GNU gprof
The gnu Profiler(GNU Binutils)
Version 2.42

Jay Fenlason and Richard Stallman

This manual describes the gnu profiler, gprof, and how you can use it todetermine 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. Eric S. Raymond made someminor corrections and additions in 2003.

Copyright c 1988-2024 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any
later version published by the Free Software Foundation; with no Invariant
Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copyof the license is included in the section entitled “GNU Free Documentation
License”.

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Table of Contents
1

Introduction to Profiling . . . . . . . . . . . . . . . . . . . . . . . 1

2

Compiling a Program for Profiling . . . . . . . . . . . . 3

3

Executing the Program . . . . . . . . . . . . . . . . . . . . . . . . 5

4gprof Command Summary . . . . . . . . . . . . . . . . . . . . . 7
4.1
4.2
4.3
4.4
4.5

5

Output Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Analysis Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Miscellaneous Options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Deprecated Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Symspecs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Interpreting gprof’s Output . . . . . . . . . . . . . . . . . . 15
5.1
5.2

The Flat Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
The Call Graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2.1 The Primary Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2.2 Lines for a Function’s Callers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2.3 Lines for a Function’s Subroutines . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2.4 How Mutually Recursive Functions Are Described . . . . . . . . 20
5.3 Line-by-line Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.4 The Annotated Source Listing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

6

Inaccuracy of gprof Output . . . . . . . . . . . . . . . . . . . 27
6.1
6.2

Statistical Sampling Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Estimating children Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

7

Answers to Common Questions . . . . . . . . . . . . . . 29

8

Incompatibilities with Unix gprof . . . . . . . . . . . . 31

9

Details of Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
9.1
9.2

Implementation of Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Profiling Data File Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.2.1 Histogram Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.2.2 Call-Graph Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.2.3 Basic-Block Execution Count Records . . . . . . . . . . . . . . . . . . . . 36
9.3 gprof’s Internal Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
9.4 Debugging gprof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

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Appendix A GNU Free Documentation License. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Chapter 1: Introduction to Profiling1

1 Introduction to Profiling
Profiling allows you to learn where your program spent its time and whichfunctions 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 lessoften than you expected. This may help you spot bugs that had otherwise
been unnoticed.
Since the profiler uses information collected during the actual execution ofyour 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 willaffect 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 feature.
Profiling has several steps:• You must compile and link your program with profiling enabled. See
Chapter 2 [Compiling a Program for Profiling], page 3.
• You must execute your program to generate a profile data file. See
Chapter 3 [Executing the Program], page 5.
• You must run gprof to analyze the profile data. See Chapter 4 [gprof
Command Summary], page 7.
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 toknow which functions burn most of the cycles, it is stated concisely here.
See Section 5.1 [The Flat Profile], page 15.
The call graph shows, for each function, which functions called it, which
other functions it called, and how many times. There is also an estimateof how much time was spent in the subroutines of each function. This cansuggest places where you might try to eliminate function calls that use a lotof time. See Section 5.2 [The Call Graph], page 17.
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.
See Section 5.4 [The Annotated Source Listing], page 24.
To better understand how profiling works, you may wish to read a description of its implementation. See Section 9.1 [Implementation of Profiling], page 33.

Chapter 2: Compiling a Program for Profiling3

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 use.)
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 forprofiling. 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 andlinks:
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 this: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 fileinstead 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’ insteadof the usual ‘-lc’. This is not absolutely necessary, but doing this gives you
number-of-calls information for standard library functions such as read andopen. 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 sharedlibrary 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:

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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 willbe 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 thatworks with gprof rather than gcov so there is still support for displaying
this kind of information in gprof. See Section 5.3 [Line-by-line Profiling],
page 22.
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.

Chapter 3: Executing the Program

5

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 giveit—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 theparticular input you use. For example, if the first command you give to yourprogram is to quit, the profile data will show the time used in initializationand in cleanup, but not much else.
Your program will write the profile data into a file called ‘gmon.out’ justbefore exiting. If there is already a file called ‘gmon.out’, its contents are
overwritten. You can rename the file afterwards if you are concerned thatit may be overwritten. If your system libc allows you may be able to write
the profile data under a different name. Set the GMON OUT PREFIX environment variable; this name will be appended with the PID of the running
program.
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 directoryat 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 thebasic-block execution counts. Unfortunately, the appearance of a humanreadable ‘bb.out’ means the basic-block counts didn’t get written into
‘gmon.out’. The Perl script bbconv.pl, included with the gprof source
distribution, will convert a ‘bb.out’ file into a format readable by gprof.
Invoke it like this:bbconv.pl < bb.out > bh-data

This translates the information in ‘bb.out’ into a form that gprof canunderstand. But you still need to tell gprof about the existence of thistranslated information. To do that, include bb-data on the gprof command
line, along with ‘gmon.out’, like this:gprof options executable-file gmon.out bb-data [yet-more-profile-data-files ...] [> outfile ]Chapter 4: gprof Command Summary

7

4 gprof Command Summary
After you have a profile data file ‘gmon.out’, you can run gprof to interpretthe information in it. The gprof program prints a flat profile and a call graphon standard output. Typically you would redirect the output of gprof into
a file with ‘>’.
You run gprof like this:gprof options [executable-file [profile-data-files ...]] [> outfile ]Here square-brackets indicate optional arguments.
If you omit the executable file name, the file ‘a.out’ is used. If you giveno profile data file name, the file ‘gmon.out’ is used. If any file is not in theproper format, or if the profile data file does not appear to belong to the
executable file, an error message is printed.
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.

4.1 Output Options
These options specify which of several output formats gprof should produce.
Many of these options take an optional symspec to specify functions to
be included or excluded. These options can be specified multiple times, withdifferent symspecs, to include or exclude sets of symbols. See Section 4.5[Symspecs], page 14.
Specifying any of these options overrides the default (‘-p -q’), which
prints a flat profile and call graph analysis for all functions.
-A[symspec ]--annotated-source[=symspec ]The ‘-A’ option causes gprof to print annotated source code.
If symspec is specified, print output only for matching symbols.
See Section 5.4 [The Annotated Source Listing], page 24.
-b--brief-B

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 tiredof seeing the blurbs.
The ‘-B’ option causes gprof to print the call graph analysis.

-C[symspec ]--exec-counts[=symspec ]The ‘-C’ option causes gprof to print a tally of functions andthe number of times each was called. If symspec is specified,print tally only for matching symbols.

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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.

-i--file-info
The ‘-i’ option causes gprof to display summary information
about the profile data file(s) and then exit. The number ofhistogram, call graph, and basic-block count records is displayed.
-I dirs--directory-path=dirs
The ‘-I’ option specifies a list of search directories in which tofind source files. Environment variable GPROF PATH can also
be used to convey this information. Used mostly for annotatedsource output.
-J[symspec ]--no-annotated-source[=symspec ]The ‘-J’ option causes gprof not to print annotated source code.
If symspec is specified, gprof prints annotated source, but excludes matching symbols.
-L--print-path
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 thedirectory in which the compiler was invoked.
-p[symspec ]--flat-profile[=symspec ]The ‘-p’ option causes gprof to print a flat profile. If symspec
is specified, print flat profile only for matching symbols. See
Section 5.1 [The Flat Profile], page 15.
-P[symspec ]--no-flat-profile[=symspec ]The ‘-P’ option causes gprof to suppress printing a flat profile.
If symspec is specified, gprof prints a flat profile, but excludes
matching symbols.
-q[symspec ]--graph[=symspec ]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. See Section 5.2 [The Call Graph], page 17.

Chapter 4: gprof Command Summary

9-Q[symspec ]--no-graph[=symspec ]The ‘-Q’ option causes gprof to suppress printing the call graph.
If symspec is specified, gprof prints a call graph, but excludes
matching symbols.
-t--table-length=num
The ‘-t’ option causes the num most active source lines in eachsource file to be listed when source annotation is enabled. The
default is 10.
-y--separate-files
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 theunderlying file system would truncate ‘filename -ann’ so that it
overwrites the original ‘filename ’, gprof generates annotatedsource in the file ‘filename .ann’ instead (if the original filename has an extension, that extension is replaced with ‘.ann’).
-Z[symspec ]--no-exec-counts[=symspec ]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.
-r--function-ordering
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 manual.
-R map_file--file-ordering map_file
The ‘--file-ordering’ option causes gprof to print a suggested .o link line ordering for the program based on profilingdata. 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.

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The map file argument is a pathname to a file which provides
function name to object file mappings. The format of the file issimilar to the output of the program nm.
c-parse.o:00000000 T yyparsec-parse.o:00000004 C yyerrflag
c-lang.o:00000000 T maybe_objc_method_namec-lang.o:00000000 T print_lang_statistics
c-lang.o:00000000 T recognize_objc_keyword
c-decl.o:00000000 T print_lang_identifierc-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-nameprogram-name.
-T--traditional
The ‘-T’ option causes gprof to print its output in “traditional”
BSD style.
-w width--width=width
Sets width of output lines to width. Currently only used whenprinting the function index at the bottom of the call graph.
-x--all-lines
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 annotatedby repeating the annotation for the first line. This behavior issimilar to tcov’s ‘-a’.
--demangle[=style ]--no-demangle
These options control whether C++ symbol names should bedemangled 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.

4.2 Analysis Options-a--no-static
The ‘-a’ option causes gprof to suppress the printing of statically declared (private) functions. (These are functions whoseChapter 4: gprof Command Summary

11

names are not listed as global, and which are not visible outside
the file/function/block where they were defined.) Time spent inthese 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.
-c--static-call-graphThe ‘-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 havebeen called, but never were. Calls to functions that were notcompiled with profiling enabled are also identified, but only ifsymbol 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 callgraph with call counts of ‘0’.
-D--ignore-non-functions
The ‘-D’ option causes gprof to ignore symbols which are notknown to be functions. This option will give more accurate
profile data on systems where it is supported (Solaris and HPUX
for example).
-k from /toThe ‘-k’ option allows you to delete from the call graph any
arcs from symbols matching symspec from to those matching
symspec to.
-l--lineThe ‘-l’ option enables line-by-line profiling, which causes histogram hits to be charged to individual source code lines, insteadof functions. This feature only works with programs compiled
by older versions of the gcc compiler. Newer versions of gcc aredesigned 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. See Section 6.1 [Statistical Sampling
Error], page 27.

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This option causes gprof to print the source file after each symbol in both the flat profile and the call graph. The full path tothe file is printed if used with the ‘-L’ option.
-m num--min-count=num
This option affects execution count output only. Symbols thatare executed less than num times are suppressed.
-nsymspec--time=symspec
The ‘-n’ option causes gprof, in its call graph analysis, to only
propagate times for symbols matching symspec.
-Nsymspec--no-time=symspec
The ‘-n’ option causes gprof, in its call graph analysis, not to
propagate times for symbols matching symspec.
-Sfilename--external-symbol-table=filename
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 isuseful for profiling kernel modules.
-z--display-unused-functions
If you give the ‘-z’ option, gprof will mention all functions in
the flat profile, even those that were never called, and that hadno time spent in them. This is useful in conjunction with the‘-c’ option for discovering which routines were never called.

4.3 Miscellaneous Options-d[num ]--debug[=num ]The ‘-d num ’ option specifies debugging options. If num is not
specified, enable all debugging. See Section 9.4 [Debugging
gprof], page 39.
-h--help

The ‘-h’ option prints command line usage.

-Oname--file-format=name
Selects the format of the profile data files. Recognized formats
are ‘auto’ (the default), ‘bsd’, ‘4.4bsd’, ‘magic’, and ‘prof’ (notyet supported).

Chapter 4: gprof Command Summary
-s
--sum

13

The ‘-s’ option causes gprof to summarize the information in
the profile data files it read in, and write out a profile data filecalled ‘gmon.sum’, which contains all the information from theprofile 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 thedata in the other input files into ‘gmon.sum’.
Eventually you can run gprof again without ‘-s’ to analyze thecumulative data in the file ‘gmon.sum’.

-v--version
The ‘-v’ flag causes gprof to print the current version number,and then exit.

4.4 Deprecated Options
These options have been replaced with newer versions that use symspecs.
-e function_name
The ‘-e function ’ option tells gprof to not print information
about the function function name (and its children. . . ) in thecall 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.
-E function_name
The -E function option works like the -e option, but timespent in the function (and children who were not called fromanywhere else), will not be used to compute the percentages-oftime for the call graph. More than one ‘-E’ option may be given;
only one function name may be indicated with each ‘-E’ option.
-f function_name
The ‘-f function ’ option causes gprof to limit the call graphto the function function name and its children (and theirchildren. . . ). More than one ‘-f’ option may be given; only
one function name may be indicated with each ‘-f’ option.
-F function_name
The ‘-F function ’ option works like the -f option, but
only time spent in the function and its children (and theirchildren. . . ) will be used to determine total-time andpercentages-of-time for the call graph. More than one ‘-F’
option may be given; only one function name may be indicatedwith each ‘-F’ option. The ‘-F’ option overrides the ‘-E’ option.

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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.

4.5 Symspecs
Many of the output options allow functions to be included or excluded using
symspecs (symbol specifications), which observe the following syntax:
filename_containing_a_dot| funcname_not_containing_a_dot| linenumber| ( [ any_filename ] ‘:’ ( any_funcname | linenumber ) )Here are some sample symspecs:‘main.c’Selects everything in file ‘main.c’—the dot in the string tells
gprof to interpret the string as a filename, rather than as afunction name. To select a file whose name does not contain adot, a trailing colon should be specified. For example, ‘odd:’ isinterpreted as the file named ‘odd’.

‘main’

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 isnecessary 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 youname a symbol in a symspec, you should type it exactly as gprof
prints it in its output. For example, if the compiler produces asymbol ‘_main’ from your main function, gprof still prints it as‘main’ in its output, so you should use ‘main’ in symspecs.

‘main.c:main’
Selects function ‘main’ in file ‘main.c’.
‘main.c:134’
Selects line 134 in file ‘main.c’.

Chapter 5: Interpreting gprof’s Output

15

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. See Section 4.1[Output Options], page 7.

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. Notethat if a function was not compiled for profiling, and didn’t run long enoughto 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
time
seconds
seconds
calls ms/call
33.34
0.02
0.02
7208
0.00
16.67
0.03
0.01
244
0.04
16.67
0.04
0.01
8
1.25
16.67
0.05
0.01
7
1.43
16.67
0.06
0.01
0.00
0.06
0.00
236
0.00
0.00
0.06
0.00
192
0.00
0.00
0.06
0.00
47
0.00
0.00
0.06
0.00
45
0.00
0.00
0.06
0.00
1
0.00
0.00
0.06
0.00
1
0.00
0.00
0.06
0.00
1
0.00
0.00
0.06
0.00
1
0.00
0.00
0.06
0.00
1
0.00
...

total
ms/call
0.00
0.12
1.25
1.43
0.00
0.00
0.00
0.00
50.00
0.00
10.11
0.00
50.00

name
open
offtimememccpywrite
mcounttzset
tolowerstrlenstrchrmain
memcpyprint
profilreport

The functions are sorted first by decreasing run-time spent in them, thenby 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 muchtime each sample counted as. This sampling period estimates the margin oferror 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

16

GNU gprof

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 inthe ‘open’ function, as indicated by the ‘self seconds’ field. Each of theother 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’ fieldfor ‘mcount’ might well be ‘0.00’ or ‘0.02’. See Section 6.1 [Statistical
Sampling Error], page 27, for a complete discussion.
The remaining functions in the listing (those whose ‘self seconds’ fieldis ‘0.00’) didn’t appear in the histogram samples at all. However, the callgraph 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 took.
Here is what the fields in each line mean:% timeThis 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 thefunctions 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.
callsThis is the total number of times the function was called. If thefunction was never called, or the number of times it was calledcannot be determined (probably because the function was notcompiled with profiling enabled), the calls field is blank.

self ms/callThis represents the average number of milliseconds spent in thisfunction per call, if this function is profiled. Otherwise, this fieldis blank for this function.
total ms/callThis represents the average number of milliseconds spent in thisfunction 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 analysis.
nameThis is the name of the function. The flat profile is sorted bythis field alphabetically after the self seconds and calls fields are
sorted.

Chapter 5: Interpreting gprof’s Output

17

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

selfchildrencalledname<spontaneous>
[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 thefollowing lines describe its subroutines (also called children when we speakof the call graph).
The entries are sorted by time spent in the function and its subroutines.
The internal profiling function mcount (see Section 5.1 [The Flat Profile],
page 15) is never mentioned in the call graph.

18

GNU gprof

5.2.1 The Primary LineThe 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 thenames of the fields:
index
...
[3]

% timeselfchildren called

100.0

0.00

0.05

1

name
report [3]Here is what the fields in the primary line mean:
index

Entries are numbered with consecutive integers. Each function
therefore has an index number, which appears at the beginningof its primary line.
Each cross-reference to a function, as a caller or subroutine ofanother, gives its index number as well as its name. The index
number guides you if you wish to look for the entry for thatfunction.

% timeThis is the percentage of the total time that was spent in thisfunction, including time spent in subroutines called from thisfunction.
The time spent in this function is counted again for the callers of
this function. Therefore, adding up these percentages is meaningless.

selfThis is the total amount of time spent in this function. Thisshould be identical to the number printed in the seconds field
for this function in the flat profile.

childrenThis is the total amount of time spent in the subroutine calls
made by this function. This should be equal to the sum of all theself and children entries of the children listed directly below
this function.

calledThis 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
main.

name

This is the name of the current function. The index number isrepeated 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(see Section 5.2.4 [How Mutually Recursive Functions Are Described], page 20). For example, if function gnurr is part ofChapter 5: Interpreting gprof’s Output

19

cycle number one, and has index number twelve, its primaryline would be end like this:gnurr <cycle 1> [12]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 aredifferent 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 headingline that shows the names of the fields:
index
...

% timeselfchildren called

[3]

100.0

0.00
0.00

0.05
0.05

1/1
1

name
main [2]report [3]Here are the meanings of the fields in the caller-line for report calledfrom main:self

An estimate of the amount of time spent in report itself when
it was called from main.

childrenAn estimate of the amount of time spent in subroutines ofreport when report was called from main.
The sum of the self and children fields is an estimate of theamount of time spent within calls to report from main.

calledTwo numbers: the number of times report was called frommain, followed by the total number of non-recursive calls toreport 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 ofthe functions it calls.
If the caller is part of a recursion cycle, the cycle number isprinted 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” andall other fields blank. This can happen for signal handlers.

5.2.3 Lines for a Function’s Subroutines
A function’s entry has a line for each of its subroutines—in other words, aline for each other function that it called. These lines’ fields correspond to

20

GNU gprof

the fields of the primary line, but their meanings are different because of thedifference 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 linethat shows the names of the fields:
index
...
[2]

% timeselfchildren called

100.0

0.00
0.00

0.05
0.05

1
1/1

name
main [2]report [3]Here are the meanings of the fields in the subroutine-line for main calling
report:self

An estimate of the amount of time spent directly within reportwhen report was called from main.

childrenAn estimate of the amount of time spent in subroutines ofreport when report was called from main.
The sum of the self and children fields is an estimate of thetotal time spent in calls to report from main.

calledTwo 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 andchildren time gets credited to main. See Section 6.2 [Estimating children Times], page 28.

name

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 isprinted between the name and the index number.

5.2.4 How Mutually Recursive Functions Are
Described
The graph may be complicated by the presence of cycles of recursion in thecall 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 calleach 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 callgraph paradoxical. The “time spent in children” of a should include the timeChapter 5: Interpreting gprof’s Output

21

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 ais indirectly recursive?
The way gprof resolves this paradox is by creating a single entry for thecycle as a whole. The primary line of this entry describes the total timespent directly in the functions of the cycle. The “subroutines” of the cycle
are the individual functions of the cycle, and all other functions that werecalled 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 aand 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
03b <cycle 1> [4]0.75
02a <cycle 1> [5]---------------------------------------3
a <cycle 1> [5]
[4]
52.85
1.02
0
0
b <cycle 1> [4]2a <cycle 1> [5]
0
0
3/6
c [6]
---------------------------------------1.77
0
1/1
main [2]2b <cycle 1> [4]
[5]
38.86
0.75
0
1
a <cycle 1> [5]3b <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 b.)index% time[1]100.00self

children called

name<spontaneous>
start [1]main [2]

0
1.93
0
0.16
1.77
1/1
---------------------------------------0.16
1.77
1/1
start [1][2]100.000.16
1.77
1main [2]1.77
01/1
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
03b <cycle 1> [4]0.75
02a <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]22

GNU gprof2
a <cycle 1> [5]
0
0
3/6
c [6]
---------------------------------------1.77
0
1/1
main [2]2b <cycle 1> [4]
[5]
38.86
0.75
0
1
a <cycle 1> [5]3b <cycle 1> [4]
0
0
3/6
c [6]
---------------------------------------00
3/6
b <cycle 1> [4]00
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 thefunctions of the cycle. It equals the sum of the self fields for the individualfunctions in the cycle, found in the entry in the subroutine lines for thesefunctions.
The children fields of the cycle’s primary line and subroutine lines countonly subroutines outside the cycle. Even though a calls b, the time spentin 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 theamount of time spent in the whole cycle, and its other subroutines, on thetimes when that caller called a function in the cycle.
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 thecycle (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 thecycle. 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 inthe 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 eachother function in the cycle. The self and children fields in these lines areblank because of the difficulty of defining meanings for them when recursion
is going on.

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

Chapter 5: Interpreting gprof’s Output

23

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 eachfunction, along with a count.
Here is a section of gprof’s output, without line-by-line profiling. Notethat 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
time
seconds
seconds
calls us/call
30.77
0.13
0.04
6335
6.31

total
us/call
6.31

namect_initCall graph (explanation follows)

granularity: each sample hit covers 4 byte(s) for 7.69% of 0.13 seconds
index % time

selfchildrencalled

[7]

0.00
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00

1/13496
40/13496
128/13496
13327/13496
13496

0.0

name
name_too_long
deflatedeflate_fast
ct_initinit_block

Now let’s look at some of gprof’s output from the same program run, thistime 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 eachof 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 callsfrom 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)

24

GNU gprof
7.69
7.69

0.12
0.13

0.01
0.01

ct_init (trees.c:382)
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

[6]

0.0

self

childrencalled

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00

1/13496
1/13496
1/13496
2/13496
4/13496
5/13496
12/13496
16/13496
128/13496
3071/13496
3730/13496
6525/13496
13496

name
name_too_long (gzip.c:1440)
deflate (deflate.c:763)
ct_init (trees.c:396)
deflate (deflate.c:727)
deflate (deflate.c:686)
deflate (deflate.c:675)
deflate (deflate.c:679)
deflate (deflate.c:730)
deflate_fast (deflate.c:654)
ct_init (trees.c:384)
ct_init (trees.c:385)
ct_init (trees.c:387)
init_block (trees.c:408)

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 wascalled. You may also need to specify the ‘-I’ option, if gprof can’t find thesource 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 program.
For example, consider the following function, taken from gzip, with line
numbers added:
1 ulg updcrc(s, n)2uch *s;3
unsigned n;
4 {5register ulg c;
6
7
static ulg crc = (ulg)0xffffffffL;89
if (s == NULL) {10
c = 0xffffffffL;
11} else {Chapter 5: Interpreting gprof’s Output
12
13
14
15
16
17
18
19 }

25

c = crc;
if (n) do {c = crc_32_tab[...];} while (--n);}crc = c;return c ^ 0xffffffffL;

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 contentsof 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 ofcode is labeled at least once. Here is updcrc’s annotated source listing for asample gzip run:ulg updcrc(s, n)uch *s;unsigned n;
2 ->{register ulg c;static ulg crc = (ulg)0xffffffffL;
2 ->
1 ->
1 ->
1 ->
1 ->
26312 ->
26312,1,26311 ->
2 ->
2 ->
2 ->}

if (s == NULL) {c = 0xffffffffL;} else {c = crc;
if (n) do {c = crc_32_tab[...];} while (--n);}crc = c;return c ^ 0xffffffffL;

In this example, the function was called twice, passing once through eachbranch 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.

Chapter 6: Inaccuracy of gprof Output

27

6 Inaccuracy of gprof Output
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 catchthat function in the act only once, there is a pretty good chance it will
actually find that function zero times, or twice.
By contrast, the number-of-calls and basic-block figures are derived bycounting, not sampling. They are completely accurate and will not varyfrom run to run if your program is deterministic and single threaded. In
multi-threaded applications, or single threaded applications that link withmulti-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). See Section 9.1 [Implementation of Profiling],
page 33.
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 itwill 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 fromseveral runs, using the ‘-s’ option of gprof. Here is how:
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.

28

GNU gprof

6. Analyze the cumulative data using this command:
gprof executable-file gmon.sum > output-file

6.2 Estimating children Times
Some of the figures in the call graph are estimates—for example, thechildren 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 inall, and 2/5 of the calls to foo came from a, then foo contributes 2 seconds
to a’s children time, by assumption.
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 thecalls from callers other than a. But gprof has no way of knowing this; itwill blindly and incorrectly charge 2 seconds of time in foo to the childrenof 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.

Chapter 7: Answers to Common Questions

29

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 theprogram is spending its time is to re-factor large functions intosequences of calls to smaller ones. Beware however that this canintroduce artificial hot spots since compiling with ‘-pg’ adds asignificant 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‘; dofastprog
mv gmon.out gmon.out.$i
done
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 ineach run will appear with a call count of 100).

Chapter 8: Incompatibilities with Unix gprof

31

8 Incompatibilities with Unix gprof
gnu gprof and Berkeley Unix gprof use the same data file ‘gmon.out’, andprovide essentially the same information. But there are a few differences.
• gnu gprof uses a new, generalized file format with support for basicblock 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. See Section 9.2 [Profiling Data File
Format], page 34.
• For a recursive function, Unix gprof lists the function as a parent andas 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 thesame 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 withoutskipping the blurbs.

Chapter 9: Details of Profiling33

9 Details of Profiling
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, whichcauses every function to call mcount (or _mcount, or __mcount, dependingon the OS and compiler) as one of its first operations.
The mcount routine, included in the profiling library, is responsible forrecording 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 istypically 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 formaintaining the in-memory call graph, which records frompc, selfpc, andthe 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, mostnotably the SPARC, using this builtin can be very computationallyexpensive, 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 theusual C library, but they were compiled with ‘-pg’. If you link your programwith ‘gcc ... -pg’, it automatically uses the profiling version of the library.
Profiling also involves watching your program as it runs, and keeping ahistogram 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 spacemaps 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.

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GNU gprof

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 thismethod requires a signal to be delivered to user space every time a sample istaken, it uses considerably more overhead than kernel-based profiling. Also,
due to the added delay required to deliver the signal, this method is lessaccurate 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 thedefault 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 (typicallywhen 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 wasconstructed 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-grapharcs and the basic-block counts.
The output from gprof gives no indication of parts of your programthat are limited by I/O or swapping bandwidth. This is because samples ofthe program counter are taken at fixed intervals of the program’s run time.
Therefore, the time measurements in gprof output say nothing about timethat your program was not running. For example, a part of the programthat 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 amountof load due to other users won’t directly affect the output you get.

9.2 Profiling Data File Format
The old BSD-derived file format used for profile data does not contain amagic cookie that allows one to check whether a data file really is a gprof file.
Furthermore, it does not provide a version number, thus rendering changes

Chapter 9: Details of Profiling35

to the file format almost impossible. gnu gprof uses a new file format thatprovides these features. For backward compatibility, gnu gprof continues
to support the old BSD-derived format, but not all features are supportedwith it. For example, basic-block execution counts cannot be accommodated
by the old file format.
The new file format is defined in header file ‘gmon_out.h’. It consists ofa header containing the magic cookie and a version number, as well as somespare bytes available for future extensions. All data in a profile data file isin 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, callgraph arc records, and basic-block execution count records. Each file cancontain 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 computethe union of all records. For example, for basic-block execution counts, theunion 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, thesize 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 physicaldimension that the bin counts represent after being scaled by the profilingclock 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” andthe 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 canbe used to produce a histogram of, say, instruction cache misses. In thiscase, 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, nota physical dimension). Also, the profiling rate would have to be set to 1 inthis case.
Histogram bins are 16-bit numbers and each bin represent an equalamount of text-space. For example, if the text-segment is one thousandbytes 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 countindicating the number of times the arc was traversed during program execution. Arcs are specified by a pair of addresses: the first must be within

36

GNU gprof

caller’s function and the second must be within the callee’s function. Whenperforming 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 whichline 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 thesequence. In an address/count pair, the address identifies a basic-block andthe count specifies the number of times that basic-block was executed. Any
address within the basic-address can be used.

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), thecall graph arcs (INCL ARCS/EXCL ARCS), printing in the callgraph (INCL GRAPH/EXCL GRAPH), timing propagation in thecall graph (INCL TIME/EXCL TIME), the annotated source listing (INCL ANNO/EXCL ANNO), and the execution count listing(INCL EXEC/EXCL EXEC).
After option processing, gprof finishes building the symspec list byadding 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
EXCL ANNO, EXCL ARCS, and EXCL EXEC.
Next, the BFD library is called to open the object file, verify that it isan 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 (ifthe ‘-c’ option was given).
gprof’s own symbol table, an array of Sym structures, is now built. Thisis done in one of two ways, by one of two routines, depending on whetherline-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 throughthe symbol table—one to count the size of the symbol table required, andChapter 9: Details of Profiling37

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, theqsort 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 indicatedwith 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 canmatch multiple symbols. An array of symbol tables (syms) is created, eachentry of which is a symbol table of Syms to be included or excluded from aparticular listing. The master symbol table and the symspecs are examinedby 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 ofeach required symbol table, and again to build the tables, which have beenmalloced 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 grapharc is created by cg_arcs.c:arc_add, unless the arc fails a symspec checkagainst 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. Boththe 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 ifline-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

38

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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 andassigning 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 checkagainst INCL FLAT/EXCL FLAT. Depending on the histogram scale factor, a sample bin may span multiple symbols, in which case a fraction of thesample 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 becredited 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 througheach symbol’s machine code, looking for subroutine call instructions, andadding 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 areassigned the same topological number. Two passes are now made throughthis 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, frombeginning 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 sourcecode itself (cg_print.c). Basically, the functions with the most use and themost parents are placed first, followed by other functions with the most use,followed by lower use functions, followed by unused functions at the end.

Chapter 9: Details of Profiling39

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 graphanalysis
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 callgraph entry
64 - Reading histogram and call graph records
Shows address ranges of histograms as they are read, and eachcall 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 graphTrace 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 graph2048 - 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

Appendix A: GNU Free Documentation License

41

Appendix A GNU Free Documentation
LicenseVersion 1.3, 3 November 2008
Copyright c 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
http://fsf.org/
Everyone is permitted to copy and distribute verbatim copiesof this license document, but changing it is not allowed.
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The purpose of this License is to make a manual, textbook, or otherfunctional and useful document free in the sense of freedom: to assure
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this License preserves for the author and publisher a way to get credit
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This License is a kind of “copyleft”, which means that derivative worksof the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft licensedesigned for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program
should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it canbe used for any textual work, regardless of subject matter or whether it
is published as a printed book. We recommend this License principally
for works whose purpose is instruction or reference.
1. APPLICABILITY AND DEFINITIONSThis License applies to any manual or other work, in any medium,that contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License. Such a notice grants aworld-wide, royalty-free license, unlimited in duration, to use that work
under the conditions stated herein. The “Document”, below, refers to
any such manual or work. Any member of the public is a licensee, andis addressed as “you”. You accept the license if you copy, modify or
distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing theDocument or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
A “Secondary Section” is a named appendix or a front-matter sectionof the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document’s overall subject (or42

GNU gprof
to related matters) and contains nothing that could fall directly withinthat overall subject. (Thus, if the Document is in part a textbook ofmathematics, a Secondary Section may not explain any mathematics.)
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Appendix A: GNU Free Documentation License

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The “publisher” means any person or entity that distributes copies of
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Appendix A: GNU Free Documentation License

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it was based on. These may be placed in the “History” section. You
may omit a network location for a work that was published at least
four years before the Document itself, or if the original publisher ofthe version it refers to gives permission.
K. For any section Entitled “Acknowledgements” or “Dedications”,
Preserve the Title of the section, and preserve in the section all thesubstance and tone of each of the contributor acknowledgements
and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document, unaltered intheir text and in their titles. Section numbers or the equivalent are
not considered part of the section titles.
M. Delete any section Entitled “Endorsements”. Such a section may
not be included in the Modified Version.
N. Do not retitle any existing section to be Entitled “Endorsements”
or to conflict in title with any Invariant Section.
O. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or appendicesthat qualify as Secondary Sections and contain no material copied fromthe Document, you may at your option designate some or all of these
sections as invariant. To do this, add their titles to the list of Invariant
Sections in the Modified Version’s license notice. These titles must be
distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains
nothing but endorsements of your Modified Version by various parties—
for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and apassage of up to 25 words as a Back-Cover Text, to the end of the list of
Cover Texts in the Modified Version. Only one passage of Front-CoverText and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes
a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not

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add another; but you may replace the old one, on explicit permission
from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or imply
endorsement of any Modified Version.
5. COMBINING DOCUMENTS
You may combine the Document with other documents released under
this License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them
all as Invariant Sections of your combined work in its license notice, andthat you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, andmultiple identical Invariant Sections may be replaced with a single copy.
If there are multiple Invariant Sections with the same name but different
contents, make the title of each such section unique by adding at the endof it, in parentheses, the name of the original author or publisher of that
section if known, or else a unique number. Make the same adjustment
to the section titles in the list of Invariant Sections in the license noticeof the combined work.
In the combination, you must combine any sections Entitled “History”
in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, andany sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies ofthis License in the various documents with a single copy that is includedin the collection, provided that you follow the rules of this License forverbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distributeit individually under this License, provided you insert a copy of thisLicense into the extracted document, and follow this License in all otherrespects regarding verbatim copying of that document.
7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an “aggregate” if the copyright resulting
from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When theDocument is included in an aggregate, this License does not apply tothe other works in the aggregate which are not themselves derivative
works of the Document.

Appendix A: GNU Free Documentation License

47

If the Cover Text requirement of section 3 is applicable to these copiesof the Document, then if the Document is less than one half of the entire
aggregate, the Document’s Cover Texts may be placed on covers thatbracket the Document within the aggregate, or the electronic equivalentof covers if the Document is in electronic form. Otherwise they must
appear on printed covers that bracket the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may distributetranslations of the Document under the terms of section 4. Replacing
Invariant Sections with translations requires special permission fromtheir copyright holders, but you may include translations of some or all
Invariant Sections in addition to the original versions of these Invariant
Sections. You may include a translation of this License, and all thelicense notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License
and the original versions of those notices and disclaimers. In case ofa disagreement between the translation and the original version of thisLicense or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title(section 1) will typically require changing the actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise tocopy, modify, sublicense, or distribute it is void, and will automaticallyterminate your rights under this License.
However, if you cease all violation of this License, then your license froma particular copyright holder is reinstated (a) provisionally, unless anduntil the copyright holder explicitly and finally terminates your license,and (b) permanently, if the copyright holder fails to notify you of theviolation by some reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstatedpermanently if the copyright holder notifies you of the violation by somereasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, andyou cure the violation prior to 30 days after your receipt of the notice.
Termination of your rights under this section does not terminate thelicenses of parties who have received copies or rights from you under
this License. If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versionsof the GNU Free Documentation License from time to time. Such

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new versions will be similar in spirit to the present version, but
may differ in detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of thisLicense “or any later version” applies to it, you have the option offollowing the terms and conditions either of that specified version orof any later version that has been published (not as a draft) by theFree Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (notas a draft) by the Free Software Foundation. If the Document specifies
that a proxy can decide which future versions of this License can be used,that proxy’s public statement of acceptance of a version permanently
authorizes you to choose that version for the Document.
11. RELICENSING“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A public
wiki that anybody can edit is an example of such a server. A “Massive
Multiauthor Collaboration” (or “MMC”) contained in the site means
any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by thatsame organization.
“Incorporate” means to publish or republish a Document, in whole orin part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License,
and if all works that were first published under this License somewhereother than this MMC, and subsequently incorporated in whole or inpart into the MMC, (1) had no cover texts or invariant sections, and(2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in thesite under CC-BY-SA on the same site at any time before August 1,
2009, provided the MMC is eligible for relicensing.

Appendix A: GNU Free Documentation License

49

ADDENDUM: How to use this License for yourdocuments
To use this License in a document you have written, include a copy of theLicense in the document and put the following copyright and license notices
just after the title page:
Copyright (C) year your name .
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;with no Invariant Sections, no Front-Cover Texts, and no Back-CoverTexts. A copy of the license is included in the section entitled ‘‘GNUFree Documentation License’’.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the “with. . . Texts.” line with this:with the Invariant Sections being list their titles , with
the Front-Cover Texts being list , and with the Back-Cover Texts
being list .

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use
in free software.