xref: /netbsd-src/external/gpl3/gcc.old/dist/gcc/doc/gcov.texi (revision 8feb0f0b7eaff0608f8350bbfa3098827b4bb91b)

@c Copyright (C) 1996-2020 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@ignore
@c man begin COPYRIGHT
Copyright @copyright{} 1996-2020 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'' and ``Funding
Free Software'', the Front-Cover texts being (a) (see below), and with
the Back-Cover Texts being (b) (see below).  A copy of the license is
included in the gfdl(7) man page.

(a) The FSF's Front-Cover Text is:

     A GNU Manual

(b) The FSF's Back-Cover Text is:

     You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development.
@c man end
@c Set file name and title for the man page.
@setfilename gcov
@settitle coverage testing tool
@end ignore

@node Gcov
@chapter @command{gcov}---a Test Coverage Program

@command{gcov} is a tool you can use in conjunction with GCC to
test code coverage in your programs.

@menu
* Gcov Intro::                  Introduction to gcov.
* Invoking Gcov::               How to use gcov.
* Gcov and Optimization::       Using gcov with GCC optimization.
* Gcov Data Files::             The files used by gcov.
* Cross-profiling::             Data file relocation.
@end menu

@node Gcov Intro
@section Introduction to @command{gcov}
@c man begin DESCRIPTION

@command{gcov} is a test coverage program.  Use it in concert with GCC
to analyze your programs to help create more efficient, faster running
code and to discover untested parts of your program.  You can use
@command{gcov} as a profiling tool to help discover where your
optimization efforts will best affect your code.  You can also use
@command{gcov} along with the other profiling tool, @command{gprof}, to
assess which parts of your code use the greatest amount of computing
time.

Profiling tools help you analyze your code's performance.  Using a
profiler such as @command{gcov} or @command{gprof}, you can find out some
basic performance statistics, such as:

@itemize @bullet
@item
how often each line of code executes

@item
what lines of code are actually executed

@item
how much computing time each section of code uses
@end itemize

Once you know these things about how your code works when compiled, you
can look at each module to see which modules should be optimized.
@command{gcov} helps you determine where to work on optimization.

Software developers also use coverage testing in concert with
testsuites, to make sure software is actually good enough for a release.
Testsuites can verify that a program works as expected; a coverage
program tests to see how much of the program is exercised by the
testsuite.  Developers can then determine what kinds of test cases need
to be added to the testsuites to create both better testing and a better
final product.

You should compile your code without optimization if you plan to use
@command{gcov} because the optimization, by combining some lines of code
into one function, may not give you as much information as you need to
look for `hot spots' where the code is using a great deal of computer
time.  Likewise, because @command{gcov} accumulates statistics by line (at
the lowest resolution), it works best with a programming style that
places only one statement on each line.  If you use complicated macros
that expand to loops or to other control structures, the statistics are
less helpful---they only report on the line where the macro call
appears.  If your complex macros behave like functions, you can replace
them with inline functions to solve this problem.

@command{gcov} creates a logfile called @file{@var{sourcefile}.gcov} which
indicates how many times each line of a source file @file{@var{sourcefile}.c}
has executed.  You can use these logfiles along with @command{gprof} to aid
in fine-tuning the performance of your programs.  @command{gprof} gives
timing information you can use along with the information you get from
@command{gcov}.

@command{gcov} works only on code compiled with GCC@.  It is not
compatible with any other profiling or test coverage mechanism.

@c man end

@node Invoking Gcov
@section Invoking @command{gcov}

@smallexample
gcov @r{[}@var{options}@r{]} @var{files}
@end smallexample

@command{gcov} accepts the following options:

@ignore
@c man begin SYNOPSIS
gcov [@option{-v}|@option{--version}] [@option{-h}|@option{--help}]
     [@option{-a}|@option{--all-blocks}]
     [@option{-b}|@option{--branch-probabilities}]
     [@option{-c}|@option{--branch-counts}]
     [@option{-d}|@option{--display-progress}]
     [@option{-f}|@option{--function-summaries}]
     [@option{-i}|@option{--json-format}]
     [@option{-j}|@option{--human-readable}]
     [@option{-k}|@option{--use-colors}]
     [@option{-l}|@option{--long-file-names}]
     [@option{-m}|@option{--demangled-names}]
     [@option{-n}|@option{--no-output}]
     [@option{-o}|@option{--object-directory} @var{directory|file}]
     [@option{-p}|@option{--preserve-paths}]
     [@option{-q}|@option{--use-hotness-colors}]
     [@option{-r}|@option{--relative-only}]
     [@option{-s}|@option{--source-prefix} @var{directory}]
     [@option{-t}|@option{--stdout}]
     [@option{-u}|@option{--unconditional-branches}]
     [@option{-x}|@option{--hash-filenames}]
     @var{files}
@c man end
@c man begin SEEALSO
gpl(7), gfdl(7), fsf-funding(7), gcc(1) and the Info entry for @file{gcc}.
@c man end
@end ignore

@c man begin OPTIONS
@table @gcctabopt

@item -a
@itemx --all-blocks
Write individual execution counts for every basic block.  Normally gcov
outputs execution counts only for the main blocks of a line.  With this
option you can determine if blocks within a single line are not being
executed.

@item -b
@itemx --branch-probabilities
Write branch frequencies to the output file, and write branch summary
info to the standard output.  This option allows you to see how often
each branch in your program was taken.  Unconditional branches will not
be shown, unless the @option{-u} option is given.

@item -c
@itemx --branch-counts
Write branch frequencies as the number of branches taken, rather than
the percentage of branches taken.

@item -d
@itemx --display-progress
Display the progress on the standard output.

@item -f
@itemx --function-summaries
Output summaries for each function in addition to the file level summary.

@item -h
@itemx --help
Display help about using @command{gcov} (on the standard output), and
exit without doing any further processing.

@item -i
@itemx --json-format
Output gcov file in an easy-to-parse JSON intermediate format
which does not require source code for generation.  The JSON
file is compressed with gzip compression algorithm
and the files have @file{.gcov.json.gz} extension.

Structure of the JSON is following:

@smallexample
@{
  "current_working_directory": @var{current_working_directory},
  "data_file": @var{data_file},
  "format_version": @var{format_version},
  "gcc_version": @var{gcc_version}
  "files": [@var{file}]
@}
@end smallexample

Fields of the root element have following semantics:

@itemize @bullet
@item
@var{current_working_directory}: working directory where
a compilation unit was compiled

@item
@var{data_file}: name of the data file (GCDA)

@item
@var{format_version}: semantic version of the format

@item
@var{gcc_version}: version of the GCC compiler
@end itemize

Each @var{file} has the following form:

@smallexample
@{
  "file": @var{file_name},
  "functions": [@var{function}],
  "lines": [@var{line}]
@}
@end smallexample

Fields of the @var{file} element have following semantics:

@itemize @bullet
@item
@var{file_name}: name of the source file
@end itemize

Each @var{function} has the following form:

@smallexample
@{
  "blocks": @var{blocks},
  "blocks_executed": @var{blocks_executed},
  "demangled_name": "@var{demangled_name},
  "end_column": @var{end_column},
  "end_line": @var{end_line},
  "execution_count": @var{execution_count},
  "name": @var{name},
  "start_column": @var{start_column}
  "start_line": @var{start_line}
@}
@end smallexample

Fields of the @var{function} element have following semantics:

@itemize @bullet
@item
@var{blocks}: number of blocks that are in the function

@item
@var{blocks_executed}: number of executed blocks of the function

@item
@var{demangled_name}: demangled name of the function

@item
@var{end_column}: column in the source file where the function ends

@item
@var{end_line}: line in the source file where the function ends

@item
@var{execution_count}: number of executions of the function

@item
@var{name}: name of the function

@item
@var{start_column}: column in the source file where the function begins

@item
@var{start_line}: line in the source file where the function begins
@end itemize

Note that line numbers and column numbers number from 1.  In the current
implementation, @var{start_line} and @var{start_column} do not include
any template parameters and the leading return type but that
this is likely to be fixed in the future.

Each @var{line} has the following form:

@smallexample
@{
  "branches": [@var{branch}],
  "count": @var{count},
  "line_number": @var{line_number},
  "unexecuted_block": @var{unexecuted_block}
  "function_name": @var{function_name},
@}
@end smallexample

Branches are present only with @var{-b} option.
Fields of the @var{line} element have following semantics:

@itemize @bullet
@item
@var{count}: number of executions of the line

@item
@var{line_number}: line number

@item
@var{unexecuted_block}: flag whether the line contains an unexecuted block
(not all statements on the line are executed)

@item
@var{function_name}: a name of a function this @var{line} belongs to
(for a line with an inlined statements can be not set)
@end itemize

Each @var{branch} has the following form:

@smallexample
@{
  "count": @var{count},
  "fallthrough": @var{fallthrough},
  "throw": @var{throw}
@}
@end smallexample

Fields of the @var{branch} element have following semantics:

@itemize @bullet
@item
@var{count}: number of executions of the branch

@item
@var{fallthrough}: true when the branch is a fall through branch

@item
@var{throw}: true when the branch is an exceptional branch
@end itemize

@item -j
@itemx --human-readable
Write counts in human readable format (like 24.6k).

@item -k
@itemx --use-colors

Use colors for lines of code that have zero coverage.  We use red color for
non-exceptional lines and cyan for exceptional.  Same colors are used for
basic blocks with @option{-a} option.

@item -l
@itemx --long-file-names
Create long file names for included source files.  For example, if the
header file @file{x.h} contains code, and was included in the file
@file{a.c}, then running @command{gcov} on the file @file{a.c} will
produce an output file called @file{a.c##x.h.gcov} instead of
@file{x.h.gcov}.  This can be useful if @file{x.h} is included in
multiple source files and you want to see the individual
contributions.  If you use the @samp{-p} option, both the including
and included file names will be complete path names.

@item -m
@itemx --demangled-names
Display demangled function names in output. The default is to show
mangled function names.

@item -n
@itemx --no-output
Do not create the @command{gcov} output file.

@item -o @var{directory|file}
@itemx --object-directory @var{directory}
@itemx --object-file @var{file}
Specify either the directory containing the gcov data files, or the
object path name.  The @file{.gcno}, and
@file{.gcda} data files are searched for using this option.  If a directory
is specified, the data files are in that directory and named after the
input file name, without its extension.  If a file is specified here,
the data files are named after that file, without its extension.

@item -p
@itemx --preserve-paths
Preserve complete path information in the names of generated
@file{.gcov} files.  Without this option, just the filename component is
used.  With this option, all directories are used, with @samp{/} characters
translated to @samp{#} characters, @file{.} directory components
removed and unremoveable @file{..}
components renamed to @samp{^}.  This is useful if sourcefiles are in several
different directories.

@item -q
@itemx --use-hotness-colors

Emit perf-like colored output for hot lines.  Legend of the color scale
is printed at the very beginning of the output file.

@item -r
@itemx --relative-only
Only output information about source files with a relative pathname
(after source prefix elision).  Absolute paths are usually system
header files and coverage of any inline functions therein is normally
uninteresting.

@item -s @var{directory}
@itemx --source-prefix @var{directory}
A prefix for source file names to remove when generating the output
coverage files.  This option is useful when building in a separate
directory, and the pathname to the source directory is not wanted when
determining the output file names.  Note that this prefix detection is
applied before determining whether the source file is absolute.

@item -t
@itemx --stdout
Output to standard output instead of output files.

@item -u
@itemx --unconditional-branches
When branch probabilities are given, include those of unconditional branches.
Unconditional branches are normally not interesting.

@item -v
@itemx --version
Display the @command{gcov} version number (on the standard output),
and exit without doing any further processing.

@item -w
@itemx --verbose
Print verbose informations related to basic blocks and arcs.

@item -x
@itemx --hash-filenames
When using @var{--preserve-paths},
gcov uses the full pathname of the source files to create
an output filename.  This can lead to long filenames that can overflow
filesystem limits.  This option creates names of the form
@file{@var{source-file}##@var{md5}.gcov},
where the @var{source-file} component is the final filename part and
the @var{md5} component is calculated from the full mangled name that
would have been used otherwise.  The option is an alternative
to the @var{--preserve-paths} on systems which have a filesystem limit.

@end table

@command{gcov} should be run with the current directory the same as that
when you invoked the compiler.  Otherwise it will not be able to locate
the source files.  @command{gcov} produces files called
@file{@var{mangledname}.gcov} in the current directory.  These contain
the coverage information of the source file they correspond to.
One @file{.gcov} file is produced for each source (or header) file
containing code,
which was compiled to produce the data files.  The @var{mangledname} part
of the output file name is usually simply the source file name, but can
be something more complicated if the @samp{-l} or @samp{-p} options are
given.  Refer to those options for details.

If you invoke @command{gcov} with multiple input files, the
contributions from each input file are summed.  Typically you would
invoke it with the same list of files as the final link of your executable.

The @file{.gcov} files contain the @samp{:} separated fields along with
program source code.  The format is

@smallexample
@var{execution_count}:@var{line_number}:@var{source line text}
@end smallexample

Additional block information may succeed each line, when requested by
command line option.  The @var{execution_count} is @samp{-} for lines
containing no code.  Unexecuted lines are marked @samp{#####} or
@samp{=====}, depending on whether they are reachable by
non-exceptional paths or only exceptional paths such as C++ exception
handlers, respectively. Given the @samp{-a} option, unexecuted blocks are
marked @samp{$$$$$} or @samp{%%%%%}, depending on whether a basic block
is reachable via non-exceptional or exceptional paths.
Executed basic blocks having a statement with zero @var{execution_count}
end with @samp{*} character and are colored with magenta color with
the @option{-k} option.  This functionality is not supported in Ada.

Note that GCC can completely remove the bodies of functions that are
not needed -- for instance if they are inlined everywhere.  Such functions
are marked with @samp{-}, which can be confusing.
Use the @option{-fkeep-inline-functions} and @option{-fkeep-static-functions}
options to retain these functions and
allow gcov to properly show their @var{execution_count}.

Some lines of information at the start have @var{line_number} of zero.
These preamble lines are of the form

@smallexample
-:0:@var{tag}:@var{value}
@end smallexample

The ordering and number of these preamble lines will be augmented as
@command{gcov} development progresses --- do not rely on them remaining
unchanged.  Use @var{tag} to locate a particular preamble line.

The additional block information is of the form

@smallexample
@var{tag} @var{information}
@end smallexample

The @var{information} is human readable, but designed to be simple
enough for machine parsing too.

When printing percentages, 0% and 100% are only printed when the values
are @emph{exactly} 0% and 100% respectively.  Other values which would
conventionally be rounded to 0% or 100% are instead printed as the
nearest non-boundary value.

When using @command{gcov}, you must first compile your program
with a special GCC option @samp{--coverage}.
This tells the compiler to generate additional information needed by
gcov (basically a flow graph of the program) and also includes
additional code in the object files for generating the extra profiling
information needed by gcov.  These additional files are placed in the
directory where the object file is located.

Running the program will cause profile output to be generated.  For each
source file compiled with @option{-fprofile-arcs}, an accompanying
@file{.gcda} file will be placed in the object file directory.

Running @command{gcov} with your program's source file names as arguments
will now produce a listing of the code along with frequency of execution
for each line.  For example, if your program is called @file{tmp.cpp}, this
is what you see when you use the basic @command{gcov} facility:

@smallexample
$ g++ --coverage tmp.cpp
$ a.out
$ gcov tmp.cpp -m
File 'tmp.cpp'
Lines executed:92.86% of 14
Creating 'tmp.cpp.gcov'
@end smallexample

The file @file{tmp.cpp.gcov} contains output from @command{gcov}.
Here is a sample:

@smallexample
        -:    0:Source:tmp.cpp
        -:    0:Working directory:/home/gcc/testcase
        -:    0:Graph:tmp.gcno
        -:    0:Data:tmp.gcda
        -:    0:Runs:1
        -:    0:Programs:1
        -:    1:#include <stdio.h>
        -:    2:
        -:    3:template<class T>
        -:    4:class Foo
        -:    5:@{
        -:    6:  public:
       1*:    7:  Foo(): b (1000) @{@}
------------------
Foo<char>::Foo():
    #####:    7:  Foo(): b (1000) @{@}
------------------
Foo<int>::Foo():
        1:    7:  Foo(): b (1000) @{@}
------------------
       2*:    8:  void inc () @{ b++; @}
------------------
Foo<char>::inc():
    #####:    8:  void inc () @{ b++; @}
------------------
Foo<int>::inc():
        2:    8:  void inc () @{ b++; @}
------------------
        -:    9:
        -:   10:  private:
        -:   11:  int b;
        -:   12:@};
        -:   13:
        -:   14:template class Foo<int>;
        -:   15:template class Foo<char>;
        -:   16:
        -:   17:int
        1:   18:main (void)
        -:   19:@{
        -:   20:  int i, total;
        1:   21:  Foo<int> counter;
        -:   22:
        1:   23:  counter.inc();
        1:   24:  counter.inc();
        1:   25:  total = 0;
        -:   26:
       11:   27:  for (i = 0; i < 10; i++)
       10:   28:    total += i;
        -:   29:
       1*:   30:  int v = total > 100 ? 1 : 2;
        -:   31:
        1:   32:  if (total != 45)
    #####:   33:    printf ("Failure\n");
        -:   34:  else
        1:   35:    printf ("Success\n");
        1:   36:  return 0;
        -:   37:@}
@end smallexample

Note that line 7 is shown in the report multiple times.  First occurrence
presents total number of execution of the line and the next two belong
to instances of class Foo constructors.  As you can also see, line 30 contains
some unexecuted basic blocks and thus execution count has asterisk symbol.

When you use the @option{-a} option, you will get individual block
counts, and the output looks like this:

@smallexample
        -:    0:Source:tmp.cpp
        -:    0:Working directory:/home/gcc/testcase
        -:    0:Graph:tmp.gcno
        -:    0:Data:tmp.gcda
        -:    0:Runs:1
        -:    0:Programs:1
        -:    1:#include <stdio.h>
        -:    2:
        -:    3:template<class T>
        -:    4:class Foo
        -:    5:@{
        -:    6:  public:
       1*:    7:  Foo(): b (1000) @{@}
------------------
Foo<char>::Foo():
    #####:    7:  Foo(): b (1000) @{@}
------------------
Foo<int>::Foo():
        1:    7:  Foo(): b (1000) @{@}
------------------
       2*:    8:  void inc () @{ b++; @}
------------------
Foo<char>::inc():
    #####:    8:  void inc () @{ b++; @}
------------------
Foo<int>::inc():
        2:    8:  void inc () @{ b++; @}
------------------
        -:    9:
        -:   10:  private:
        -:   11:  int b;
        -:   12:@};
        -:   13:
        -:   14:template class Foo<int>;
        -:   15:template class Foo<char>;
        -:   16:
        -:   17:int
        1:   18:main (void)
        -:   19:@{
        -:   20:  int i, total;
        1:   21:  Foo<int> counter;
        1:   21-block  0
        -:   22:
        1:   23:  counter.inc();
        1:   23-block  0
        1:   24:  counter.inc();
        1:   24-block  0
        1:   25:  total = 0;
        -:   26:
       11:   27:  for (i = 0; i < 10; i++)
        1:   27-block  0
       11:   27-block  1
       10:   28:    total += i;
       10:   28-block  0
        -:   29:
       1*:   30:  int v = total > 100 ? 1 : 2;
        1:   30-block  0
    %%%%%:   30-block  1
        1:   30-block  2
        -:   31:
        1:   32:  if (total != 45)
        1:   32-block  0
    #####:   33:    printf ("Failure\n");
    %%%%%:   33-block  0
        -:   34:  else
        1:   35:    printf ("Success\n");
        1:   35-block  0
        1:   36:  return 0;
        1:   36-block  0
        -:   37:@}
@end smallexample

In this mode, each basic block is only shown on one line -- the last
line of the block.  A multi-line block will only contribute to the
execution count of that last line, and other lines will not be shown
to contain code, unless previous blocks end on those lines.
The total execution count of a line is shown and subsequent lines show
the execution counts for individual blocks that end on that line.  After each
block, the branch and call counts of the block will be shown, if the
@option{-b} option is given.

Because of the way GCC instruments calls, a call count can be shown
after a line with no individual blocks.
As you can see, line 33 contains a basic block that was not executed.

@need 450
When you use the @option{-b} option, your output looks like this:

@smallexample
        -:    0:Source:tmp.cpp
        -:    0:Working directory:/home/gcc/testcase
        -:    0:Graph:tmp.gcno
        -:    0:Data:tmp.gcda
        -:    0:Runs:1
        -:    0:Programs:1
        -:    1:#include <stdio.h>
        -:    2:
        -:    3:template<class T>
        -:    4:class Foo
        -:    5:@{
        -:    6:  public:
       1*:    7:  Foo(): b (1000) @{@}
------------------
Foo<char>::Foo():
function Foo<char>::Foo() called 0 returned 0% blocks executed 0%
    #####:    7:  Foo(): b (1000) @{@}
------------------
Foo<int>::Foo():
function Foo<int>::Foo() called 1 returned 100% blocks executed 100%
        1:    7:  Foo(): b (1000) @{@}
------------------
       2*:    8:  void inc () @{ b++; @}
------------------
Foo<char>::inc():
function Foo<char>::inc() called 0 returned 0% blocks executed 0%
    #####:    8:  void inc () @{ b++; @}
------------------
Foo<int>::inc():
function Foo<int>::inc() called 2 returned 100% blocks executed 100%
        2:    8:  void inc () @{ b++; @}
------------------
        -:    9:
        -:   10:  private:
        -:   11:  int b;
        -:   12:@};
        -:   13:
        -:   14:template class Foo<int>;
        -:   15:template class Foo<char>;
        -:   16:
        -:   17:int
function main called 1 returned 100% blocks executed 81%
        1:   18:main (void)
        -:   19:@{
        -:   20:  int i, total;
        1:   21:  Foo<int> counter;
call    0 returned 100%
branch  1 taken 100% (fallthrough)
branch  2 taken 0% (throw)
        -:   22:
        1:   23:  counter.inc();
call    0 returned 100%
branch  1 taken 100% (fallthrough)
branch  2 taken 0% (throw)
        1:   24:  counter.inc();
call    0 returned 100%
branch  1 taken 100% (fallthrough)
branch  2 taken 0% (throw)
        1:   25:  total = 0;
        -:   26:
       11:   27:  for (i = 0; i < 10; i++)
branch  0 taken 91% (fallthrough)
branch  1 taken 9%
       10:   28:    total += i;
        -:   29:
       1*:   30:  int v = total > 100 ? 1 : 2;
branch  0 taken 0% (fallthrough)
branch  1 taken 100%
        -:   31:
        1:   32:  if (total != 45)
branch  0 taken 0% (fallthrough)
branch  1 taken 100%
    #####:   33:    printf ("Failure\n");
call    0 never executed
branch  1 never executed
branch  2 never executed
        -:   34:  else
        1:   35:    printf ("Success\n");
call    0 returned 100%
branch  1 taken 100% (fallthrough)
branch  2 taken 0% (throw)
        1:   36:  return 0;
        -:   37:@}
@end smallexample

For each function, a line is printed showing how many times the function
is called, how many times it returns and what percentage of the
function's blocks were executed.

For each basic block, a line is printed after the last line of the basic
block describing the branch or call that ends the basic block.  There can
be multiple branches and calls listed for a single source line if there
are multiple basic blocks that end on that line.  In this case, the
branches and calls are each given a number.  There is no simple way to map
these branches and calls back to source constructs.  In general, though,
the lowest numbered branch or call will correspond to the leftmost construct
on the source line.

For a branch, if it was executed at least once, then a percentage
indicating the number of times the branch was taken divided by the
number of times the branch was executed will be printed.  Otherwise, the
message ``never executed'' is printed.

For a call, if it was executed at least once, then a percentage
indicating the number of times the call returned divided by the number
of times the call was executed will be printed.  This will usually be
100%, but may be less for functions that call @code{exit} or @code{longjmp},
and thus may not return every time they are called.

The execution counts are cumulative.  If the example program were
executed again without removing the @file{.gcda} file, the count for the
number of times each line in the source was executed would be added to
the results of the previous run(s).  This is potentially useful in
several ways.  For example, it could be used to accumulate data over a
number of program runs as part of a test verification suite, or to
provide more accurate long-term information over a large number of
program runs.

The data in the @file{.gcda} files is saved immediately before the program
exits.  For each source file compiled with @option{-fprofile-arcs}, the
profiling code first attempts to read in an existing @file{.gcda} file; if
the file doesn't match the executable (differing number of basic block
counts) it will ignore the contents of the file.  It then adds in the
new execution counts and finally writes the data to the file.

@node Gcov and Optimization
@section Using @command{gcov} with GCC Optimization

If you plan to use @command{gcov} to help optimize your code, you must
first compile your program with a special GCC option
@samp{--coverage}.  Aside from that, you can use any
other GCC options; but if you want to prove that every single line
in your program was executed, you should not compile with optimization
at the same time.  On some machines the optimizer can eliminate some
simple code lines by combining them with other lines.  For example, code
like this:

@smallexample
if (a != b)
  c = 1;
else
  c = 0;
@end smallexample

@noindent
can be compiled into one instruction on some machines.  In this case,
there is no way for @command{gcov} to calculate separate execution counts
for each line because there isn't separate code for each line.  Hence
the @command{gcov} output looks like this if you compiled the program with
optimization:

@smallexample
      100:   12:if (a != b)
      100:   13:  c = 1;
      100:   14:else
      100:   15:  c = 0;
@end smallexample

The output shows that this block of code, combined by optimization,
executed 100 times.  In one sense this result is correct, because there
was only one instruction representing all four of these lines.  However,
the output does not indicate how many times the result was 0 and how
many times the result was 1.

Inlineable functions can create unexpected line counts.  Line counts are
shown for the source code of the inlineable function, but what is shown
depends on where the function is inlined, or if it is not inlined at all.

If the function is not inlined, the compiler must emit an out of line
copy of the function, in any object file that needs it.  If
@file{fileA.o} and @file{fileB.o} both contain out of line bodies of a
particular inlineable function, they will also both contain coverage
counts for that function.  When @file{fileA.o} and @file{fileB.o} are
linked together, the linker will, on many systems, select one of those
out of line bodies for all calls to that function, and remove or ignore
the other.  Unfortunately, it will not remove the coverage counters for
the unused function body.  Hence when instrumented, all but one use of
that function will show zero counts.

If the function is inlined in several places, the block structure in
each location might not be the same.  For instance, a condition might
now be calculable at compile time in some instances.  Because the
coverage of all the uses of the inline function will be shown for the
same source lines, the line counts themselves might seem inconsistent.

Long-running applications can use the @code{__gcov_reset} and @code{__gcov_dump}
facilities to restrict profile collection to the program region of
interest. Calling @code{__gcov_reset(void)} will clear all profile counters
to zero, and calling @code{__gcov_dump(void)} will cause the profile information
collected at that point to be dumped to @file{.gcda} output files.
Instrumented applications use a static destructor with priority 99
to invoke the @code{__gcov_dump} function. Thus @code{__gcov_dump}
is executed after all user defined static destructors,
as well as handlers registered with @code{atexit}.
If an executable loads a dynamic shared object via dlopen functionality,
@option{-Wl,--dynamic-list-data} is needed to dump all profile data.

Profiling run-time library reports various errors related to profile
manipulation and profile saving.  Errors are printed into standard error output
or @samp{GCOV_ERROR_FILE} file, if environment variable is used.
In order to terminate immediately after an errors occurs
set @samp{GCOV_EXIT_AT_ERROR} environment variable.
That can help users to find profile clashing which leads
to a misleading profile.

@c man end

@node Gcov Data Files
@section Brief Description of @command{gcov} Data Files

@command{gcov} uses two files for profiling.  The names of these files
are derived from the original @emph{object} file by substituting the
file suffix with either @file{.gcno}, or @file{.gcda}.  The files
contain coverage and profile data stored in a platform-independent format.
The @file{.gcno} files are placed in the same directory as the object
file.  By default, the @file{.gcda} files are also stored in the same
directory as the object file, but the GCC @option{-fprofile-dir} option
may be used to store the @file{.gcda} files in a separate directory.

The @file{.gcno} notes file is generated when the source file is compiled
with the GCC @option{-ftest-coverage} option.  It contains information to
reconstruct the basic block graphs and assign source line numbers to
blocks.

The @file{.gcda} count data file is generated when a program containing
object files built with the GCC @option{-fprofile-arcs} option is executed.
A separate @file{.gcda} file is created for each object file compiled with
this option.  It contains arc transition counts, value profile counts, and
some summary information.

It is not recommended to access the coverage files directly.
Consumers should use the intermediate format that is provided
by @command{gcov} tool via @option{--json-format} option.

@node Cross-profiling
@section Data File Relocation to Support Cross-Profiling

Running the program will cause profile output to be generated.  For each
source file compiled with @option{-fprofile-arcs}, an accompanying @file{.gcda}
file will be placed in the object file directory. That implicitly requires
running the program on the same system as it was built or having the same
absolute directory structure on the target system. The program will try
to create the needed directory structure, if it is not already present.

To support cross-profiling, a program compiled with @option{-fprofile-arcs}
can relocate the data files based on two environment variables:

@itemize @bullet
@item
GCOV_PREFIX contains the prefix to add to the absolute paths
in the object file. Prefix can be absolute, or relative.  The
default is no prefix.

@item
GCOV_PREFIX_STRIP indicates the how many initial directory names to strip off
the hardwired absolute paths. Default value is 0.

@emph{Note:} If GCOV_PREFIX_STRIP is set without GCOV_PREFIX is undefined,
 then a relative path is made out of the hardwired absolute paths.
@end itemize

For example, if the object file @file{/user/build/foo.o} was built with
@option{-fprofile-arcs}, the final executable will try to create the data file
@file{/user/build/foo.gcda} when running on the target system.  This will
fail if the corresponding directory does not exist and it is unable to create
it.  This can be overcome by, for example, setting the environment as
@samp{GCOV_PREFIX=/target/run} and @samp{GCOV_PREFIX_STRIP=1}.  Such a
setting will name the data file @file{/target/run/build/foo.gcda}.

You must move the data files to the expected directory tree in order to
use them for profile directed optimizations (@option{-fprofile-use}), or to
use the @command{gcov} tool.