gcov − coverage testing tool

gcov [−b] [−c] [−v] [−n] [−l] [−f] [−o directory] sourcefile

gcov is a test coverage program.  Use it in concert with GCC
to analyze your programs to help create more efficient,
faster running code.  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:

•   how often each line of code executes

•   what lines of code are actually executed

•   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.

     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

−b  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.

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

−v  Display the gcov version number (on the standard error

−n  Do not create the gcov output file.

−l  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.

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

−o  The directory where the object files live.  Gcov will
    search for .bb, .bbg, and .da files in this directory.

     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 source code is located.

     Running the program will cause profile output to be
generated.  For each source file compiled with
−fprofile−arcs, an accompanying .da file will be placed in
the source 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
         87.50% of 8 source lines executed in file tmp.c
        Creating tmp.c.gcov.

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

                   1      int i, total;

                        1      total = 0;

                       11      for (i = 0; i < 10; i++)
                  10        total += i;

                        1      if (total != 45)
              ######        printf ("Failure\n");
                   1        printf ("Success\n");
                   1    }

     When you use the −b option, your output looks like

             $ gcov ‐b tmp.c
         87.50% of 8 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:

                   1      int i, total;

                        1      total = 0;

                       11      for (i = 0; i < 10; i++)
        branch 0 taken = 91%
        branch 1 taken = 100%
        branch 2 taken = 100%
                  10        total += i;


                        1      if (total != 45)
        branch 0 taken = 100%
              ######        printf ("Failure\n");
        call 0 never executed
        branch 1 never executed
                   1        printf ("Success\n");
        call 0 returns = 100%
                   1    }

     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 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 .da 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 .da 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 .da 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.


     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 this:

             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  if (a != b)
              100    c = 1;
              100  else
              100    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.

gpl(7), gfdl(7), fsf−funding(7), gcc(1) and the Info entry
for gcc.

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