thus the full capabilities of the debugger are
not available during data gathering, and then at data examination time,
you will be limited by only having what was collected. The following
items describe some common problems, but it is not exhaustive, and you
may run into additional difficulties not mentioned here.
* Tracepoint expressions are intended to gather objects (lvalues).
Thus the full flexibility of GDB's expression evaluator is not
available. You cannot call functions, cast objects to aggregate
types, access convenience variables or modify values (except by
assignment to trace state variables). Some language features may
implicitly call functions (for instance Objective-C fields with
accessors), and therefore cannot be collected either.
* Collection of local variables, either individually or in bulk with
'$locals' or '$args', during 'while-stepping' may behave
erratically. The stepping action may enter a new scope (for
instance by stepping into a function), or the location of the
variable may change (for instance it is loaded into a register).
The tracepoint data recorded uses the location information for the
variables that is correct for the tracepoint location. When the
tracepoint is created, it is not possible, in general, to determine
where the steps of a 'while-stepping' sequence will advance the
program--particularly if a conditional branch is stepped.
* Collection of an incompletely-initialized or partially-destroyed
object may result in something that GDB cannot display, or displays
in a misleading way.
* When GDB displays a pointer to character it automatically
dereferences the pointer to also display characters of the string
being pointed to. However, collecting the pointer during tracing
does not automatically collect the string. You need to explicitly
dereference the pointer and provide size information if you want to
collect not only the pointer, but the memory pointed to. For
example, '*ptr@50' can be used to collect the 50 element array
pointed to by 'ptr'.
* It is not possible to collect a complete stack backtrace at a
tracepoint. Instead, you may collect the registers and a few
hundred bytes from the stack pointer with something like
'*(unsigned char *)$esp@300' (adjust to use the name of the actual
stack pointer register on your target architecture, and the amount
of stack you wish to capture). Then the 'backtrace' command will
show a partial backtrace when using a trace frame. The number of
stack frames that can be examined depends on the sizes of the
frames in the collected stack. Note that if you ask for a block so
large that it goes past the bottom of the stack, the target agent
may report an error trying to read from an invalid address.
* If you do not collect registers at a tracepoint, GDB can infer that
the value of '$pc' must be the same as the address of the
tracepoint and use that when you are looking at a trace frame for
that tracepoint. However, this cannot work if the tracepoint has
multiple locations (for instance if it was set in a function that
was inlined), or if it has a 'while-stepping' loop. In those cases
GDB will warn you that it can't infer '$pc', and default it to
zero.

File: gdb.info, Node: Analyze Collected Data, Next: Tracepoint Variables, Prev: Set Tracepoints, Up: Tracepoints
13.2 Using the Collected Data
=============================
After the tracepoint experiment ends, you use GDB commands for examining
the trace data. The basic idea is that each tracepoint collects a trace
"snapshot" every time it is hit and another snapshot every time it
single-steps. All these snapshots are consecutively numbered from zero
and go into a buffer, and you can examine them later. The way you
examine them is to "focus" on a specific trace snapshot. When the
remote stub is focused on a trace snapshot, it will respond to all GDB
requests for memory and registers by reading from the buffer which
belongs to that snapshot, rather than from _real_ memory or registers of
the program being debugged. This means that *all* GDB commands
('print', 'info registers', 'backtrace', etc.) will behave as if we
were currently debugging the program state as it was when the tracepoint
occurred. Any requests for data that are not in the buffer will fail.
* Menu:
* tfind:: How to select a trace snapshot
* tdump:: How to display all data for a snapshot
* save tracepoints:: How to save tracepoints for a future run

File: gdb.info, Node: tfind, Next: tdump, Up: Analyze Collected Data
13.2.1 'tfind N'
----------------
The basic command for selecting a trace snapshot from the buffer is
'tfind N', which finds trace snapshot number N, counting from zero. If
no argument N is given, the next snapshot is selected.
Here are the various forms of using the 'tfind' command.
'tfind start'
Find the first snapshot in the buffer. This is a synonym for
'tfind 0' (since 0 is the number of the first snapshot).
'tfind none'
Stop debugging trace snapshots, resume _live_ debugging.
'tfind end'
Same as 'tfind none'.
'tfind'
No argument means find the next trace snapshot.
'tfind -'
Find the previous trace snapshot before the current one. This
permits retracing earlier steps.
'tfind tracepoint NUM'
Find the next snapshot associated with tracepoint NUM. Search
proceeds forward from the last examined trace snapshot. If no
argument NUM is given, it means find the next snapshot collected
for the same tracepoint as the current snapshot.
'tfind pc ADDR'
Find the next snapshot associated with the value ADDR of the
program counter. Search proceeds forward from the last examined
trace snapshot. If no argument ADDR is given, it means find the
next snapshot with the same value of PC as the current snapshot.
'tfind outside ADDR1, ADDR2'
Find the next snapshot whose PC is outside the given range of
addresses (exclusive).
'tfind range ADDR1, ADDR2'
Find the next snapshot whose PC is between ADDR1 and ADDR2
(inclusive).
'tfind line [FILE:]N'
Find the next snapshot associated with the source line N. If the
optional argument FILE is given, refer to line N in that source
file. Search proceeds forward from the last examined trace
snapshot. If no argument N is given, it means find the next line
other than the one currently being examined; thus saying 'tfind
line' repeatedly can appear to have the same effect as stepping
from line to line in a _live_ debugging session.
The default arguments for the 'tfind' commands are specifically
designed to make it easy to scan through the trace buffer. For
instance, 'tfind' with no argument selects the next trace snapshot, and
'tfind -' with no argument selects the previous trace snapshot. So, by
giving one 'tfind' command, and then simply hitting <RET> repeatedly you
can examine all the trace snapshots in order. Or, by saying 'tfind -'
and then hitting <RET> repeatedly you can examine the snapshots in
reverse order. The 'tfind line' command with no argument selects the
snapshot for the next source line executed. The 'tfind pc' command with
no argument selects the next snapshot with the same program counter (PC)
as the current frame. The 'tfind tracepoint' command with no argument
selects the next trace snapshot collected by the same tracepoint as the
current one.
In addition to letting you scan through the trace buffer manually,
these commands make it easy to construct GDB scripts that scan through
the trace buffer and print out whatever collected data you are
interested in. Thus, if we want to examine the PC, FP, and SP registers
from each trace frame in the buffer, we can say this:
(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \
$trace_frame, $pc, $sp, $fp
> tfind
> end
Frame 0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44
Frame 1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44
Frame 2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44
Frame 3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44
Frame 4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44
Frame 5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44
Frame 6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44
Frame 7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44
Frame 8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44
Frame 9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44
Frame 10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14
Or, if we want to examine the variable 'X' at each source line in the
buffer:
(gdb) tfind start
(gdb) while ($trace_frame != -1)
> printf "Frame %d, X == %d\n", $trace_frame, X
> tfind line
> end
Frame 0, X = 1
Frame 7, X = 2
Frame 13, X = 255

File: gdb.info, Node: tdump, Next: save tracepoints, Prev: tfind, Up: Analyze Collected Data
13.2.2 'tdump'
--------------
This command takes no arguments. It prints all the data collected at
the current trace snapshot.
(gdb) trace 444
(gdb) actions
Enter actions for tracepoint #2, one per line:
> collect $regs, $locals, $args, gdb_long_test
> end
(gdb) tstart
(gdb) tfind line 444
#0 gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66)
at gdb_test.c:444
444 printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", )
(gdb) tdump
Data collected at tracepoint 2, trace frame 1:
d0 0xc4aa0085 -995491707
d1 0x18 24
d2 0x80 128
d3 0x33 51
d4 0x71aea3d 119204413
d5 0x22 34
d6 0xe0 224
d7 0x380035 3670069
a0 0x19e24a 1696330
a1 0x3000668 50333288
a2 0x100 256
a3 0x322000 3284992
a4 0x3000698 50333336
a5 0x1ad3cc 1758156
fp 0x30bf3c 0x30bf3c
sp 0x30bf34 0x30bf34
ps 0x0 0
pc 0x20b2c8 0x20b2c8
fpcontrol 0x0 0
fpstatus 0x0 0
fpiaddr 0x0 0
p = 0x20e5b4 "gdb-test"
p1 = (void *) 0x11
p2 = (void *) 0x22
p3 = (void *) 0x33
p4 = (void *) 0x44
p5 = (void *) 0x55
p6 = (void *) 0x66
gdb_long_test = 17 '\021'
(gdb)
'tdump' works by scanning the tracepoint's current collection actions
and printing the value of each expression listed. So 'tdump' can fail,
if after a run, you change the tracepoint's actions to mention variables
that were not collected during the run.
Also, for tracepoints with 'while-stepping' loops, 'tdump' uses the
collected value of '$pc' to distinguish between trace frames that were
collected at the tracepoint hit, and frames that were collected while
stepping. This allows it to correctly choose whether to display the
basic list of collections, or the collections from the body of the
while-stepping loop. However, if '$pc' was not collected, then 'tdump'
will always attempt to dump using the basic collection list, and may
fail if a while-stepping frame does not include all the same data that
is collected at the tracepoint hit.

File: gdb.info, Node: save tracepoints, Prev: tdump, Up: Analyze Collected Data
13.2.3 'save tracepoints FILENAME'
----------------------------------
This command saves all current tracepoint definitions together with
their actions and passcounts, into a file 'FILENAME' suitable for use in
a later debugging session. To read the saved tracepoint definitions,
use the 'source' command (*note Command Files::). The 'save-tracepoints'
command is a deprecated alias for 'save tracepoints'

File: gdb.info, Node: Tracepoint Variables, Next: Trace Files, Prev: Analyze Collected Data, Up: Tracepoints
13.3 Convenience Variables for Tracepoints
==========================================
'(int) $trace_frame'
The current trace snapshot (a.k.a. "frame") number, or -1 if no
snapshot is selected.
'(int) $tracepoint'
The tracepoint for the current trace snapshot.
'(int) $trace_line'
The line number for the current trace snapshot.
'(char []) $trace_file'
The source file for the current trace snapshot.
'(char []) $trace_func'
The name of the function containing '$tracepoint'.
Note: '$trace_file' is not suitable for use in 'printf', use 'output'
instead.
Here's a simple example of using these convenience variables for
stepping through all the trace snapshots and printing some of their
data. Note that these are not the same as trace state variables, which
are managed by the target.
(gdb) tfind start
(gdb) while $trace_frame != -1
> output $trace_file
> printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint
> tfind
> end

File: gdb.info, Node: Trace Files, Prev: Tracepoint Variables, Up: Tracepoints
13.4 Using Trace Files
======================
In some situations, the target running a trace experiment may no longer
be available; perhaps it crashed, or the hardware was needed for a
different activity. To handle these cases, you can arrange to dump the
trace data into a file, and later use that file as a source of trace
data, via the 'target tfile' command.
'tsave [ -r ] FILENAME'
'tsave [-ctf] DIRNAME'
Save the trace data to FILENAME. By default, this command assumes
that FILENAME refers to the host filesystem, so if necessary GDB
will copy raw trace data up from the target and then save it. If
the target supports it, you can also supply the optional argument
'-r' ("remote") to direct the target to save the data directly into
FILENAME in its own filesystem, which may be more efficient if the
trace buffer is very large. (Note, however, that 'target tfile'
can only read from files accessible to the host.) By default, this
command will save trace frame in tfile format. You can supply the
optional argument '-ctf' to save date in CTF format. The "Common
Trace Format" (CTF) is proposed as a trace format that can be
shared by multiple debugging and tracing tools. Please go to
'http://www.efficios.com/ctf' to get more information.
'target tfile FILENAME'
'target ctf DIRNAME'
Use the file named FILENAME or directory named DIRNAME as a source
of trace data. Commands that examine data work as they do with a
live target, but it is not possible to run any new trace
experiments. 'tstatus' will report the state of the trace run at
the moment the data was saved, as well as the current trace frame
you are examining. Both FILENAME and DIRNAME must be on a
filesystem accessible to the host.
(gdb) target ctf ctf.ctf
(gdb) tfind
Found trace frame 0, tracepoint 2
39 ++a; /* set tracepoint 1 here */
(gdb) tdump
Data collected at tracepoint 2, trace frame 0:
i = 0
a = 0
b = 1 '\001'
c = {"123", "456", "789", "123", "456", "789"}
d = {{{a = 1, b = 2}, {a = 3, b = 4}}, {{a = 5, b = 6}, {a = 7, b = 8}}}
(gdb) p b
$1 = 1

File: gdb.info, Node: Overlays, Next: Languages, Prev: Tracepoints, Up: Top
14 Debugging Programs That Use Overlays
***************************************
If your program is too large to fit completely in your target system's
memory, you can sometimes use "overlays" to work around this problem.
GDB provides some support for debugging programs that use overlays.
* Menu:
* How Overlays Work:: A general explanation of overlays.
* Overlay Commands:: Managing overlays in GDB.
* Automatic Overlay Debugging:: GDB can find out which overlays are
mapped by asking the inferior.
* Overlay Sample Program:: A sample program using overlays.

File: gdb.info, Node: How Overlays Work, Next: Overlay Commands, Up: Overlays
14.1 How Overlays Work
======================
Suppose you have a computer whose instruction address space is only 64
kilobytes long, but which has much more memory which can be accessed by
other means: special instructions, segment registers, or memory
management hardware, for example. Suppose further that you want to
adapt a program which is larger than 64 kilobytes to run on this system.
One solution is to identify modules of your program which are
relatively independent, and need not call each other directly; call
these modules "overlays". Separate the overlays from the main program,
and place their machine code in the larger memory. Place your main
program in instruction memory, but leave at least enough space there to
hold the largest overlay as well.
Now, to call a function located in an overlay, you must first copy
that overlay's machine code from the large memory into the space set
aside for it in the instruction memory, and then jump to its entry point
there.
Data Instruction Larger
Address Space Address Space Address Space
+-----------+ +-----------+ +-----------+
| | | | | |
+-----------+ +-----------+ +-----------+<-- overlay 1
| program | | main | .----| overlay 1 | load address
| variables | | program | | +-----------+
| and heap | | | | | |
+-----------+ | | | +-----------+<-- overlay 2
| | +-----------+ | | | load address
+-----------+ | | | .-| overlay 2 |
| | | | | |
mapped --->+-----------+ | | +-----------+
address | | | | | |
| overlay | <-' | | |
| area | <---' +-----------+<-- overlay 3
| | <---. | | load address
+-----------+ `--| overlay 3 |
| | | |
+-----------+ | |
+-----------+
| |
+-----------+
A code overlay
The diagram (*note A code overlay::) shows a system with separate
data and instruction address spaces. To map an overlay, the program
copies its code from the larger address space to the instruction address
space. Since the overlays shown here all use the same mapped address,
only one may be mapped at a time. For a system with a single address
space for data and instructions, the diagram would be similar, except
that the program variables and heap would share an address space with
the main program and the overlay area.
An overlay loaded into instruction memory and ready for use is called
a "mapped" overlay; its "mapped address" is its address in the
instruction memory. An overlay not present (or only partially present)
in instruction memory is called "unmapped"; its "load address" is its
address in the larger memory. The mapped address is also called the
"virtual memory address", or "VMA"; the load address is also called the
"load memory address", or "LMA".
Unfortunately, overlays are not a completely transparent way to adapt
a program to limited instruction memory. They introduce a new set of
global constraints you must keep in mind as you design your program:
* Before calling or returning to a function in an overlay, your
program must make sure that overlay is actually mapped. Otherwise,
the call or return will transfer control to the right address, but
in the wrong overlay, and your program will probably crash.
* If the process of mapping an overlay is expensive on your system,
you will need to choose your overlays carefully to minimize their
effect on your program's performance.
* The executable file you load onto your system must contain each
overlay's instructions, appearing at the overlay's load address,
not its mapped address. However, each overlay's instructions must
be relocated and its symbols defined as if the overlay were at its
mapped address. You can use GNU linker scripts to specify
different load and relocation addresses for pieces of your program;
see *note (ld.info)Overlay Description::.
* The procedure for loading executable files onto your system must be
able to load their contents into the larger address space as well
as the instruction and data spaces.
The overlay system described above is rather simple, and could be
improved in many ways:
* If your system has suitable bank switch registers or memory
management hardware, you could use those facilities to make an
overlay's load area contents simply appear at their mapped address
in instruction space. This would probably be faster than copying
the overlay to its mapped area in the usual way.
* If your overlays are small enough, you could set aside more than
one overlay area, and have more than one overlay mapped at a time.
* You can use overlays to manage data, as well as instructions. In
general, data overlays are even less transparent to your design
than code overlays: whereas code overlays only require care when
you call or return to functions, data overlays require care every
time you access the data. Also, if you change the contents of a
data overlay, you must copy its contents back out to its load
address before you can copy a different data overlay into the same
mapped area.

File: gdb.info, Node: Overlay Commands, Next: Automatic Overlay Debugging, Prev: How Overlays Work, Up: Overlays
14.2 Overlay Commands
=====================
To use GDB's overlay support, each overlay in your program must
correspond to a separate section of the executable file. The section's
virtual memory address and load memory address must be the overlay's
mapped and load addresses. Identifying overlays with sections allows
GDB to determine the appropriate address of a function or variable,
depending on whether the overlay is mapped or not.
GDB's overlay commands all start with the word 'overlay'; you can
abbreviate this as 'ov' or 'ovly'. The commands are:
'overlay off'
Disable GDB's overlay support. When overlay support is disabled,
GDB assumes that all functions and variables are always present at
their mapped addresses. By default, GDB's overlay support is
disabled.
'overlay manual'
Enable "manual" overlay debugging. In this mode, GDB relies on you
to tell it which overlays are mapped, and which are not, using the
'overlay map-overlay' and 'overlay unmap-overlay' commands
described below.
'overlay map-overlay OVERLAY'
'overlay map OVERLAY'
Tell GDB that OVERLAY is now mapped; OVERLAY must be the name of
the object file section containing the overlay. When an overlay is
mapped, GDB assumes it can find the overlay's functions and
variables at their mapped addresses. GDB assumes that any other
overlays whose mapped ranges overlap that of OVERLAY are now
unmapped.
'overlay unmap-overlay OVERLAY'
'overlay unmap OVERLAY'
Tell GDB that OVERLAY is no longer mapped; OVERLAY must be the name
of the object file section containing the overlay. When an overlay
is unmapped, GDB assumes it can find the overlay's functions and
variables at their load addresses.
'overlay auto'
Enable "automatic" overlay debugging. In this mode, GDB consults a
data structure the overlay manager maintains in the inferior to see
which overlays are mapped. For details, see *note Automatic
Overlay Debugging::.
'overlay load-target'
'overlay load'
Re-read the overlay table from the inferior. Normally, GDB
re-reads the table GDB automatically each time the inferior stops,
so this command should only be necessary if you have changed the
overlay mapping yourself using GDB. This command is only useful
when using automatic overlay debugging.
'overlay list-overlays'
'overlay list'
Display a list of the overlays currently mapped, along with their
mapped addresses, load addresses, and sizes.
Normally, when GDB prints a code address, it includes the name of the
function the address falls in:
(gdb) print main
$3 = {int ()} 0x11a0 <main>
When overlay debugging is enabled, GDB recognizes code in unmapped
overlays, and prints the names of unmapped functions with asterisks
around them. For example, if 'foo' is a function in an unmapped
overlay, GDB prints it this way:
(gdb) overlay list
No sections are mapped.
(gdb) print foo
$5 = {int (int)} 0x100000 <*foo*>
When 'foo''s overlay is mapped, GDB prints the function's name normally:
(gdb) overlay list
Section .ov.foo.text, loaded at 0x100000 - 0x100034,
mapped at 0x1016 - 0x104a
(gdb) print foo
$6 = {int (int)} 0x1016 <foo>
When overlay debugging is enabled, GDB can find the correct address
for functions and variables in an overlay, whether or not the overlay is
mapped. This allows most GDB commands, like 'break' and 'disassemble',
to work normally, even on unmapped code. However, GDB's breakpoint
support has some limitations:
* You can set breakpoints in functions in unmapped overlays, as long
as GDB can write to the overlay at its load address.
* GDB can not set hardware or simulator-based breakpoints in unmapped
overlays. However, if you set a breakpoint at the end of your
overlay manager (and tell GDB which overlays are now mapped, if you
are using manual overlay management), GDB will re-set its
breakpoints properly.

File: gdb.info, Node: Automatic Overlay Debugging, Next: Overlay Sample Program, Prev: Overlay Commands, Up: Overlays
14.3 Automatic Overlay Debugging
================================
GDB can automatically track which overlays are mapped and which are not,
given some simple co-operation from the overlay manager in the inferior.
If you enable automatic overlay debugging with the 'overlay auto'
command (*note Overlay Commands::), GDB looks in the inferior's memory
for certain variables describing the current state of the overlays.
Here are the variables your overlay manager must define to support
GDB's automatic overlay debugging:
'_ovly_table':
This variable must be an array of the following structures:
struct
{
/* The overlay's mapped address. */
unsigned long vma;
/* The size of the overlay, in bytes. */
unsigned long size;
/* The overlay's load address. */
unsigned long lma;
/* Non-zero if the overlay is currently mapped;
zero otherwise. */
unsigned long mapped;
}
'_novlys':
This variable must be a four-byte signed integer, holding the total
number of elements in '_ovly_table'.
To decide whether a particular overlay is mapped or not, GDB looks
for an entry in '_ovly_table' whose 'vma' and 'lma' members equal the
VMA and LMA of the overlay's section in the executable file. When GDB
finds a matching entry, it consults the entry's 'mapped' member to
determine whether the overlay is currently mapped.
In addition, your overlay manager may define a function called
'_ovly_debug_event'. If this function is defined, GDB will silently set
a breakpoint there. If the overlay manager then calls this function
whenever it has changed the overlay table, this will enable GDB to
accurately keep track of which overlays are in program memory, and
update any breakpoints that may be set in overlays. This will allow
breakpoints to work even if the overlays are kept in ROM or other
non-writable memory while they are not being executed.

File: gdb.info, Node: Overlay Sample Program, Prev: Automatic Overlay Debugging, Up: Overlays
14.4 Overlay Sample Program
===========================
When linking a program which uses overlays, you must place the overlays
at their load addresses, while relocating them to run at their mapped
addresses. To do this, you must write a linker script (*note
(ld.info)Overlay Description::). Unfortunately, since linker scripts
are specific to a particular host system, target architecture, and
target memory layout, this manual cannot provide portable sample code
demonstrating GDB's overlay support.
However, the GDB source distribution does contain an overlaid
program, with linker scripts for a few systems, as part of its test
suite. The program consists of the following files from
'gdb/testsuite/gdb.base':
'overlays.c'
The main program file.
'ovlymgr.c'
A simple overlay manager, used by 'overlays.c'.
'foo.c'
'bar.c'
'baz.c'
'grbx.c'
Overlay modules, loaded and used by 'overlays.c'.
'd10v.ld'
'm32r.ld'
Linker scripts for linking the test program on the 'd10v-elf' and
'm32r-elf' targets.
You can build the test program using the 'd10v-elf' GCC
cross-compiler like this:
$ d10v-elf-gcc -g -c overlays.c
$ d10v-elf-gcc -g -c ovlymgr.c
$ d10v-elf-gcc -g -c foo.c
$ d10v-elf-gcc -g -c bar.c
$ d10v-elf-gcc -g -c baz.c
$ d10v-elf-gcc -g -c grbx.c
$ d10v-elf-gcc -g overlays.o ovlymgr.o foo.o bar.o \
baz.o grbx.o -Wl,-Td10v.ld -o overlays
The build process is identical for any other architecture, except
that you must substitute the appropriate compiler and linker script for
the target system for 'd10v-elf-gcc' and 'd10v.ld'.

File: gdb.info, Node: Languages, Next: Symbols, Prev: Overlays, Up: Top
15 Using GDB with Different Languages
*************************************
Although programming languages generally have common aspects, they are
rarely expressed in the same manner. For instance, in ANSI C,
dereferencing a pointer 'p' is accomplished by '*p', but in Modula-2, it
is accomplished by 'p^'. Values can also be represented (and displayed)
differently. Hex numbers in C appear as '0x1ae', while in Modula-2 they
appear as '1AEH'.
Language-specific information is built into GDB for some languages,
allowing you to express operations like the above in your program's
native language, and allowing GDB to output values in a manner
consistent with the syntax of your program's native language. The
language you use to build expressions is called the "working language".
* Menu:
* Setting:: Switching between source languages
* Show:: Displaying the language
* Checks:: Type and range checks
* Supported Languages:: Supported languages
* Unsupported Languages:: Unsupported languages

File: gdb.info, Node: Setting, Next: Show, Up: Languages
15.1 Switching Between Source Languages
=======================================
There are two ways to control the working language--either have GDB set
it automatically, or select it manually yourself. You can use the 'set
language' command for either purpose. On startup, GDB defaults to
setting the language automatically. The working language is used to
determine how expressions you type are interpreted, how values are
printed, etc.
In addition to the working language, every source file that GDB knows
about has its own working language. For some object file formats, the
compiler might indicate which language a particular source file is in.
However, most of the time GDB infers the language from the name of the
file. The language of a source file controls whether C++ names are
demangled--this way 'backtrace' can show each frame appropriately for
its own language. There is no way to set the language of a source file
from within GDB, but you can set the language associated with a filename
extension. *Note Displaying the Language: Show.
This is most commonly a problem when you use a program, such as
'cfront' or 'f2c', that generates C but is written in another language.
In that case, make the program use '#line' directives in its C output;
that way GDB will know the correct language of the source code of the
original program, and will display that source code, not the generated C
code.
* Menu:
* Filenames:: Filename extensions and languages.
* Manually:: Setting the working language manually
* Automatically:: Having GDB infer the source language

File: gdb.info, Node: Filenames, Next: Manually, Up: Setting
15.1.1 List of Filename Extensions and Languages
------------------------------------------------
If a source file name ends in one of the following extensions, then GDB
infers that its language is the one indicated.
'.ada'
'.ads'
'.adb'
'.a'
Ada source file.
'.c'
C source file
'.C'
'.cc'
'.cp'
'.cpp'
'.cxx'
'.c++'
C++ source file
'.d'
D source file
'.m'
Objective-C source file
'.f'
'.F'
Fortran source file
'.mod'
Modula-2 source file
'.s'
'.S'
Assembler source file. This actually behaves almost like C, but
GDB does not skip over function prologues when stepping.
In addition, you may set the language associated with a filename
extension. *Note Displaying the Language: Show.

File: gdb.info, Node: Manually, Next: Automatically, Prev: Filenames, Up: Setting
15.1.2 Setting the Working Language
-----------------------------------
If you allow GDB to set the language automatically, expressions are
interpreted the same way in your debugging session and your program.
If you wish, you may set the language manually. To do this, issue
the command 'set language LANG', where LANG is the name of a language,
such as 'c' or 'modula-2'. For a list of the supported languages, type
'set language'.
Setting the language manually prevents GDB from updating the working
language automatically. This can lead to confusion if you try to debug
a program when the working language is not the same as the source
language, when an expression is acceptable to both languages--but means
different things. For instance, if the current source file were written
in C, and GDB was parsing Modula-2, a command such as:
print a = b + c
might not have the effect you intended. In C, this means to add 'b' and
'c' and place the result in 'a'. The result printed would be the value
of 'a'. In Modula-2, this means to compare 'a' to the result of 'b+c',
yielding a 'BOOLEAN' value.

File: gdb.info, Node: Automatically, Prev: Manually, Up: Setting
15.1.3 Having GDB Infer the Source Language
-------------------------------------------
To have GDB set the working language automatically, use 'set language
local' or 'set language auto'. GDB then infers the working language.
That is, when your program stops in a frame (usually by encountering a
breakpoint), GDB sets the working language to the language recorded for
the function in that frame. If the language for a frame is unknown
(that is, if the function or block corresponding to the frame was
defined in a source file that does not have a recognized extension), the
current working language is not changed, and GDB issues a warning.
This may not seem necessary for most programs, which are written
entirely in one source language. However, program modules and libraries
written in one source language can be used by a main program written in
a different source language. Using 'set language auto' in this case
frees you from having to set the working language manually.

File: gdb.info, Node: Show, Next: Checks, Prev: Setting, Up: Languages
15.2 Displaying the Language
============================
The following commands help you find out which language is the working
language, and also what language source files were written in.
'show language'
Display the current working language. This is the language you can
use with commands such as 'print' to build and compute expressions
that may involve variables in your program.
'info frame'
Display the source language for this frame. This language becomes
the working language if you use an identifier from this frame.
*Note Information about a Frame: Frame Info, to identify the other
information listed here.
'info source'
Display the source language of this source file. *Note Examining
the Symbol Table: Symbols, to identify the other information listed
here.
In unusual circumstances, you may have source files with extensions
not in the standard list. You can then set the extension associated
with a language explicitly:
'set extension-language EXT LANGUAGE'
Tell GDB that source files with extension EXT are to be assumed as
written in the source language LANGUAGE.
'info extensions'
List all the filename extensions and the associated languages.

File: gdb.info, Node: Checks, Next: Supported Languages, Prev: Show, Up: Languages
15.3 Type and Range Checking
============================
Some languages are designed to guard you against making seemingly common
errors through a series of compile- and run-time checks. These include
checking the type of arguments to functions and operators and making
sure mathematical overflows are caught at run time. Checks such as
these help to ensure a program's correctness once it has been compiled
by eliminating type mismatches and providing active checks for range
errors when your program is running.
By default GDB checks for these errors according to the rules of the
current source language. Although GDB does not check the statements in
your program, it can check expressions entered directly into GDB for
evaluation via the 'print' command, for example.
* Menu:
* Type Checking:: An overview of type checking
* Range Checking:: An overview of range checking

File: gdb.info, Node: Type Checking, Next: Range Checking, Up: Checks
15.3.1 An Overview of Type Checking
-----------------------------------
Some languages, such as C and C++, are strongly typed, meaning that the
arguments to operators and functions have to be of the correct type,
otherwise an error occurs. These checks prevent type mismatch errors
from ever causing any run-time problems. For example,
int klass::my_method(char *b) { return b ? 1 : 2; }
(gdb) print obj.my_method (0)
$1 = 2
but
(gdb) print obj.my_method (0x1234)
Cannot resolve method klass::my_method to any overloaded instance
The second example fails because in C++ the integer constant '0x1234'
is not type-compatible with the pointer parameter type.
For the expressions you use in GDB commands, you can tell GDB to not
enforce strict type checking or to treat any mismatches as errors and
abandon the expression; When type checking is disabled, GDB successfully
evaluates expressions like the second example above.
Even if type checking is off, there may be other reasons related to
type that prevent GDB from evaluating an expression. For instance, GDB
does not know how to add an 'int' and a 'struct foo'. These particular
type errors have nothing to do with the language in use and usually
arise from expressions which make little sense to evaluate anyway.
GDB provides some additional commands for controlling type checking:
'set check type on'
'set check type off'
Set strict type checking on or off. If any type mismatches occur
in evaluating an expression while type checking is on, GDB prints a
message and aborts evaluation of the expression.
'show check type'
Show the current setting of type checking and whether GDB is
enforcing strict type checking rules.

File: gdb.info, Node: Range Checking, Prev: Type Checking, Up: Checks
15.3.2 An Overview of Range Checking
------------------------------------
In some languages (such as Modula-2), it is an error to exceed the
bounds of a type; this is enforced with run-time checks. Such range
checking is meant to ensure program correctness by making sure
computations do not overflow, or indices on an array element access do
not exceed the bounds of the array.
For expressions you use in GDB commands, you can tell GDB to treat
range errors in one of three ways: ignore them, always treat them as
errors and abandon the expression, or issue warnings but evaluate the
expression anyway.
A range error can result from numerical overflow, from exceeding an
array index bound, or when you type a constant that is not a member of
any type. Some languages, however, do not treat overflows as an error.
In many implementations of C, mathematical overflow causes the result to
"wrap around" to lower values--for example, if M is the largest integer
value, and S is the smallest, then
M + 1 => S
This, too, is specific to individual languages, and in some cases
specific to individual compilers or machines. *Note Supported
Languages: Supported Languages, for further details on specific
languages.
GDB provides some additional commands for controlling the range
checker:
'set check range auto'
Set range checking on or off based on the current working language.
*Note Supported Languages: Supported Languages, for the default
settings for each language.
'set check range on'
'set check range off'
Set range checking on or off, overriding the default setting for
the current working language. A warning is issued if the setting
does not match the language default. If a range error occurs and
range checking is on, then a message is printed and evaluation of
the expression is aborted.
'set check range warn'
Output messages when the GDB range checker detects a range error,
but attempt to evaluate the expression anyway. Evaluating the
expression may still be impossible for other reasons, such as
accessing memory that the process does not own (a typical example
from many Unix systems).
'show range'
Show the current setting of the range checker, and whether or not
it is being set automatically by GDB.

File: gdb.info, Node: Supported Languages, Next: Unsupported Languages, Prev: Checks, Up: Languages
15.4 Supported Languages
========================
GDB supports C, C++, D, Go, Objective-C, Fortran, Java, OpenCL C,
Pascal, assembly, Modula-2, and Ada. Some GDB features may be used in
expressions regardless of the language you use: the GDB '@' and '::'
operators, and the '{type}addr' construct (*note Expressions:
Expressions.) can be used with the constructs of any supported language.
The following sections detail to what degree each source language is
supported by GDB. These sections are not meant to be language tutorials
or references, but serve only as a reference guide to what the GDB
expression parser accepts, and what input and output formats should look
like for different languages. There are many good books written on each
of these languages; please look to these for a language reference or
tutorial.
* Menu:
* C:: C and C++
* D:: D
* Go:: Go
* Objective-C:: Objective-C
* OpenCL C:: OpenCL C
* Fortran:: Fortran
* Pascal:: Pascal
* Modula-2:: Modula-2
* Ada:: Ada

File: gdb.info, Node: C, Next: D, Up: Supported Languages
15.4.1 C and C++
----------------
Since C and C++ are so closely related, many features of GDB apply to
both languages. Whenever this is the case, we discuss those languages
together.
The C++ debugging facilities are jointly implemented by the C++
compiler and GDB. Therefore, to debug your C++ code effectively, you
must compile your C++ programs with a supported C++ compiler, such as
GNU 'g++', or the HP ANSI C++ compiler ('aCC').
* Menu:
* C Operators:: C and C++ operators
* C Constants:: C and C++ constants
* C Plus Plus Expressions:: C++ expressions
* C Defaults:: Default settings for C and C++
* C Checks:: C and C++ type and range checks
* Debugging C:: GDB and C
* Debugging C Plus Plus:: GDB features for C++
* Decimal Floating Point:: Numbers in Decimal Floating Point format

File: gdb.info, Node: C Operators, Next: C Constants, Up: C
15.4.1.1 C and C++ 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 C and C++, the following definitions hold:
* _Integral types_ include 'int' with any of its storage-class
specifiers; 'char'; 'enum'; and, for C++, 'bool'.
* _Floating-point types_ include 'float', 'double', and 'long double'
(if supported by the target platform).
* _Pointer types_ include all types defined as '(TYPE *)'.
* _Scalar types_ include all of the above.
The following operators are supported. They are listed here in order of
increasing precedence:
','
The comma or sequencing operator. Expressions in a comma-separated
list are evaluated from left to right, with the result of the
entire expression being the last expression evaluated.
'='
Assignment. The value of an assignment expression is the value
assigned. Defined on scalar types.
'OP='
Used in an expression of the form 'A OP= B', and translated to
'A = A OP B'. 'OP=' and '=' have the same precedence. The
operator OP is any one of the operators '|', '^', '&', '<<', '>>',
'+', '-', '*', '/', '%'.
'?:'
The ternary operator. 'A ? B : C' can be thought of as: if A then
B else C. The argument A should be of an integral type.
'||'
Logical OR. Defined on integral types.
'&&'
Logical AND. Defined on integral types.
'|'
Bitwise OR. Defined on integral types.
'^'
Bitwise exclusive-OR. Defined on integral types.
'&'
Bitwise AND. Defined on integral types.
'==, !='
Equality and inequality. Defined on scalar types. The value of
these expressions is 0 for false and non-zero for true.
'<, >, <=, >='
Less than, greater than, less than or equal, greater than or equal.
Defined on scalar types. The value of these expressions is 0 for
false and non-zero for true.
'<<, >>'
left shift, and right shift. Defined on integral types.
'@'
The GDB "artificial array" operator (*note Expressions:
Expressions.).
'+, -'
Addition and subtraction. Defined on integral types,
floating-point types and pointer types.
'*, /, %'
Multiplication, division, and modulus. Multiplication and division
are defined on integral and floating-point types. Modulus is
defined on integral types.
'++, --'
Increment and decrement. When appearing before a variable, the
operation is performed before the variable is used in an
expression; when appearing after it, the variable's value is used
before the operation takes place.
'*'
Pointer dereferencing. Defined on pointer types. Same precedence
as '++'.
'&'
Address operator. Defined on variables. Same precedence as '++'.
For debugging C++, GDB implements a use of '&' beyond what is
allowed in the C++ language itself: you can use '&(&REF)' to
examine the address where a C++ reference variable (declared with
'&REF') is stored.
'-'
Negative. Defined on integral and floating-point types. Same
precedence as '++'.
'!'
Logical negation. Defined on integral types. Same precedence as
'++'.
'~'
Bitwise complement operator. Defined on integral types. Same
precedence as '++'.
'., ->'
Structure member, and pointer-to-structure member. For
convenience, GDB regards the two as equivalent, choosing whether to
dereference a pointer based on the stored type information.
Defined on 'struct' and 'union' data.
'.*, ->*'
Dereferences of pointers to members.
'[]'
Array indexing. 'A[I]' is defined as '*(A+I)'. Same precedence as
'->'.
'()'
Function parameter list. Same precedence as '->'.
'::'
C++ scope resolution operator. Defined on 'struct', 'union', and
'class' types.
'::'
Doubled colons also represent the GDB scope operator (*note
Expressions: Expressions.). Same precedence as '::', above.
If an operator is redefined in the user code, GDB usually attempts to
invoke the redefined version instead of using the operator's predefined
meaning.

File: gdb.info, Node: C Constants, Next: C Plus Plus Expressions, Prev: C Operators, Up: C
15.4.1.2 C and C++ Constants
............................
GDB allows you to express the constants of C and C++ in the following
ways:
* Integer constants are a sequence of digits. Octal constants are
specified by a leading '0' (i.e. zero), and hexadecimal constants
by a leading '0x' or '0X'. Constants may also end with a letter
'l', specifying that the constant should be treated as a 'long'
value.
* Floating point constants are a sequence of digits, followed by a
decimal point, followed by a sequence of digits, and optionally
followed by an exponent. An exponent is of the form:
'e[[+]|-]NNN', where NNN is another sequence of digits. The '+' is
optional for positive exponents. A floating-point constant may
also end with a letter 'f' or 'F', specifying that the constant
should be treated as being of the 'float' (as opposed to the
default 'double') type; or with a letter 'l' or 'L', which
specifies a 'long double' constant.
* Enumerated constants consist of enumerated identifiers, or their
integral equivalents.
* Character constants are a single character surrounded by single
quotes ('''), or a number--the ordinal value of the corresponding
character (usually its ASCII value). Within quotes, the single
character may be represented by a letter or by "escape sequences",
which are of the form '\NNN', where NNN is the octal representation
of the character's ordinal value; or of the form '\X', where 'X' is
a predefined special character--for example, '\n' for newline.
Wide character constants can be written by prefixing a character
constant with 'L', as in C. For example, 'L'x'' is the wide form of
'x'. The target wide character set is used when computing the
value of this constant (*note Character Sets::).
* String constants are a sequence of character constants surrounded
by double quotes ('"'). Any valid character constant (as described
above) may appear. Double quotes within the string must be
preceded by a backslash, so for instance '"a\"b'c"' is a string of
five characters.
Wide string constants can be written by prefixing a string constant
with 'L', as in C. The target wide character set is used when
computing the value of this constant (*note Character Sets::).
* Pointer constants are an integral value. You can also write
pointers to constants using the C operator '&'.
* Array constants are comma-separated lists surrounded by braces '{'
and '}'; for example, '{1,2,3}' is a three-element array of
integers, '{{1,2}, {3,4}, {5,6}}' is a three-by-two array, and
'{&"hi", &"there", &"fred"}' is a three-element array of pointers.

File: gdb.info, Node: C Plus Plus Expressions, Next: C Defaults, Prev: C Constants, Up: C
15.4.1.3 C++ Expressions
........................
GDB expression handling can interpret most C++ expressions.
_Warning:_ GDB can only debug C++ code if you use the proper
compiler and the proper debug format. Currently, GDB works best
when debugging C++ code that is compiled with the most recent
version of GCC possible. The DWARF debugging format is preferred;
GCC defaults to this on most popular platforms. Other compilers
and/or debug formats are likely to work badly or not at all when
using GDB to debug C++ code. *Note Compilation::.
1. Member function calls are allowed; you can use expressions like
count = aml->GetOriginal(x, y)
2. While a member function is active (in the selected stack frame),
your expressions have the same namespace available as the member
function; that is, GDB allows implicit references to the class
instance pointer 'this' following the same rules as C++. 'using'
declarations in the current scope are also respected by GDB.
3. You can call overloaded functions; GDB resolves the function call
to the right definition, with some restrictions. GDB does not
perform overload resolution involving user-defined type
conversions, calls to constructors, or instantiations of templates
that do not exist in the program. It also cannot handle ellipsis
argument lists or default arguments.
It does perform integral conversions and promotions, floating-point
promotions, arithmetic conversions, pointer conversions,
conversions of class objects to base classes, and standard
conversions such as those of functions or arrays to pointers; it
requires an exact match on the number of function arguments.
Overload resolution is always performed, unless you have specified
'set overload-resolution off'. *Note GDB Features for C++:
Debugging C Plus Plus.
You must specify 'set overload-resolution off' in order to use an
explicit function signature to call an overloaded function, as in
p 'foo(char,int)'('x', 13)
The GDB command-completion facility can simplify this; see *note
Command Completion: Completion.
4. GDB understands variables declared as C++ references; you can use
them in expressions just as you do in C++ source--they are
automatically dereferenced.
In the parameter list shown when GDB displays a frame, the values
of reference variables are not displayed (unlike other variables);
this avoids clutter, since references are often used for large
structures. The _address_ of a reference variable is always shown,
unless you have specified 'set print address off'.
5. GDB supports the C++ name resolution operator '::'--your
expressions can use it just as expressions in your program do.
Since one scope may be defined in another, you can use '::'
repeatedly if necessary, for example in an expression like
'SCOPE1::SCOPE2::NAME'. GDB also allows resolving name scope by
reference to source files, in both C and C++ debugging (*note
Program Variables: Variables.).
6. GDB performs argument-dependent lookup, following the C++
specification.

File: gdb.info, Node: C Defaults, Next: C Checks, Prev: C Plus Plus Expressions, Up: C
15.4.1.4 C and C++ Defaults
...........................
If you allow GDB to set range checking automatically, it defaults to
'off' whenever the working language changes to C or C++. This happens
regardless of whether you or GDB selects the working language.
If you allow GDB to set the language automatically, it recognizes
source files whose names end with '.c', '.C', or '.cc', etc, and when
GDB enters code compiled from one of these files, it sets the working
language to C or C++. *Note Having GDB Infer the Source Language:
Automatically, for further details.

File: gdb.info, Node: C Checks, Next: Debugging C, Prev: C Defaults, Up: C
15.4.1.5 C and C++ Type and Range Checks
........................................
By default, when GDB parses C or C++ expressions, strict type checking
is used. However, if you turn type checking off, GDB will allow certain
non-standard conversions, such as promoting integer constants to
pointers.
Range checking, if turned on, is done on mathematical operations.
Array indices are not checked, since they are often used to index a
pointer that is not itself an array.

File: gdb.info, Node: Debugging C, Next: Debugging C Plus Plus, Prev: C Checks, Up: C
15.4.1.6 GDB and C
..................
The 'set print union' and 'show print union' commands apply to the
'union' type. When set to 'on', any 'union' that is inside a 'struct'
or 'class' is also printed. Otherwise, it appears as '{...}'.
The '@' operator aids in the debugging of dynamic arrays, formed with
pointers and a memory allocation function. *Note Expressions:
Expressions.

File: gdb.info, Node: Debugging C Plus Plus, Next: Decimal Floating Point, Prev: Debugging C, Up: C
15.4.1.7 GDB Features for C++
.............................
Some GDB commands are particularly useful with C++, and some are
designed specifically for use with C++. Here is a summary:
'breakpoint menus'
When you want a breakpoint in a function whose name is overloaded,
GDB has the capability to display a menu of possible breakpoint
locations to help you specify which function definition you want.
*Note Ambiguous Expressions: Ambiguous Expressions.
'rbreak REGEX'
Setting breakpoints using regular expressions is helpful for
setting breakpoints on overloaded functions that are not members of
any special classes. *Note Setting Breakpoints: Set Breaks.
'catch throw'
'catch rethrow'
'catch catch'
Debug C++ exception handling using these commands. *Note Setting
Catchpoints: Set Catchpoints.
'ptype TYPENAME'
Print inheritance relationships as well as other information for
type TYPENAME. *Note Examining the Symbol Table: Symbols.
'info vtbl EXPRESSION.'
The 'info vtbl' command can be used to display the virtual method
tables of the object computed by EXPRESSION. This shows one entry
per virtual table; there may be multiple virtual tables when
multiple inheritance is in use.
'demangle NAME'
Demangle NAME. *Note Symbols::, for a more complete description of
the 'demangle' command.
'set print demangle'
'show print demangle'
'set print asm-demangle'
'show print asm-demangle'
Control whether C++ symbols display in their source form, both when
displaying code as C++ source and when displaying disassemblies.
*Note Print Settings: Print Settings.
'set print object'
'show print object'
Choose whether to print derived (actual) or declared types of
objects. *Note Print Settings: Print Settings.
'set print vtbl'
'show print vtbl'
Control the format for printing virtual function tables. *Note
Print Settings: Print Settings. (The 'vtbl' commands do not work
on programs compiled with the HP ANSI C++ compiler ('aCC').)
'set overload-resolution on'
Enable overload resolution for C++ expression evaluation. The
default is on. For overloaded functions, GDB evaluates the
arguments and searches for a function whose signature matches the
argument types, using the standard C++ conversion rules (see *note
C++ Expressions: C Plus Plus Expressions, for details). If it
cannot find a match, it emits a message.
'set overload-resolution off'
Disable overload resolution for C++ expression evaluation. For
overloaded functions that are not class member functions, GDB
chooses the first function of the specified name that it finds in
the symbol table, whether or not its arguments are of the correct
type. For overloaded functions that are class member functions,
GDB searches for a function whose signature _exactly_ matches the
argument types.
'show overload-resolution'
Show the current setting of overload resolution.
'Overloaded symbol names'
You can specify a particular definition of an overloaded symbol,
using the same notation that is used to declare such symbols in
C++: type 'SYMBOL(TYPES)' rather than just SYMBOL. You can also
use the GDB command-line word completion facilities to list the
available choices, or to finish the type list for you. *Note
Command Completion: Completion, for details on how to do this.

File: gdb.info, Node: Decimal Floating Point, Prev: Debugging C Plus Plus, Up: C
15.4.1.8 Decimal Floating Point format
......................................
GDB can examine, set and perform computations with numbers in decimal
floating point format, which in the C language correspond to the
'_Decimal32', '_Decimal64' and '_Decimal128' types as specified by the
extension to support decimal floating-point arithmetic.
There are two encodings in use, depending on the architecture: BID
(Binary Integer Decimal) for x86 and x86-64, and DPD (Densely Packed
Decimal) for PowerPC and S/390. GDB will use the appropriate encoding
for the configured target.
Because of a limitation in 'libdecnumber', the library used by GDB to
manipulate decimal floating point numbers, it is not possible to convert
(using a cast, for example) integers wider than 32-bit to decimal float.
In addition, in order to imitate GDB's behaviour with binary floating
point computations, error checking in decimal float operations ignores
underflow, overflow and divide by zero exceptions.
In the PowerPC architecture, GDB provides a set of pseudo-registers
to inspect '_Decimal128' values stored in floating point registers. See
*note PowerPC: PowerPC. for more details.

File: gdb.info, Node: D, Next: Go, Prev: C, Up: Supported Languages
15.4.2 D
--------
GDB can be used to debug programs written in D and compiled with GDC,
LDC or DMD compilers. Currently GDB supports only one D specific
feature -- dynamic arrays.

File: gdb.info, Node: Go, Next: Objective-C, Prev: D, Up: Supported Languages
15.4.3 Go
---------
GDB can be used to debug programs written in Go and compiled with
'gccgo' or '6g' compilers.
Here is a summary of the Go-specific features and restrictions:
'The current Go package'
The name of the current package does not need to be specified when
specifying global variables and functions.
For example, given the program:
package main
var myglob = "Shall we?"
func main () {
// ...
}
When stopped inside 'main' either of these work:
(gdb) p myglob
(gdb) p main.myglob
'Builtin Go types'
The 'string' type is recognized by GDB and is printed as a string.
'Builtin Go functions'
The GDB expression parser recognizes the 'unsafe.Sizeof' function
and handles it internally.
'Restrictions on Go expressions'
All Go operators are supported except '&^'. The Go '_' "blank
identifier" is not supported. Automatic dereferencing of pointers
is not supported.

File: gdb.info, Node: Objective-C, Next: OpenCL C, Prev: Go, Up: Supported Languages
15.4.4 Objective-C
------------------
This section provides information about some commands and command
options that are useful for debugging Objective-C code. See also *note
info classes: Symbols, and *note info selectors: Symbols, for a few more
commands specific to Objective-C support.
* Menu:
* Method Names in Commands::
* The Print Command with Objective-C::

File: gdb.info, Node: Method Names in Commands, Next: The Print Command with Objective-C, Up: Objective-C
15.4.4.1 Method Names in Commands
.................................
The following commands have been extended to accept Objective-C method
names as line specifications:
* 'clear'
* 'break'
* 'info line'
* 'jump'
* 'list'
A fully qualified Objective-C method name is specified as
-[CLASS METHODNAME]
where the minus sign is used to indicate an instance method and a
plus sign (not shown) is used to indicate a class method. The class
name CLASS and method name METHODNAME are enclosed in brackets, similar
to the way messages are specified in Objective-C source code. For
example, to set a breakpoint at the 'create' instance method of class
'Fruit' in the program currently being debugged, enter:
break -[Fruit create]
To list ten program lines around the 'initialize' class method,
enter:
list +[NSText initialize]
In the current version of GDB, the plus or minus sign is required.
In future versions of GDB, the plus or minus sign will be optional, but
you can use it to narrow the search. It is also possible to specify
just a method name:
break create
You must specify the complete method name, including any colons. If
your program's source files contain more than one 'create' method,
you'll be presented with a numbered list of classes that implement that
method. Indicate your choice by number, or type '0' to exit if none
apply.
As another example, to clear a breakpoint established at the
'makeKeyAndOrderFront:' method of the 'NSWindow' class, enter:
clear -[NSWindow makeKeyAndOrderFront:]

File: gdb.info, Node: The Print Command with Objective-C, Prev: Method Names in Commands, Up: Objective-C
15.4.4.2 The Print Command With Objective-C
...........................................
The print command has also been extended to accept methods. For
example:
print -[OBJECT hash]
will tell GDB to send the 'hash' message to OBJECT and print the result.
Also, an additional command has been added, 'print-object' or 'po' for
short, which is meant to print the description of an object. However,
this command may only work with certain Objective-C libraries that have
a particular hook function, '_NSPrintForDebugger', defined.

File: gdb.info, Node: OpenCL C, Next: Fortran, Prev: Objective-C, Up: Supported Languages
15.4.5 OpenCL C
---------------
This section provides information about GDBs OpenCL C support.
* Menu:
* OpenCL C Datatypes::
* OpenCL C Expressions::
* OpenCL C Operators::

File: gdb.info, Node: OpenCL C Datatypes, Next: OpenCL C Expressions, Up: OpenCL C
15.4.5.1 OpenCL C Datatypes
...........................
GDB supports the builtin scalar and vector datatypes specified by OpenCL
1.1. In addition the half- and double-precision floating point data
types of the 'cl_khr_fp16' and 'cl_khr_fp64' OpenCL extensions are also
known to GDB.

File: gdb.info, Node: OpenCL C Expressions, Next: OpenCL C Operators, Prev: OpenCL C Datatypes, Up: OpenCL C
15.4.5.2 OpenCL C Expressions
.............................
GDB supports accesses to vector components including the access as
lvalue where possible. Since OpenCL C is based on C99 most C
expressions supported by GDB can be used as well.

File: gdb.info, Node: OpenCL C Operators, Prev: OpenCL C Expressions, Up: OpenCL C
15.4.5.3 OpenCL C Operators
...........................
GDB supports the operators specified by OpenCL 1.1 for scalar and vector
data types.

File: gdb.info, Node: Fortran, Next: Pascal, Prev: OpenCL C, Up: Supported Languages
15.4.6 Fortran
--------------
GDB can be used to debug programs written in Fortran, but it currently
supports only the features of Fortran 77 language.
Some Fortran compilers (GNU Fortran 77 and Fortran 95 compilers among
them) append an underscore to the names of variables and functions.
When you debug programs compiled by those compilers, you will need to
refer to variables and functions with a trailing underscore.
* Menu:
* Fortran Operators:: Fortran operators and expressions
* Fortran Defaults:: Default settings for Fortran
* Special Fortran Commands:: Special GDB commands for Fortran

File: gdb.info, Node: Fortran Operators, Next: Fortran Defaults, Up: Fortran
15.4.6.1 Fortran Operators and Expressions
..........................................
Operators must be defined on values of specific types. For instance,
'+' is defined on numbers, but not on characters or other non-
arithmetic types. Operators are often defined on groups of types.
'**'
The exponentiation operator. It raises the first operand to the
power of the second one.
':'
The range operator. Normally used in the form of array(low:high)
to represent a section of array.
'%'
The access component operator. Normally used to access elements in
derived types. Also suitable for unions. As unions aren't part of
regular Fortran, this can only happen when accessing a register
that uses a gdbarch-defined union type.

File: gdb.info, Node: Fortran Defaults, Next: Special Fortran Commands, Prev: Fortran Operators, Up: Fortran
15.4.6.2 Fortran Defaults
.........................
Fortran symbols are usually case-insensitive, so GDB by default uses
case-insensitive matches for Fortran symbols. You can change that with
the 'set case-insensitive' command, see *note Symbols::, for the
details.

File: gdb.info, Node: Special Fortran Commands, Prev: Fortran Defaults, Up: Fortran
15.4.6.3 Special Fortran Commands
.................................
GDB has some commands to support Fortran-specific features, such as
displaying common blocks.
'info common [COMMON-NAME]'
This command prints the values contained in the Fortran 'COMMON'
block whose name is COMMON-NAME. With no argument, the names of
all 'COMMON' blocks visible at the current program location are
printed.

File: gdb.info, Node: Pascal, Next: Modula-2, 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: Modula-2, Next: Ada, Prev: Pascal, Up: Supported Languages
15.4.8 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
15.4.8.1 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
15.4.8.2 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
15.4.8.3 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
15.4.8.4 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
15.4.8.5 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
15.4.8.6 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
15.4.8.7 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
15.4.8.8 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
15.4.8.9 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
15.4.9 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.
* 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 Glitches:: Known peculiarities of Ada mode.

File: gdb.info, Node: Ada Mode Intro, Next: Omissions from Ada, Up: Ada
15.4.9.1 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).
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.

File: gdb.info, Node: Omissions from Ada, Next: Additions to Ada, Prev: Ada Mode Intro, Up: Ada
15.4.9.2 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: Stopping Before Main Program, Prev: Omissions from Ada, Up: Ada
15.4.9.3 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: Stopping Before Main Program, Next: Ada Exceptions, Prev: Additions to Ada, Up: Ada
15.4.9.4 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
15.4.9.5 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
15.4.9.6 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 detailled 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: 0x807f378
Parent: 1 (main_task)
Base Priority: 15
State: Runnable
'task'
This command prints the ID 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 t
(gdb) task
[Current task is 2]
'task TASKNO'
This command is like the 'thread THREADNO' 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 t
(gdb) task 1
[Switching to task 1]
#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 LINESPEC task TASKNO'
'break LINESPEC task TASKNO if ...'
These commands are like the 'break ... thread ...' command (*note
Thread Stops::). The LINESPEC 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
15.4.9.7 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 Glitches, Prev: Ada Tasks and Core Files, Up: Ada
15.4.9.8 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 Glitches, Prev: Ravenscar Profile, Up: Ada
15.4.9.9 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 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'.
'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
$1 = <incomplete type>
"Incomplete type" is C terminology for data types that are not
completely specified.
'info types REGEXP'
'info types'
Print a brief description of all types whose names match the
regular expression REGEXP (or all types in your program, if you
supply no argument). Each complete typename is matched as though
it were a complete line; thus, 'i type value' gives information on
all types in your program whose names include the string 'value',
but 'i type ^value$' gives information only on types whose complete
name is 'value'.
This command differs from 'ptype' in two ways: first, like
'whatis', it does not print a detailed description; second, it
lists all source files where a type is defined.
'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 functions'
Print the names and data types of all defined functions.
'info functions REGEXP'
Print the names and data types of all defined functions whose names
contain a match for 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.
'info variables'
Print the names and data types of all variables that are defined
outside of functions (i.e. excluding local variables).
'info variables REGEXP'
Print the names and data types of all variables (except for local
variables) whose names contain a match for regular expression
REGEXP.
'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 FILENAME'
'maint print psymbols FILENAME'
'maint print msymbols FILENAME'
Write a dump of debugging symbol data into the file FILENAME.
These commands are used to debug the GDB symbol-reading code. Only
symbols with debugging data are included. If you use 'maint print
symbols', GDB includes all the symbols for which it has already
collected full details: that is, FILENAME reflects symbols for only
those files whose symbols GDB has read. 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' dumps just the minimal symbol information
required for each object file from which GDB has read some 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 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 LINESPEC'
'j LINESPEC'
'jump LOCATION'
'j LOCATION'
Resume execution at line LINESPEC or at address given by LOCATION.
Execution stops again immediately if there is a breakpoint there.
*Note Specify Location::, for a description of the different forms
of LINESPEC and 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 line LINESPEC 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.