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Invoking gcov

The following documentation discusses invoking gcov.

    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 filenames 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 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 and -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 th directory where the source code is located.

Running gcov will cause profile output to be generated. For each source file compiled with -fprofile-arcs, and accompanying file, .da, will be placed in the source directory.

Running gcov with your programs source file names as arguments will now produce a listing of the code along with frquency of execution for each line. For example, if your program is called tmp.c, youll see the following when you use the gcov facility.

For example, if your program is called tmp.c, the following 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. The following 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    }

    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, 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 doesnt 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.