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* gprof: (gprof). Profiling your program's execution
This file documents the gprof profiler of the GNU system.
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File:, Node: Top, Next: Introduction, Up: (dir)
Profiling a Program: Where Does It Spend Its Time?
This manual describes the GNU profiler, `gprof', and how you can use it
to determine which parts of a program are taking most of the execution
time. We assume that you know how to write, compile, and execute
programs. GNU `gprof' was written by Jay Fenlason.
This manual is for `gprof' (GNU Binutils) version 2.21.
This document is distributed under the terms of the GNU Free
Documentation License version 1.3. A copy of the license is included
in the section entitled "GNU Free Documentation License".
* Menu:
* Introduction:: What profiling means, and why it is useful.
* Compiling:: How to compile your program for profiling.
* Executing:: Executing your program to generate profile data
* Invoking:: How to run `gprof', and its options
* Output:: Interpreting `gprof''s output
* Inaccuracy:: Potential problems you should be aware of
* How do I?:: Answers to common questions
* Incompatibilities:: (between GNU `gprof' and Unix `gprof'.)
* Details:: Details of how profiling is done
* GNU Free Documentation License:: GNU Free Documentation License

File:, Node: Introduction, Next: Compiling, Prev: Top, Up: Top
1 Introduction to Profiling
Profiling allows you to learn where your program spent its time and
which functions called which other functions while it was executing.
This information can show you which pieces of your program are slower
than you expected, and might be candidates for rewriting to make your
program execute faster. It can also tell you which functions are being
called more or less often than you expected. This may help you spot
bugs that had otherwise been unnoticed.
Since the profiler uses information collected during the actual
execution of your program, it can be used on programs that are too
large or too complex to analyze by reading the source. However, how
your program is run will affect the information that shows up in the
profile data. If you don't use some feature of your program while it
is being profiled, no profile information will be generated for that
Profiling has several steps:
* You must compile and link your program with profiling enabled.
*Note Compiling a Program for Profiling: Compiling.
* You must execute your program to generate a profile data file.
*Note Executing the Program: Executing.
* You must run `gprof' to analyze the profile data. *Note `gprof'
Command Summary: Invoking.
The next three chapters explain these steps in greater detail.
Several forms of output are available from the analysis.
The "flat profile" shows how much time your program spent in each
function, and how many times that function was called. If you simply
want to know which functions burn most of the cycles, it is stated
concisely here. *Note The Flat Profile: Flat Profile.
The "call graph" shows, for each function, which functions called
it, which other functions it called, and how many times. There is also
an estimate of how much time was spent in the subroutines of each
function. This can suggest places where you might try to eliminate
function calls that use a lot of time. *Note The Call Graph: Call
The "annotated source" listing is a copy of the program's source
code, labeled with the number of times each line of the program was
executed. *Note The Annotated Source Listing: Annotated Source.
To better understand how profiling works, you may wish to read a
description of its implementation. *Note Implementation of Profiling:

File:, Node: Compiling, Next: Executing, Prev: Introduction, Up: Top
2 Compiling a Program for Profiling
The first step in generating profile information for your program is to
compile and link it with profiling enabled.
To compile a source file for profiling, specify the `-pg' option when
you run the compiler. (This is in addition to the options you normally
To link the program for profiling, if you use a compiler such as `cc'
to do the linking, simply specify `-pg' in addition to your usual
options. The same option, `-pg', alters either compilation or linking
to do what is necessary for profiling. Here are examples:
cc -g -c myprog.c utils.c -pg
cc -o myprog myprog.o utils.o -pg
The `-pg' option also works with a command that both compiles and
cc -o myprog myprog.c utils.c -g -pg
Note: The `-pg' option must be part of your compilation options as
well as your link options. If it is not then no call-graph data will
be gathered and when you run `gprof' you will get an error message like
gprof: gmon.out file is missing call-graph data
If you add the `-Q' switch to suppress the printing of the call
graph data you will still be able to see the time samples:
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls Ts/call Ts/call name
44.12 0.07 0.07 zazLoop
35.29 0.14 0.06 main
20.59 0.17 0.04 bazMillion
If you run the linker `ld' directly instead of through a compiler
such as `cc', you may have to specify a profiling startup file
`gcrt0.o' as the first input file instead of the usual startup file
`crt0.o'. In addition, you would probably want to specify the
profiling C library, `libc_p.a', by writing `-lc_p' instead of the
usual `-lc'. This is not absolutely necessary, but doing this gives
you number-of-calls information for standard library functions such as
`read' and `open'. For example:
ld -o myprog /lib/gcrt0.o myprog.o utils.o -lc_p
If you are running the program on a system which supports shared
libraries you may run into problems with the profiling support code in
a shared library being called before that library has been fully
initialised. This is usually detected by the program encountering a
segmentation fault as soon as it is run. The solution is to link
against a static version of the library containing the profiling
support code, which for `gcc' users can be done via the `-static' or
`-static-libgcc' command line option. For example:
gcc -g -pg -static-libgcc myprog.c utils.c -o myprog
If you compile only some of the modules of the program with `-pg',
you can still profile the program, but you won't get complete
information about the modules that were compiled without `-pg'. The
only information you get for the functions in those modules is the
total time spent in them; there is no record of how many times they
were called, or from where. This will not affect the flat profile
(except that the `calls' field for the functions will be blank), but
will greatly reduce the usefulness of the call graph.
If you wish to perform line-by-line profiling you should use the
`gcov' tool instead of `gprof'. See that tool's manual or info pages
for more details of how to do this.
Note, older versions of `gcc' produce line-by-line profiling
information that works with `gprof' rather than `gcov' so there is
still support for displaying this kind of information in `gprof'. *Note
Line-by-line Profiling: Line-by-line.
It also worth noting that `gcc' implements a
`-finstrument-functions' command line option which will insert calls to
special user supplied instrumentation routines at the entry and exit of
every function in their program. This can be used to implement an
alternative profiling scheme.

File:, Node: Executing, Next: Invoking, Prev: Compiling, Up: Top
3 Executing the Program
Once the program is compiled for profiling, you must run it in order to
generate the information that `gprof' needs. Simply run the program as
usual, using the normal arguments, file names, etc. The program should
run normally, producing the same output as usual. It will, however, run
somewhat slower than normal because of the time spent collecting and
writing the profile data.
The way you run the program--the arguments and input that you give
it--may have a dramatic effect on what the profile information shows.
The profile data will describe the parts of the program that were
activated for the particular input you use. For example, if the first
command you give to your program is to quit, the profile data will show
the time used in initialization and in cleanup, but not much else.
Your program will write the profile data into a file called
`gmon.out' just before exiting. If there is already a file called
`gmon.out', its contents are overwritten. There is currently no way to
tell the program to write the profile data under a different name, but
you can rename the file afterwards if you are concerned that it may be
In order to write the `gmon.out' file properly, your program must
exit normally: by returning from `main' or by calling `exit'. Calling
the low-level function `_exit' does not write the profile data, and
neither does abnormal termination due to an unhandled signal.
The `gmon.out' file is written in the program's _current working
directory_ at the time it exits. This means that if your program calls
`chdir', the `gmon.out' file will be left in the last directory your
program `chdir''d to. If you don't have permission to write in this
directory, the file is not written, and you will get an error message.
Older versions of the GNU profiling library may also write a file
called `bb.out'. This file, if present, contains an human-readable
listing of the basic-block execution counts. Unfortunately, the
appearance of a human-readable `bb.out' means the basic-block counts
didn't get written into `gmon.out'. The Perl script `',
included with the `gprof' source distribution, will convert a `bb.out'
file into a format readable by `gprof'. Invoke it like this: < bb.out > BH-DATA
This translates the information in `bb.out' into a form that `gprof'
can understand. But you still need to tell `gprof' about the existence
of this translated information. To do that, include BB-DATA on the
`gprof' command line, _along with `gmon.out'_, like this:

File:, Node: Invoking, Next: Output, Prev: Executing, Up: Top
4 `gprof' Command Summary
After you have a profile data file `gmon.out', you can run `gprof' to
interpret the information in it. The `gprof' program prints a flat
profile and a call graph on standard output. Typically you would
redirect the output of `gprof' into a file with `>'.
You run `gprof' like this:
Here square-brackets indicate optional arguments.
If you omit the executable file name, the file `a.out' is used. If
you give no profile data file name, the file `gmon.out' is used. If
any file is not in the proper format, or if the profile data file does
not appear to belong to the executable file, an error message is
You can give more than one profile data file by entering all their
names after the executable file name; then the statistics in all the
data files are summed together.
The order of these options does not matter.
* Menu:
* Output Options:: Controlling `gprof''s output style
* Analysis Options:: Controlling how `gprof' analyzes its data
* Miscellaneous Options::
* Deprecated Options:: Options you no longer need to use, but which
have been retained for compatibility
* Symspecs:: Specifying functions to include or exclude

File:, Node: Output Options, Next: Analysis Options, Up: Invoking
4.1 Output Options
These options specify which of several output formats `gprof' should
Many of these options take an optional "symspec" to specify
functions to be included or excluded. These options can be specified
multiple times, with different symspecs, to include or exclude sets of
symbols. *Note Symspecs: Symspecs.
Specifying any of these options overrides the default (`-p -q'),
which prints a flat profile and call graph analysis for all functions.
The `-A' option causes `gprof' to print annotated source code. If
SYMSPEC is specified, print output only for matching symbols.
*Note The Annotated Source Listing: Annotated Source.
If the `-b' option is given, `gprof' doesn't print the verbose
blurbs that try to explain the meaning of all of the fields in the
tables. This is useful if you intend to print out the output, or
are tired of seeing the blurbs.
The `-C' option causes `gprof' to print a tally of functions and
the number of times each was called. If SYMSPEC is specified,
print tally only for matching symbols.
If the profile data file contains basic-block count records,
specifying the `-l' option, along with `-C', will cause basic-block
execution counts to be tallied and displayed.
The `-i' option causes `gprof' to display summary information
about the profile data file(s) and then exit. The number of
histogram, call graph, and basic-block count records is displayed.
The `-I' option specifies a list of search directories in which to
find source files. Environment variable GPROF_PATH can also be
used to convey this information. Used mostly for annotated source
The `-J' option causes `gprof' not to print annotated source code.
If SYMSPEC is specified, `gprof' prints annotated source, but
excludes matching symbols.
Normally, source filenames are printed with the path component
suppressed. The `-L' option causes `gprof' to print the full
pathname of source filenames, which is determined from symbolic
debugging information in the image file and is relative to the
directory in which the compiler was invoked.
The `-p' option causes `gprof' to print a flat profile. If
SYMSPEC is specified, print flat profile only for matching symbols.
*Note The Flat Profile: Flat Profile.
The `-P' option causes `gprof' to suppress printing a flat profile.
If SYMSPEC is specified, `gprof' prints a flat profile, but
excludes matching symbols.
The `-q' option causes `gprof' to print the call graph analysis.
If SYMSPEC is specified, print call graph only for matching symbols
and their children. *Note The Call Graph: Call Graph.
The `-Q' option causes `gprof' to suppress printing the call graph.
If SYMSPEC is specified, `gprof' prints a call graph, but excludes
matching symbols.
The `-t' option causes the NUM most active source lines in each
source file to be listed when source annotation is enabled. The
default is 10.
This option affects annotated source output only. Normally,
`gprof' prints annotated source files to standard-output. If this
option is specified, annotated source for a file named
`path/FILENAME' is generated in the file `FILENAME-ann'. If the
underlying file system would truncate `FILENAME-ann' so that it
overwrites the original `FILENAME', `gprof' generates annotated
source in the file `FILENAME.ann' instead (if the original file
name has an extension, that extension is _replaced_ with `.ann').
The `-Z' option causes `gprof' not to print a tally of functions
and the number of times each was called. If SYMSPEC is specified,
print tally, but exclude matching symbols.
The `--function-ordering' option causes `gprof' to print a
suggested function ordering for the program based on profiling
data. This option suggests an ordering which may improve paging,
tlb and cache behavior for the program on systems which support
arbitrary ordering of functions in an executable.
The exact details of how to force the linker to place functions in
a particular order is system dependent and out of the scope of this
`--file-ordering MAP_FILE'
The `--file-ordering' option causes `gprof' to print a suggested
.o link line ordering for the program based on profiling data.
This option suggests an ordering which may improve paging, tlb and
cache behavior for the program on systems which do not support
arbitrary ordering of functions in an executable.
Use of the `-a' argument is highly recommended with this option.
The MAP_FILE argument is a pathname to a file which provides
function name to object file mappings. The format of the file is
similar to the output of the program `nm'.
c-parse.o:00000000 T yyparse
c-parse.o:00000004 C yyerrflag
c-lang.o:00000000 T maybe_objc_method_name
c-lang.o:00000000 T print_lang_statistics
c-lang.o:00000000 T recognize_objc_keyword
c-decl.o:00000000 T print_lang_identifier
c-decl.o:00000000 T print_lang_type
To create a MAP_FILE with GNU `nm', type a command like `nm
--extern-only --defined-only -v --print-file-name program-name'.
The `-T' option causes `gprof' to print its output in
"traditional" BSD style.
`-w WIDTH'
Sets width of output lines to WIDTH. Currently only used when
printing the function index at the bottom of the call graph.
This option affects annotated source output only. By default,
only the lines at the beginning of a basic-block are annotated.
If this option is specified, every line in a basic-block is
annotated by repeating the annotation for the first line. This
behavior is similar to `tcov''s `-a'.
These options control whether C++ symbol names should be demangled
when printing output. The default is to demangle symbols. The
`--no-demangle' option may be used to turn off demangling.
Different compilers have different mangling styles. The optional
demangling style argument can be used to choose an appropriate
demangling style for your compiler.

File:, Node: Analysis Options, Next: Miscellaneous Options, Prev: Output Options, Up: Invoking
4.2 Analysis Options
The `-a' option causes `gprof' to suppress the printing of
statically declared (private) functions. (These are functions
whose names are not listed as global, and which are not visible
outside the file/function/block where they were defined.) Time
spent in these functions, calls to/from them, etc., will all be
attributed to the function that was loaded directly before it in
the executable file. This option affects both the flat profile
and the call graph.
The `-c' option causes the call graph of the program to be
augmented by a heuristic which examines the text space of the
object file and identifies function calls in the binary machine
code. Since normal call graph records are only generated when
functions are entered, this option identifies children that could
have been called, but never were. Calls to functions that were
not compiled with profiling enabled are also identified, but only
if symbol table entries are present for them. Calls to dynamic
library routines are typically _not_ found by this option.
Parents or children identified via this heuristic are indicated in
the call graph with call counts of `0'.
The `-D' option causes `gprof' to ignore symbols which are not
known to be functions. This option will give more accurate
profile data on systems where it is supported (Solaris and HPUX for
`-k FROM/TO'
The `-k' option allows you to delete from the call graph any arcs
from symbols matching symspec FROM to those matching symspec TO.
The `-l' option enables line-by-line profiling, which causes
histogram hits to be charged to individual source code lines,
instead of functions. This feature only works with programs
compiled by older versions of the `gcc' compiler. Newer versions
of `gcc' are designed to work with the `gcov' tool instead.
If the program was compiled with basic-block counting enabled,
this option will also identify how many times each line of code
was executed. While line-by-line profiling can help isolate where
in a large function a program is spending its time, it also
significantly increases the running time of `gprof', and magnifies
statistical inaccuracies. *Note Statistical Sampling Error:
Sampling Error.
`-m NUM'
This option affects execution count output only. Symbols that are
executed less than NUM times are suppressed.
The `-n' option causes `gprof', in its call graph analysis, to
only propagate times for symbols matching SYMSPEC.
The `-n' option causes `gprof', in its call graph analysis, not to
propagate times for symbols matching SYMSPEC.
The `-S' option causes `gprof' to read an external symbol table
file, such as `/proc/kallsyms', rather than read the symbol table
from the given object file (the default is `a.out'). This is useful
for profiling kernel modules.
If you give the `-z' option, `gprof' will mention all functions in
the flat profile, even those that were never called, and that had
no time spent in them. This is useful in conjunction with the
`-c' option for discovering which routines were never called.

File:, Node: Miscellaneous Options, Next: Deprecated Options, Prev: Analysis Options, Up: Invoking
4.3 Miscellaneous Options
The `-d NUM' option specifies debugging options. If NUM is not
specified, enable all debugging. *Note Debugging `gprof':
The `-h' option prints command line usage.
Selects the format of the profile data files. Recognized formats
are `auto' (the default), `bsd', `4.4bsd', `magic', and `prof'
(not yet supported).
The `-s' option causes `gprof' to summarize the information in the
profile data files it read in, and write out a profile data file
called `gmon.sum', which contains all the information from the
profile data files that `gprof' read in. The file `gmon.sum' may
be one of the specified input files; the effect of this is to
merge the data in the other input files into `gmon.sum'.
Eventually you can run `gprof' again without `-s' to analyze the
cumulative data in the file `gmon.sum'.
The `-v' flag causes `gprof' to print the current version number,
and then exit.

File:, Node: Deprecated Options, Next: Symspecs, Prev: Miscellaneous Options, Up: Invoking
4.4 Deprecated Options
These options have been replaced with newer versions that use symspecs.
The `-e FUNCTION' option tells `gprof' to not print information
about the function FUNCTION_NAME (and its children...) in the call
graph. The function will still be listed as a child of any
functions that call it, but its index number will be shown as
`[not printed]'. More than one `-e' option may be given; only one
FUNCTION_NAME may be indicated with each `-e' option.
The `-E FUNCTION' option works like the `-e' option, but time
spent in the function (and children who were not called from
anywhere else), will not be used to compute the
percentages-of-time for the call graph. More than one `-E' option
may be given; only one FUNCTION_NAME may be indicated with each
`-E' option.
The `-f FUNCTION' option causes `gprof' to limit the call graph to
the function FUNCTION_NAME and its children (and their
children...). More than one `-f' option may be given; only one
FUNCTION_NAME may be indicated with each `-f' option.
The `-F FUNCTION' option works like the `-f' option, but only time
spent in the function and its children (and their children...)
will be used to determine total-time and percentages-of-time for
the call graph. More than one `-F' option may be given; only one
FUNCTION_NAME may be indicated with each `-F' option. The `-F'
option overrides the `-E' option.
Note that only one function can be specified with each `-e', `-E',
`-f' or `-F' option. To specify more than one function, use multiple
options. For example, this command:
gprof -e boring -f foo -f bar myprogram > gprof.output
lists in the call graph all functions that were reached from either
`foo' or `bar' and were not reachable from `boring'.

File:, Node: Symspecs, Prev: Deprecated Options, Up: Invoking
4.5 Symspecs
Many of the output options allow functions to be included or excluded
using "symspecs" (symbol specifications), which observe the following
| funcname_not_containing_a_dot
| linenumber
| ( [ any_filename ] `:' ( any_funcname | linenumber ) )
Here are some sample symspecs:
Selects everything in file `main.c'--the dot in the string tells
`gprof' to interpret the string as a filename, rather than as a
function name. To select a file whose name does not contain a
dot, a trailing colon should be specified. For example, `odd:' is
interpreted as the file named `odd'.
Selects all functions named `main'.
Note that there may be multiple instances of the same function name
because some of the definitions may be local (i.e., static).
Unless a function name is unique in a program, you must use the
colon notation explained below to specify a function from a
specific source file.
Sometimes, function names contain dots. In such cases, it is
necessary to add a leading colon to the name. For example,
`:.mul' selects function `.mul'.
In some object file formats, symbols have a leading underscore.
`gprof' will normally not print these underscores. When you name a
symbol in a symspec, you should type it exactly as `gprof' prints
it in its output. For example, if the compiler produces a symbol
`_main' from your `main' function, `gprof' still prints it as
`main' in its output, so you should use `main' in symspecs.
Selects function `main' in file `main.c'.
Selects line 134 in file `main.c'.

File:, Node: Output, Next: Inaccuracy, Prev: Invoking, Up: Top
5 Interpreting `gprof''s Output
`gprof' can produce several different output styles, the most important
of which are described below. The simplest output styles (file
information, execution count, and function and file ordering) are not
described here, but are documented with the respective options that
trigger them. *Note Output Options: Output Options.
* Menu:
* Flat Profile:: The flat profile shows how much time was spent
executing directly in each function.
* Call Graph:: The call graph shows which functions called which
others, and how much time each function used
when its subroutine calls are included.
* Line-by-line:: `gprof' can analyze individual source code lines
* Annotated Source:: The annotated source listing displays source code
labeled with execution counts

File:, Node: Flat Profile, Next: Call Graph, Up: Output
5.1 The Flat Profile
The "flat profile" shows the total amount of time your program spent
executing each function. Unless the `-z' option is given, functions
with no apparent time spent in them, and no apparent calls to them, are
not mentioned. Note that if a function was not compiled for profiling,
and didn't run long enough to show up on the program counter histogram,
it will be indistinguishable from a function that was never called.
This is part of a flat profile for a small program:
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls ms/call ms/call name
33.34 0.02 0.02 7208 0.00 0.00 open
16.67 0.03 0.01 244 0.04 0.12 offtime
16.67 0.04 0.01 8 1.25 1.25 memccpy
16.67 0.05 0.01 7 1.43 1.43 write
16.67 0.06 0.01 mcount
0.00 0.06 0.00 236 0.00 0.00 tzset
0.00 0.06 0.00 192 0.00 0.00 tolower
0.00 0.06 0.00 47 0.00 0.00 strlen
0.00 0.06 0.00 45 0.00 0.00 strchr
0.00 0.06 0.00 1 0.00 50.00 main
0.00 0.06 0.00 1 0.00 0.00 memcpy
0.00 0.06 0.00 1 0.00 10.11 print
0.00 0.06 0.00 1 0.00 0.00 profil
0.00 0.06 0.00 1 0.00 50.00 report
The functions are sorted first by decreasing run-time spent in them,
then by decreasing number of calls, then alphabetically by name. The
functions `mcount' and `profil' are part of the profiling apparatus and
appear in every flat profile; their time gives a measure of the amount
of overhead due to profiling.
Just before the column headers, a statement appears indicating how
much time each sample counted as. This "sampling period" estimates the
margin of error in each of the time figures. A time figure that is not
much larger than this is not reliable. In this example, each sample
counted as 0.01 seconds, suggesting a 100 Hz sampling rate. The
program's total execution time was 0.06 seconds, as indicated by the
`cumulative seconds' field. Since each sample counted for 0.01
seconds, this means only six samples were taken during the run. Two of
the samples occurred while the program was in the `open' function, as
indicated by the `self seconds' field. Each of the other four samples
occurred one each in `offtime', `memccpy', `write', and `mcount'.
Since only six samples were taken, none of these values can be regarded
as particularly reliable. In another run, the `self seconds' field for
`mcount' might well be `0.00' or `0.02'. *Note Statistical Sampling
Error: Sampling Error, for a complete discussion.
The remaining functions in the listing (those whose `self seconds'
field is `0.00') didn't appear in the histogram samples at all.
However, the call graph indicated that they were called, so therefore
they are listed, sorted in decreasing order by the `calls' field.
Clearly some time was spent executing these functions, but the paucity
of histogram samples prevents any determination of how much time each
Here is what the fields in each line mean:
`% time'
This is the percentage of the total execution time your program
spent in this function. These should all add up to 100%.
`cumulative seconds'
This is the cumulative total number of seconds the computer spent
executing this functions, plus the time spent in all the functions
above this one in this table.
`self seconds'
This is the number of seconds accounted for by this function alone.
The flat profile listing is sorted first by this number.
This is the total number of times the function was called. If the
function was never called, or the number of times it was called
cannot be determined (probably because the function was not
compiled with profiling enabled), the "calls" field is blank.
`self ms/call'
This represents the average number of milliseconds spent in this
function per call, if this function is profiled. Otherwise, this
field is blank for this function.
`total ms/call'
This represents the average number of milliseconds spent in this
function and its descendants per call, if this function is
profiled. Otherwise, this field is blank for this function. This
is the only field in the flat profile that uses call graph
This is the name of the function. The flat profile is sorted by
this field alphabetically after the "self seconds" and "calls"
fields are sorted.

File:, Node: Call Graph, Next: Line-by-line, Prev: Flat Profile, Up: Output
5.2 The Call Graph
The "call graph" shows how much time was spent in each function and its
children. From this information, you can find functions that, while
they themselves may not have used much time, called other functions
that did use unusual amounts of time.
Here is a sample call from a small program. This call came from the
same `gprof' run as the flat profile example in the previous section.
granularity: each sample hit covers 2 byte(s) for 20.00% of 0.05 seconds
index % time self children called name
[1] 100.0 0.00 0.05 start [1]
0.00 0.05 1/1 main [2]
0.00 0.00 1/2 on_exit [28]
0.00 0.00 1/1 exit [59]
0.00 0.05 1/1 start [1]
[2] 100.0 0.00 0.05 1 main [2]
0.00 0.05 1/1 report [3]
0.00 0.05 1/1 main [2]
[3] 100.0 0.00 0.05 1 report [3]
0.00 0.03 8/8 timelocal [6]
0.00 0.01 1/1 print [9]
0.00 0.01 9/9 fgets [12]
0.00 0.00 12/34 strncmp <cycle 1> [40]
0.00 0.00 8/8 lookup [20]
0.00 0.00 1/1 fopen [21]
0.00 0.00 8/8 chewtime [24]
0.00 0.00 8/16 skipspace [44]
[4] 59.8 0.01 0.02 8+472 <cycle 2 as a whole> [4]
0.01 0.02 244+260 offtime <cycle 2> [7]
0.00 0.00 236+1 tzset <cycle 2> [26]
The lines full of dashes divide this table into "entries", one for
each function. Each entry has one or more lines.
In each entry, the primary line is the one that starts with an index
number in square brackets. The end of this line says which function
the entry is for. The preceding lines in the entry describe the
callers of this function and the following lines describe its
subroutines (also called "children" when we speak of the call graph).
The entries are sorted by time spent in the function and its
The internal profiling function `mcount' (*note The Flat Profile:
Flat Profile.) is never mentioned in the call graph.
* Menu:
* Primary:: Details of the primary line's contents.
* Callers:: Details of caller-lines' contents.
* Subroutines:: Details of subroutine-lines' contents.
* Cycles:: When there are cycles of recursion,
such as `a' calls `b' calls `a'...

File:, Node: Primary, Next: Callers, Up: Call Graph
5.2.1 The Primary Line
The "primary line" in a call graph entry is the line that describes the
function which the entry is about and gives the overall statistics for
this function.
For reference, we repeat the primary line from the entry for function
`report' in our main example, together with the heading line that shows
the names of the fields:
index % time self children called name
[3] 100.0 0.00 0.05 1 report [3]
Here is what the fields in the primary line mean:
Entries are numbered with consecutive integers. Each function
therefore has an index number, which appears at the beginning of
its primary line.
Each cross-reference to a function, as a caller or subroutine of
another, gives its index number as well as its name. The index
number guides you if you wish to look for the entry for that
`% time'
This is the percentage of the total time that was spent in this
function, including time spent in subroutines called from this
The time spent in this function is counted again for the callers of
this function. Therefore, adding up these percentages is
This is the total amount of time spent in this function. This
should be identical to the number printed in the `seconds' field
for this function in the flat profile.
This is the total amount of time spent in the subroutine calls
made by this function. This should be equal to the sum of all the
`self' and `children' entries of the children listed directly
below this function.
This is the number of times the function was called.
If the function called itself recursively, there are two numbers,
separated by a `+'. The first number counts non-recursive calls,
and the second counts recursive calls.
In the example above, the function `report' was called once from
This is the name of the current function. The index number is
repeated after it.
If the function is part of a cycle of recursion, the cycle number
is printed between the function's name and the index number (*note
How Mutually Recursive Functions Are Described: Cycles.). For
example, if function `gnurr' is part of cycle number one, and has
index number twelve, its primary line would be end like this:
gnurr <cycle 1> [12]

File:, Node: Callers, Next: Subroutines, Prev: Primary, Up: Call Graph
5.2.2 Lines for a Function's Callers
A function's entry has a line for each function it was called by.
These lines' fields correspond to the fields of the primary line, but
their meanings are different because of the difference in context.
For reference, we repeat two lines from the entry for the function
`report', the primary line and one caller-line preceding it, together
with the heading line that shows the names of the fields:
index % time self children called name
0.00 0.05 1/1 main [2]
[3] 100.0 0.00 0.05 1 report [3]
Here are the meanings of the fields in the caller-line for `report'
called from `main':
An estimate of the amount of time spent in `report' itself when it
was called from `main'.
An estimate of the amount of time spent in subroutines of `report'
when `report' was called from `main'.
The sum of the `self' and `children' fields is an estimate of the
amount of time spent within calls to `report' from `main'.
Two numbers: the number of times `report' was called from `main',
followed by the total number of non-recursive calls to `report'
from all its callers.
`name and index number'
The name of the caller of `report' to which this line applies,
followed by the caller's index number.
Not all functions have entries in the call graph; some options to
`gprof' request the omission of certain functions. When a caller
has no entry of its own, it still has caller-lines in the entries
of the functions it calls.
If the caller is part of a recursion cycle, the cycle number is
printed between the name and the index number.
If the identity of the callers of a function cannot be determined, a
dummy caller-line is printed which has `<spontaneous>' as the "caller's
name" and all other fields blank. This can happen for signal handlers.

File:, Node: Subroutines, Next: Cycles, Prev: Callers, Up: Call Graph
5.2.3 Lines for a Function's Subroutines
A function's entry has a line for each of its subroutines--in other
words, a line for each other function that it called. These lines'
fields correspond to the fields of the primary line, but their meanings
are different because of the difference in context.
For reference, we repeat two lines from the entry for the function
`main', the primary line and a line for a subroutine, together with the
heading line that shows the names of the fields:
index % time self children called name
[2] 100.0 0.00 0.05 1 main [2]
0.00 0.05 1/1 report [3]
Here are the meanings of the fields in the subroutine-line for `main'
calling `report':
An estimate of the amount of time spent directly within `report'
when `report' was called from `main'.
An estimate of the amount of time spent in subroutines of `report'
when `report' was called from `main'.
The sum of the `self' and `children' fields is an estimate of the
total time spent in calls to `report' from `main'.
Two numbers, the number of calls to `report' from `main' followed
by the total number of non-recursive calls to `report'. This
ratio is used to determine how much of `report''s `self' and
`children' time gets credited to `main'. *Note Estimating
`children' Times: Assumptions.
The name of the subroutine of `main' to which this line applies,
followed by the subroutine's index number.
If the caller is part of a recursion cycle, the cycle number is
printed between the name and the index number.

File:, Node: Cycles, Prev: Subroutines, Up: Call Graph
5.2.4 How Mutually Recursive Functions Are Described
The graph may be complicated by the presence of "cycles of recursion"
in the call graph. A cycle exists if a function calls another function
that (directly or indirectly) calls (or appears to call) the original
function. For example: if `a' calls `b', and `b' calls `a', then `a'
and `b' form a cycle.
Whenever there are call paths both ways between a pair of functions,
they belong to the same cycle. If `a' and `b' call each other and `b'
and `c' call each other, all three make one cycle. Note that even if
`b' only calls `a' if it was not called from `a', `gprof' cannot
determine this, so `a' and `b' are still considered a cycle.
The cycles are numbered with consecutive integers. When a function
belongs to a cycle, each time the function name appears in the call
graph it is followed by `<cycle NUMBER>'.
The reason cycles matter is that they make the time values in the
call graph paradoxical. The "time spent in children" of `a' should
include the time spent in its subroutine `b' and in `b''s
subroutines--but one of `b''s subroutines is `a'! How much of `a''s
time should be included in the children of `a', when `a' is indirectly
The way `gprof' resolves this paradox is by creating a single entry
for the cycle as a whole. The primary line of this entry describes the
total time spent directly in the functions of the cycle. The
"subroutines" of the cycle are the individual functions of the cycle,
and all other functions that were called directly by them. The
"callers" of the cycle are the functions, outside the cycle, that
called functions in the cycle.
Here is an example portion of a call graph which shows a cycle
containing functions `a' and `b'. The cycle was entered by a call to
`a' from `main'; both `a' and `b' called `c'.
index % time self children called name
1.77 0 1/1 main [2]
[3] 91.71 1.77 0 1+5 <cycle 1 as a whole> [3]
1.02 0 3 b <cycle 1> [4]
0.75 0 2 a <cycle 1> [5]
3 a <cycle 1> [5]
[4] 52.85 1.02 0 0 b <cycle 1> [4]
2 a <cycle 1> [5]
0 0 3/6 c [6]
1.77 0 1/1 main [2]
2 b <cycle 1> [4]
[5] 38.86 0.75 0 1 a <cycle 1> [5]
3 b <cycle 1> [4]
0 0 3/6 c [6]
(The entire call graph for this program contains in addition an entry
for `main', which calls `a', and an entry for `c', with callers `a' and
index % time self children called name
[1] 100.00 0 1.93 0 start [1]
0.16 1.77 1/1 main [2]
0.16 1.77 1/1 start [1]
[2] 100.00 0.16 1.77 1 main [2]
1.77 0 1/1 a <cycle 1> [5]
1.77 0 1/1 main [2]
[3] 91.71 1.77 0 1+5 <cycle 1 as a whole> [3]
1.02 0 3 b <cycle 1> [4]
0.75 0 2 a <cycle 1> [5]
0 0 6/6 c [6]
3 a <cycle 1> [5]
[4] 52.85 1.02 0 0 b <cycle 1> [4]
2 a <cycle 1> [5]
0 0 3/6 c [6]
1.77 0 1/1 main [2]
2 b <cycle 1> [4]
[5] 38.86 0.75 0 1 a <cycle 1> [5]
3 b <cycle 1> [4]
0 0 3/6 c [6]
0 0 3/6 b <cycle 1> [4]
0 0 3/6 a <cycle 1> [5]
[6] 0.00 0 0 6 c [6]
The `self' field of the cycle's primary line is the total time spent
in all the functions of the cycle. It equals the sum of the `self'
fields for the individual functions in the cycle, found in the entry in
the subroutine lines for these functions.
The `children' fields of the cycle's primary line and subroutine
lines count only subroutines outside the cycle. Even though `a' calls
`b', the time spent in those calls to `b' is not counted in `a''s
`children' time. Thus, we do not encounter the problem of what to do
when the time in those calls to `b' includes indirect recursive calls
back to `a'.
The `children' field of a caller-line in the cycle's entry estimates
the amount of time spent _in the whole cycle_, and its other
subroutines, on the times when that caller called a function in the
The `called' field in the primary line for the cycle has two numbers:
first, the number of times functions in the cycle were called by
functions outside the cycle; second, the number of times they were
called by functions in the cycle (including times when a function in
the cycle calls itself). This is a generalization of the usual split
into non-recursive and recursive calls.
The `called' field of a subroutine-line for a cycle member in the
cycle's entry says how many time that function was called from
functions in the cycle. The total of all these is the second number in
the primary line's `called' field.
In the individual entry for a function in a cycle, the other
functions in the same cycle can appear as subroutines and as callers.
These lines show how many times each function in the cycle called or
was called from each other function in the cycle. The `self' and
`children' fields in these lines are blank because of the difficulty of
defining meanings for them when recursion is going on.

File:, Node: Line-by-line, Next: Annotated Source, Prev: Call Graph, Up: Output
5.3 Line-by-line Profiling
`gprof''s `-l' option causes the program to perform "line-by-line"
profiling. In this mode, histogram samples are assigned not to
functions, but to individual lines of source code. This only works
with programs compiled with older versions of the `gcc' compiler.
Newer versions of `gcc' use a different program - `gcov' - to display
line-by-line profiling information.
With the older versions of `gcc' the program usually has to be
compiled with a `-g' option, in addition to `-pg', in order to generate
debugging symbols for tracking source code lines. Note, in much older
versions of `gcc' the program had to be compiled with the `-a' command
line option as well.
The flat profile is the most useful output table in line-by-line
mode. The call graph isn't as useful as normal, since the current
version of `gprof' does not propagate call graph arcs from source code
lines to the enclosing function. The call graph does, however, show
each line of code that called each function, along with a count.
Here is a section of `gprof''s output, without line-by-line
profiling. Note that `ct_init' accounted for four histogram hits, and
13327 calls to `init_block'.
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls us/call us/call name
30.77 0.13 0.04 6335 6.31 6.31 ct_init
Call graph (explanation follows)
granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds
index % time self children called name
0.00 0.00 1/13496 name_too_long
0.00 0.00 40/13496 deflate
0.00 0.00 128/13496 deflate_fast
0.00 0.00 13327/13496 ct_init
[7] 0.0 0.00 0.00 13496 init_block
Now let's look at some of `gprof''s output from the same program run,
this time with line-by-line profiling enabled. Note that `ct_init''s
four histogram hits are broken down into four lines of source code--one
hit occurred on each of lines 349, 351, 382 and 385. In the call graph,
note how `ct_init''s 13327 calls to `init_block' are broken down into
one call from line 396, 3071 calls from line 384, 3730 calls from line
385, and 6525 calls from 387.
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self
time seconds seconds calls name
7.69 0.10 0.01 ct_init (trees.c:349)
7.69 0.11 0.01 ct_init (trees.c:351)
7.69 0.12 0.01 ct_init (trees.c:382)
7.69 0.13 0.01 ct_init (trees.c:385)
Call graph (explanation follows)
granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds
% time self children called name
0.00 0.00 1/13496 name_too_long (gzip.c:1440)
0.00 0.00 1/13496 deflate (deflate.c:763)
0.00 0.00 1/13496 ct_init (trees.c:396)
0.00 0.00 2/13496 deflate (deflate.c:727)
0.00 0.00 4/13496 deflate (deflate.c:686)
0.00 0.00 5/13496 deflate (deflate.c:675)
0.00 0.00 12/13496 deflate (deflate.c:679)
0.00 0.00 16/13496 deflate (deflate.c:730)
0.00 0.00 128/13496 deflate_fast (deflate.c:654)
0.00 0.00 3071/13496 ct_init (trees.c:384)
0.00 0.00 3730/13496 ct_init (trees.c:385)
0.00 0.00 6525/13496 ct_init (trees.c:387)
[6] 0.0 0.00 0.00 13496 init_block (trees.c:408)

File:, Node: Annotated Source, Prev: Line-by-line, Up: Output
5.4 The Annotated Source Listing
`gprof''s `-A' option triggers an annotated source listing, which lists
the program's source code, each function labeled with the number of
times it was called. You may also need to specify the `-I' option, if
`gprof' can't find the source code files.
With older versions of `gcc' compiling with `gcc ... -g -pg -a'
augments your program with basic-block counting code, in addition to
function counting code. This enables `gprof' to determine how many
times each line of code was executed. With newer versions of `gcc'
support for displaying basic-block counts is provided by the `gcov'
For example, consider the following function, taken from gzip, with
line numbers added:
1 ulg updcrc(s, n)
2 uch *s;
3 unsigned n;
4 {
5 register ulg c;
7 static ulg crc = (ulg)0xffffffffL;
9 if (s == NULL) {
10 c = 0xffffffffL;
11 } else {
12 c = crc;
13 if (n) do {
14 c = crc_32_tab[...];
15 } while (--n);
16 }
17 crc = c;
18 return c ^ 0xffffffffL;
19 }
`updcrc' has at least five basic-blocks. One is the function
itself. The `if' statement on line 9 generates two more basic-blocks,
one for each branch of the `if'. A fourth basic-block results from the
`if' on line 13, and the contents of the `do' loop form the fifth
basic-block. The compiler may also generate additional basic-blocks to
handle various special cases.
A program augmented for basic-block counting can be analyzed with
`gprof -l -A'. The `-x' option is also helpful, to ensure that each
line of code is labeled at least once. Here is `updcrc''s annotated
source listing for a sample `gzip' run:
ulg updcrc(s, n)
uch *s;
unsigned n;
2 ->{
register ulg c;
static ulg crc = (ulg)0xffffffffL;
2 -> if (s == NULL) {
1 -> c = 0xffffffffL;
1 -> } else {
1 -> c = crc;
1 -> if (n) do {
26312 -> c = crc_32_tab[...];
26312,1,26311 -> } while (--n);
2 -> crc = c;
2 -> return c ^ 0xffffffffL;
2 ->}
In this example, the function was called twice, passing once through
each branch of the `if' statement. The body of the `do' loop was
executed a total of 26312 times. Note how the `while' statement is
annotated. It began execution 26312 times, once for each iteration
through the loop. One of those times (the last time) it exited, while
it branched back to the beginning of the loop 26311 times.

File:, Node: Inaccuracy, Next: How do I?, Prev: Output, Up: Top
6 Inaccuracy of `gprof' Output
* Menu:
* Sampling Error:: Statistical margins of error
* Assumptions:: Estimating children times

File:, Node: Sampling Error, Next: Assumptions, Up: Inaccuracy
6.1 Statistical Sampling Error
The run-time figures that `gprof' gives you are based on a sampling
process, so they are subject to statistical inaccuracy. If a function
runs only a small amount of time, so that on the average the sampling
process ought to catch that function in the act only once, there is a
pretty good chance it will actually find that function zero times, or
By contrast, the number-of-calls and basic-block figures are derived
by counting, not sampling. They are completely accurate and will not
vary from run to run if your program is deterministic and single
threaded. In multi-threaded applications, or single threaded
applications that link with multi-threaded libraries, the counts are
only deterministic if the counting function is thread-safe. (Note:
beware that the mcount counting function in glibc is _not_
thread-safe). *Note Implementation of Profiling: Implementation.
The "sampling period" that is printed at the beginning of the flat
profile says how often samples are taken. The rule of thumb is that a
run-time figure is accurate if it is considerably bigger than the
sampling period.
The actual amount of error can be predicted. For N samples, the
_expected_ error is the square-root of N. For example, if the sampling
period is 0.01 seconds and `foo''s run-time is 1 second, N is 100
samples (1 second/0.01 seconds), sqrt(N) is 10 samples, so the expected
error in `foo''s run-time is 0.1 seconds (10*0.01 seconds), or ten
percent of the observed value. Again, if the sampling period is 0.01
seconds and `bar''s run-time is 100 seconds, N is 10000 samples,
sqrt(N) is 100 samples, so the expected error in `bar''s run-time is 1
second, or one percent of the observed value. It is likely to vary
this much _on the average_ from one profiling run to the next.
(_Sometimes_ it will vary more.)
This does not mean that a small run-time figure is devoid of
information. If the program's _total_ run-time is large, a small
run-time for one function does tell you that that function used an
insignificant fraction of the whole program's time. Usually this means
it is not worth optimizing.
One way to get more accuracy is to give your program more (but
similar) input data so it will take longer. Another way is to combine
the data from several runs, using the `-s' option of `gprof'. Here is
1. Run your program once.
2. Issue the command `mv gmon.out gmon.sum'.
3. Run your program again, the same as before.
4. Merge the new data in `gmon.out' into `gmon.sum' with this command:
gprof -s EXECUTABLE-FILE gmon.out gmon.sum
5. Repeat the last two steps as often as you wish.
6. Analyze the cumulative data using this command:

File:, Node: Assumptions, Prev: Sampling Error, Up: Inaccuracy
6.2 Estimating `children' Times
Some of the figures in the call graph are estimates--for example, the
`children' time values and all the time figures in caller and
subroutine lines.
There is no direct information about these measurements in the
profile data itself. Instead, `gprof' estimates them by making an
assumption about your program that might or might not be true.
The assumption made is that the average time spent in each call to
any function `foo' is not correlated with who called `foo'. If `foo'
used 5 seconds in all, and 2/5 of the calls to `foo' came from `a',
then `foo' contributes 2 seconds to `a''s `children' time, by
This assumption is usually true enough, but for some programs it is
far from true. Suppose that `foo' returns very quickly when its
argument is zero; suppose that `a' always passes zero as an argument,
while other callers of `foo' pass other arguments. In this program,
all the time spent in `foo' is in the calls from callers other than `a'.
But `gprof' has no way of knowing this; it will blindly and incorrectly
charge 2 seconds of time in `foo' to the children of `a'.
We hope some day to put more complete data into `gmon.out', so that
this assumption is no longer needed, if we can figure out how. For the
novice, the estimated figures are usually more useful than misleading.

File:, Node: How do I?, Next: Incompatibilities, Prev: Inaccuracy, Up: Top
7 Answers to Common Questions
How can I get more exact information about hot spots in my program?
Looking at the per-line call counts only tells part of the story.
Because `gprof' can only report call times and counts by function,
the best way to get finer-grained information on where the program
is spending its time is to re-factor large functions into sequences
of calls to smaller ones. Beware however that this can introduce
artificial hot spots since compiling with `-pg' adds a significant
overhead to function calls. An alternative solution is to use a
non-intrusive profiler, e.g. oprofile.
How do I find which lines in my program were executed the most times?
Use the `gcov' program.
How do I find which lines in my program called a particular function?
Use `gprof -l' and lookup the function in the call graph. The
callers will be broken down by function and line number.
How do I analyze a program that runs for less than a second?
Try using a shell script like this one:
for i in `seq 1 100`; do
mv gmon.out gmon.out.$i
gprof -s fastprog gmon.out.*
gprof fastprog gmon.sum
If your program is completely deterministic, all the call counts
will be simple multiples of 100 (i.e., a function called once in
each run will appear with a call count of 100).

File:, Node: Incompatibilities, Next: Details, Prev: How do I?, Up: Top
8 Incompatibilities with Unix `gprof'
GNU `gprof' and Berkeley Unix `gprof' use the same data file
`gmon.out', and provide essentially the same information. But there
are a few differences.
* GNU `gprof' uses a new, generalized file format with support for
basic-block execution counts and non-realtime histograms. A magic
cookie and version number allows `gprof' to easily identify new
style files. Old BSD-style files can still be read. *Note
Profiling Data File Format: File Format.
* For a recursive function, Unix `gprof' lists the function as a
parent and as a child, with a `calls' field that lists the number
of recursive calls. GNU `gprof' omits these lines and puts the
number of recursive calls in the primary line.
* When a function is suppressed from the call graph with `-e', GNU
`gprof' still lists it as a subroutine of functions that call it.
* GNU `gprof' accepts the `-k' with its argument in the form
`from/to', instead of `from to'.
* In the annotated source listing, if there are multiple basic
blocks on the same line, GNU `gprof' prints all of their counts,
separated by commas.
* The blurbs, field widths, and output formats are different. GNU
`gprof' prints blurbs after the tables, so that you can see the
tables without skipping the blurbs.

File:, Node: Details, Next: GNU Free Documentation License, Prev: Incompatibilities, Up: Top
9 Details of Profiling
* Menu:
* Implementation:: How a program collects profiling information
* File Format:: Format of `gmon.out' files
* Internals:: `gprof''s internal operation
* Debugging:: Using `gprof''s `-d' option

File:, Node: Implementation, Next: File Format, Up: Details
9.1 Implementation of Profiling
Profiling works by changing how every function in your program is
compiled so that when it is called, it will stash away some information
about where it was called from. From this, the profiler can figure out
what function called it, and can count how many times it was called.
This change is made by the compiler when your program is compiled with
the `-pg' option, which causes every function to call `mcount' (or
`_mcount', or `__mcount', depending on the OS and compiler) as one of
its first operations.
The `mcount' routine, included in the profiling library, is
responsible for recording in an in-memory call graph table both its
parent routine (the child) and its parent's parent. This is typically
done by examining the stack frame to find both the address of the
child, and the return address in the original parent. Since this is a
very machine-dependent operation, `mcount' itself is typically a short
assembly-language stub routine that extracts the required information,
and then calls `__mcount_internal' (a normal C function) with two
arguments--`frompc' and `selfpc'. `__mcount_internal' is responsible
for maintaining the in-memory call graph, which records `frompc',
`selfpc', and the number of times each of these call arcs was traversed.
GCC Version 2 provides a magical function
(`__builtin_return_address'), which allows a generic `mcount' function
to extract the required information from the stack frame. However, on
some architectures, most notably the SPARC, using this builtin can be
very computationally expensive, and an assembly language version of
`mcount' is used for performance reasons.
Number-of-calls information for library routines is collected by
using a special version of the C library. The programs in it are the
same as in the usual C library, but they were compiled with `-pg'. If
you link your program with `gcc ... -pg', it automatically uses the
profiling version of the library.
Profiling also involves watching your program as it runs, and
keeping a histogram of where the program counter happens to be every
now and then. Typically the program counter is looked at around 100
times per second of run time, but the exact frequency may vary from
system to system.
This is done is one of two ways. Most UNIX-like operating systems
provide a `profil()' system call, which registers a memory array with
the kernel, along with a scale factor that determines how the program's
address space maps into the array. Typical scaling values cause every
2 to 8 bytes of address space to map into a single array slot. On
every tick of the system clock (assuming the profiled program is
running), the value of the program counter is examined and the
corresponding slot in the memory array is incremented. Since this is
done in the kernel, which had to interrupt the process anyway to handle
the clock interrupt, very little additional system overhead is required.
However, some operating systems, most notably Linux 2.0 (and
earlier), do not provide a `profil()' system call. On such a system,
arrangements are made for the kernel to periodically deliver a signal
to the process (typically via `setitimer()'), which then performs the
same operation of examining the program counter and incrementing a slot
in the memory array. Since this method requires a signal to be
delivered to user space every time a sample is taken, it uses
considerably more overhead than kernel-based profiling. Also, due to
the added delay required to deliver the signal, this method is less
accurate as well.
A special startup routine allocates memory for the histogram and
either calls `profil()' or sets up a clock signal handler. This
routine (`monstartup') can be invoked in several ways. On Linux
systems, a special profiling startup file `gcrt0.o', which invokes
`monstartup' before `main', is used instead of the default `crt0.o'.
Use of this special startup file is one of the effects of using `gcc
... -pg' to link. On SPARC systems, no special startup files are used.
Rather, the `mcount' routine, when it is invoked for the first time
(typically when `main' is called), calls `monstartup'.
If the compiler's `-a' option was used, basic-block counting is also
enabled. Each object file is then compiled with a static array of
counts, initially zero. In the executable code, every time a new
basic-block begins (i.e., when an `if' statement appears), an extra
instruction is inserted to increment the corresponding count in the
array. At compile time, a paired array was constructed that recorded
the starting address of each basic-block. Taken together, the two
arrays record the starting address of every basic-block, along with the
number of times it was executed.
The profiling library also includes a function (`mcleanup') which is
typically registered using `atexit()' to be called as the program
exits, and is responsible for writing the file `gmon.out'. Profiling
is turned off, various headers are output, and the histogram is
written, followed by the call-graph arcs and the basic-block counts.
The output from `gprof' gives no indication of parts of your program
that are limited by I/O or swapping bandwidth. This is because samples
of the program counter are taken at fixed intervals of the program's
run time. Therefore, the time measurements in `gprof' output say
nothing about time that your program was not running. For example, a
part of the program that creates so much data that it cannot all fit in
physical memory at once may run very slowly due to thrashing, but
`gprof' will say it uses little time. On the other hand, sampling by
run time has the advantage that the amount of load due to other users
won't directly affect the output you get.

File:, Node: File Format, Next: Internals, Prev: Implementation, Up: Details
9.2 Profiling Data File Format
The old BSD-derived file format used for profile data does not contain a
magic cookie that allows to check whether a data file really is a
`gprof' file. Furthermore, it does not provide a version number, thus
rendering changes to the file format almost impossible. GNU `gprof'
uses a new file format that provides these features. For backward
compatibility, GNU `gprof' continues to support the old BSD-derived
format, but not all features are supported with it. For example,
basic-block execution counts cannot be accommodated by the old file
The new file format is defined in header file `gmon_out.h'. It
consists of a header containing the magic cookie and a version number,
as well as some spare bytes available for future extensions. All data
in a profile data file is in the native format of the target for which
the profile was collected. GNU `gprof' adapts automatically to the
byte-order in use.
In the new file format, the header is followed by a sequence of
records. Currently, there are three different record types: histogram
records, call-graph arc records, and basic-block execution count
records. Each file can contain any number of each record type. When
reading a file, GNU `gprof' will ensure records of the same type are
compatible with each other and compute the union of all records. For
example, for basic-block execution counts, the union is simply the sum
of all execution counts for each basic-block.
9.2.1 Histogram Records
Histogram records consist of a header that is followed by an array of
bins. The header contains the text-segment range that the histogram
spans, the size of the histogram in bytes (unlike in the old BSD
format, this does not include the size of the header), the rate of the
profiling clock, and the physical dimension that the bin counts
represent after being scaled by the profiling clock rate. The physical
dimension is specified in two parts: a long name of up to 15 characters
and a single character abbreviation. For example, a histogram
representing real-time would specify the long name as "seconds" and the
abbreviation as "s". This feature is useful for architectures that
support performance monitor hardware (which, fortunately, is becoming
increasingly common). For example, under DEC OSF/1, the "uprofile"
command can be used to produce a histogram of, say, instruction cache
misses. In this case, the dimension in the histogram header could be
set to "i-cache misses" and the abbreviation could be set to "1"
(because it is simply a count, not a physical dimension). Also, the
profiling rate would have to be set to 1 in this case.
Histogram bins are 16-bit numbers and each bin represent an equal
amount of text-space. For example, if the text-segment is one thousand
bytes long and if there are ten bins in the histogram, each bin
represents one hundred bytes.
9.2.2 Call-Graph Records
Call-graph records have a format that is identical to the one used in
the BSD-derived file format. It consists of an arc in the call graph
and a count indicating the number of times the arc was traversed during
program execution. Arcs are specified by a pair of addresses: the
first must be within caller's function and the second must be within
the callee's function. When performing profiling at the function
level, these addresses can point anywhere within the respective
function. However, when profiling at the line-level, it is better if
the addresses are as close to the call-site/entry-point as possible.
This will ensure that the line-level call-graph is able to identify
exactly which line of source code performed calls to a function.
9.2.3 Basic-Block Execution Count Records
Basic-block execution count records consist of a header followed by a
sequence of address/count pairs. The header simply specifies the
length of the sequence. In an address/count pair, the address
identifies a basic-block and the count specifies the number of times
that basic-block was executed. Any address within the basic-address can
be used.

File:, Node: Internals, Next: Debugging, Prev: File Format, Up: Details
9.3 `gprof''s Internal Operation
Like most programs, `gprof' begins by processing its options. During
this stage, it may building its symspec list (`sym_ids.c:sym_id_add'),
if options are specified which use symspecs. `gprof' maintains a
single linked list of symspecs, which will eventually get turned into
12 symbol tables, organized into six include/exclude pairs--one pair
each for the flat profile (INCL_FLAT/EXCL_FLAT), the call graph arcs
(INCL_ARCS/EXCL_ARCS), printing in the call graph
(INCL_GRAPH/EXCL_GRAPH), timing propagation in the call graph
(INCL_TIME/EXCL_TIME), the annotated source listing
(INCL_ANNO/EXCL_ANNO), and the execution count listing
After option processing, `gprof' finishes building the symspec list
by adding all the symspecs in `default_excluded_list' to the exclude
lists EXCL_TIME and EXCL_GRAPH, and if line-by-line profiling is
specified, EXCL_FLAT as well. These default excludes are not added to
Next, the BFD library is called to open the object file, verify that
it is an object file, and read its symbol table (`core.c:core_init'),
using `bfd_canonicalize_symtab' after mallocing an appropriately sized
array of symbols. At this point, function mappings are read (if the
`--file-ordering' option has been specified), and the core text space
is read into memory (if the `-c' option was given).
`gprof''s own symbol table, an array of Sym structures, is now built.
This is done in one of two ways, by one of two routines, depending on
whether line-by-line profiling (`-l' option) has been enabled. For
normal profiling, the BFD canonical symbol table is scanned. For
line-by-line profiling, every text space address is examined, and a new
symbol table entry gets created every time the line number changes. In
either case, two passes are made through the symbol table--one to count
the size of the symbol table required, and the other to actually read
the symbols. In between the two passes, a single array of type `Sym'
is created of the appropriate length. Finally,
`symtab.c:symtab_finalize' is called to sort the symbol table and
remove duplicate entries (entries with the same memory address).
The symbol table must be a contiguous array for two reasons. First,
the `qsort' library function (which sorts an array) will be used to
sort the symbol table. Also, the symbol lookup routine
(`symtab.c:sym_lookup'), which finds symbols based on memory address,
uses a binary search algorithm which requires the symbol table to be a
sorted array. Function symbols are indicated with an `is_func' flag.
Line number symbols have no special flags set. Additionally, a symbol
can have an `is_static' flag to indicate that it is a local symbol.
With the symbol table read, the symspecs can now be translated into
Syms (`sym_ids.c:sym_id_parse'). Remember that a single symspec can
match multiple symbols. An array of symbol tables (`syms') is created,
each entry of which is a symbol table of Syms to be included or
excluded from a particular listing. The master symbol table and the
symspecs are examined by nested loops, and every symbol that matches a
symspec is inserted into the appropriate syms table. This is done
twice, once to count the size of each required symbol table, and again
to build the tables, which have been malloced between passes. From now
on, to determine whether a symbol is on an include or exclude symspec
list, `gprof' simply uses its standard symbol lookup routine on the
appropriate table in the `syms' array.
Now the profile data file(s) themselves are read
(`gmon_io.c:gmon_out_read'), first by checking for a new-style
`gmon.out' header, then assuming this is an old-style BSD `gmon.out' if
the magic number test failed.
New-style histogram records are read by `hist.c:hist_read_rec'. For
the first histogram record, allocate a memory array to hold all the
bins, and read them in. When multiple profile data files (or files
with multiple histogram records) are read, the memory ranges of each
pair of histogram records must be either equal, or non-overlapping.
For each pair of histogram records, the resolution (memory region size
divided by the number of bins) must be the same. The time unit must be
the same for all histogram records. If the above containts are met, all
histograms for the same memory range are merged.
As each call graph record is read (`call_graph.c:cg_read_rec'), the
parent and child addresses are matched to symbol table entries, and a
call graph arc is created by `cg_arcs.c:arc_add', unless the arc fails
a symspec check against INCL_ARCS/EXCL_ARCS. As each arc is added, a
linked list is maintained of the parent's child arcs, and of the child's
parent arcs. Both the child's call count and the arc's call count are
incremented by the record's call count.
Basic-block records are read (`basic_blocks.c:bb_read_rec'), but
only if line-by-line profiling has been selected. Each basic-block
address is matched to a corresponding line symbol in the symbol table,
and an entry made in the symbol's bb_addr and bb_calls arrays. Again,
if multiple basic-block records are present for the same address, the
call counts are cumulative.
A gmon.sum file is dumped, if requested (`gmon_io.c:gmon_out_write').
If histograms were present in the data files, assign them to symbols
(`hist.c:hist_assign_samples') by iterating over all the sample bins
and assigning them to symbols. Since the symbol table is sorted in
order of ascending memory addresses, we can simple follow along in the
symbol table as we make our pass over the sample bins. This step
includes a symspec check against INCL_FLAT/EXCL_FLAT. Depending on the
histogram scale factor, a sample bin may span multiple symbols, in
which case a fraction of the sample count is allocated to each symbol,
proportional to the degree of overlap. This effect is rare for normal
profiling, but overlaps are more common during line-by-line profiling,
and can cause each of two adjacent lines to be credited with half a
hit, for example.
If call graph data is present, `cg_arcs.c:cg_assemble' is called.
First, if `-c' was specified, a machine-dependent routine (`find_call')
scans through each symbol's machine code, looking for subroutine call
instructions, and adding them to the call graph with a zero call count.
A topological sort is performed by depth-first numbering all the
symbols (`cg_dfn.c:cg_dfn'), so that children are always numbered less
than their parents, then making a array of pointers into the symbol
table and sorting it into numerical order, which is reverse topological
order (children appear before parents). Cycles are also detected at
this point, all members of which are assigned the same topological
number. Two passes are now made through this sorted array of symbol
pointers. The first pass, from end to beginning (parents to children),
computes the fraction of child time to propagate to each parent and a
print flag. The print flag reflects symspec handling of
INCL_GRAPH/EXCL_GRAPH, with a parent's include or exclude (print or no
print) property being propagated to its children, unless they
themselves explicitly appear in INCL_GRAPH or EXCL_GRAPH. A second
pass, from beginning to end (children to parents) actually propagates
the timings along the call graph, subject to a check against
INCL_TIME/EXCL_TIME. With the print flag, fractions, and timings now
stored in the symbol structures, the topological sort array is now
discarded, and a new array of pointers is assembled, this time sorted
by propagated time.
Finally, print the various outputs the user requested, which is now
fairly straightforward. The call graph (`cg_print.c:cg_print') and
flat profile (`hist.c:hist_print') are regurgitations of values already
computed. The annotated source listing
(`basic_blocks.c:print_annotated_source') uses basic-block information,
if present, to label each line of code with call counts, otherwise only
the function call counts are presented.
The function ordering code is marginally well documented in the
source code itself (`cg_print.c'). Basically, the functions with the
most use and the most parents are placed first, followed by other
functions with the most use, followed by lower use functions, followed
by unused functions at the end.

File:, Node: Debugging, Prev: Internals, Up: Details
9.4 Debugging `gprof'
If `gprof' was compiled with debugging enabled, the `-d' option
triggers debugging output (to stdout) which can be helpful in
understanding its operation. The debugging number specified is
interpreted as a sum of the following options:
2 - Topological sort
Monitor depth-first numbering of symbols during call graph analysis
4 - Cycles
Shows symbols as they are identified as cycle heads
16 - Tallying
As the call graph arcs are read, show each arc and how the total
calls to each function are tallied
32 - Call graph arc sorting
Details sorting individual parents/children within each call graph
64 - Reading histogram and call graph records
Shows address ranges of histograms as they are read, and each call
graph arc
128 - Symbol table
Reading, classifying, and sorting the symbol table from the object
file. For line-by-line profiling (`-l' option), also shows line
numbers being assigned to memory addresses.
256 - Static call graph
Trace operation of `-c' option
512 - Symbol table and arc table lookups
Detail operation of lookup routines
1024 - Call graph propagation
Shows how function times are propagated along the call graph
2048 - Basic-blocks
Shows basic-block records as they are read from profile data (only
meaningful with `-l' option)
4096 - Symspecs
Shows symspec-to-symbol pattern matching operation
8192 - Annotate source
Tracks operation of `-A' option

File:, Node: GNU Free Documentation License, Prev: Details, Up: Top
Appendix A GNU Free Documentation License
Version 1.3, 3 November 2008
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End Tag Table