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Updated: July 2014

gcov (1)


gcov - coverage testing tool


gcov [-v|--version] [-h|--help]
[-o|--object-directory directory|file]
[-s|--source-prefix directory]


GNU                                                       GCOV(1)

     gcov - coverage testing tool

     gcov [-v|--version] [-h|--help]
          [-o|--object-directory directory|file]
          [-s|--source-prefix directory]

     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 gcov as a profiling tool to help
     discover where your optimization efforts will best affect
     your code.  You can also use gcov along with the other
     profiling tool, 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 gcov or gprof, you can find out
     some basic performance statistics, such as:

     o   how often each line of code executes

     o   what lines of code are actually executed

     o   how much computing time each section of code uses

     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.  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.

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GNU                                                       GCOV(1)

     You should compile your code without optimization if you
     plan to use 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 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.

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

     gcov works only on code compiled with GCC.  It is not
     compatible with any other profiling or test coverage

         Display help about using gcov (on the standard output),
         and exit without doing any further processing.

         Display the gcov version number (on the standard
         output), and exit without doing any further processing.

         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.

         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 -u option is given.


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GNU                                                       GCOV(1)

         Write branch frequencies as the number of branches
         taken, rather than the percentage of branches taken.

         Do not create the gcov output file.

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

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

         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

         Output summaries for each function in addition to the
         file level summary.

     -o directory|file
     --object-directory directory
     --object-file file
         Specify either the directory containing the gcov data
         files, or the object path name.  The .gcno, and .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

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GNU                                                       GCOV(1)

         its extension.  If a file is specified here, the data
         files are named after that file, without its extension.

     -s directory
     --source-prefix 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

         When branch probabilities are given, include those of
         unconditional branches.  Unconditional branches are
         normally not interesting.

         Display the progress on the standard output.

     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.  gcov produces files
     called mangledname.gcov in the current directory.  These
     contain the coverage information of the source file they
     correspond to.  One .gcov file is produced for each source
     (or header) file containing code, which was compiled to
     produce the data files.  The mangledname part of the output
     file name is usually simply the source file name, but can be
     something more complicated if the -l or -p options are
     given.  Refer to those options for details.

     If you invoke 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 .gcov files contain the : separated fields along with
     program source code.  The format is

             <execution_count>:<line_number>:<source line text>

     Additional block information may succeed each line, when
     requested by command line option.  The execution_count is -
     for lines containing no code.  Unexecuted lines are marked
     ##### or ====, depending on whether they are reachable by
     non-exceptional paths or only exceptional paths such as C++
     exception handlers, respectively.

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GNU                                                       GCOV(1)

     Some lines of information at the start have line_number of
     zero.  These preamble lines are of the form


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

     The additional block information is of the form

             <tag> <information>

     The 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 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 gcov, you must first compile your program with
     two special GCC options: -fprofile-arcs -ftest-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
     -fprofile-arcs, an accompanying .gcda file will be placed in
     the object file directory.

     Running 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 tmp.c, this is what you see when you use
     the basic gcov facility:

             $ gcc -fprofile-arcs -ftest-coverage tmp.c
             $ a.out
             $ gcov tmp.c
             90.00% of 10 source lines executed in file tmp.c
             Creating tmp.c.gcov.

     The file tmp.c.gcov contains output from gcov.  Here is a

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GNU                                                       GCOV(1)

                     -:    0:Source:tmp.c
                     -:    0:Graph:tmp.gcno
                     -:    0:Data:tmp.gcda
                     -:    0:Runs:1
                     -:    0:Programs:1
                     -:    1:#include <stdio.h>
                     -:    2:
                     -:    3:int main (void)
                     1:    4:{
                     1:    5:  int i, total;
                     -:    6:
                     1:    7:  total = 0;
                     -:    8:
                    11:    9:  for (i = 0; i < 10; i++)
                    10:   10:    total += i;
                     -:   11:
                     1:   12:  if (total != 45)
                 #####:   13:    printf ("Failure\n");
                     -:   14:  else
                     1:   15:    printf ("Success\n");
                     1:   16:  return 0;
                     -:   17:}

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

                     -:    0:Source:tmp.c
                     -:    0:Graph:tmp.gcno
                     -:    0:Data:tmp.gcda
                     -:    0:Runs:1
                     -:    0:Programs:1
                     -:    1:#include <stdio.h>
                     -:    2:
                     -:    3:int main (void)
                     1:    4:{
                     1:    4-block  0
                     1:    5:  int i, total;
                     -:    6:
                     1:    7:  total = 0;
                     -:    8:
                    11:    9:  for (i = 0; i < 10; i++)
                    11:    9-block  0
                    10:   10:    total += i;
                    10:   10-block  0
                     -:   11:
                     1:   12:  if (total != 45)
                     1:   12-block  0
                 #####:   13:    printf ("Failure\n");
                 $$$$$:   13-block  0
                     -:   14:  else
                     1:   15:    printf ("Success\n");
                     1:   15-block  0

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GNU                                                       GCOV(1)

                     1:   16:  return 0;
                     1:   16-block  0
                     -:   17:}

     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 -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 13 contains a basic block that was not executed.

     When you use the -b option, your output looks like this:

             $ gcov -b tmp.c
             90.00% of 10 source lines executed in file tmp.c
             80.00% of 5 branches executed in file tmp.c
             80.00% of 5 branches taken at least once in file tmp.c
             50.00% of 2 calls executed in file tmp.c
             Creating tmp.c.gcov.

     Here is a sample of a resulting tmp.c.gcov file:

                     -:    0:Source:tmp.c
                     -:    0:Graph:tmp.gcno
                     -:    0:Data:tmp.gcda
                     -:    0:Runs:1
                     -:    0:Programs:1
                     -:    1:#include <stdio.h>
                     -:    2:
                     -:    3:int main (void)
             function main called 1 returned 1 blocks executed 75%
                     1:    4:{
                     1:    5:  int i, total;
                     -:    6:
                     1:    7:  total = 0;
                     -:    8:
                    11:    9:  for (i = 0; i < 10; i++)
             branch  0 taken 91% (fallthrough)
             branch  1 taken 9%
                    10:   10:    total += i;
                     -:   11:
                     1:   12:  if (total != 45)
             branch  0 taken 0% (fallthrough)
             branch  1 taken 100%
                 #####:   13:    printf ("Failure\n");

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GNU                                                       GCOV(1)

             call    0 never executed
                     -:   14:  else
                     1:   15:    printf ("Success\n");
             call    0 called 1 returned 100%
                     1:   16:  return 0;
                     -:   17:}

     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

     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 "exit" or "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 .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 .gcda files is saved immediately before the
     program exits.  For each source file compiled with
     -fprofile-arcs, the profiling code first attempts to read in
     an existing .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.

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GNU                                                       GCOV(1)

  Using gcov with GCC Optimization
     If you plan to use gcov to help optimize your code, you must
     first compile your program with two special GCC options:
     -fprofile-arcs -ftest-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

             if (a != b)
               c = 1;
               c = 0;

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

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

     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 fileA.o and fileB.o both contain out of line
     bodies of a particular inlineable function, they will also
     both contain coverage counts for that function.  When
     fileA.o and 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.

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GNU                                                       GCOV(1)

     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

     Long-running applications can use the "_gcov_reset" and
     "_gcov_dump" facilities to restrict profile collection to
     the program region of interest. Calling "_gcov_reset(void)"
     will clear all profile counters to zero, and calling
     "_gcov_dump(void)" will cause the profile information
     collected at that point to be dumped to .gcda output files.

     See attributes(5) for descriptions of the following

     |ATTRIBUTE TYPE |       ATTRIBUTE VALUE         |
     |Availability   | developer/gcc-4/gcc-common-48 |
     |Stability      | Uncommitted                   |
     gpl(7), gfdl(7), fsf-funding(7), gcc(1) and the Info entry
     for gcc.

     Copyright (c) 1996-2013 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.

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GNU                                                       GCOV(1)

     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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