gcov: a Test Coverage Program
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gcov is a tool you can use, together with GNU CC, to test code coverage in your programs.  gcov is free software, but for the moment it is only available from Cygnus Support (pending discussions with the FSF about how they think Cygnus should really write it).

This chapter describes version 1.5 of gcov.

Jim Wilson wrote gcov, and the original form of this note. Pat McGregor edited the documentation.

Introduction to gcov
********************

gcov is a test coverage program.  Use it in concert with GNU CC 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 in concert 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 SOURCENAME.gcov which indicates how many times each line of a source file SOURCENAME.c has executed.  You can use these logfiles in conjuction 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 GNU CC; it is not compatible with any other profiling or test coverage mechanism.

Invoking gcov
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     gcov [-b] [-v] [-n] [-l] [-f] [-o directory] SOURCEFILE

-b
     Write branch frequencies to the output file.  Write branch summary info to standard output.  This option allows you to see how often each branch was taken.

-v
     Display the gcov version number (on the standard error stream).

-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.

To use gcov, first compile your program with two special GNU CC options: -fprofile-arcs -ftest-coverage.  Then run the program. Then run gcov with your program's source file names as arguments. 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:

                     main()
                     {
                1      int i, total;

                1      total = 0;

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

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

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

     $ 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:

                     main()
                     {
                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
                       else
                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 everytime they are called.

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 GNU CC options: -fprofile-arcs -ftest-coverage.  Aside from that, you can use any other GNU CC 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;
     else
       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.