Using the GNU Compiler Collection (GCC)

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Invoking gcov

gcov [options] sourcefile

gcov accepts the following options:

-h
--help: Display help about using gcov (on the standard output), and exit without doing any further processing.
-v
--version: Display the gcov version number (on the standard output), and exit without doing any further processing.
-a
--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.
-b
--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 -u option is given.
-c
--branch-counts: Write branch frequencies as the number of branches taken, rather than the percentage of branches taken.
-n
--no-output: Do not create the gcov output file.
-l
--long-file-names: 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. If you uses the -p option, both the including and included file names will be complete path names.
-p
--preserve-paths: 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 '..' components renamed to '^'. This is useful if sourcefiles are in several different directories. It also affects the -l option.
-f
--function-summaries: 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 source file name, without its extension. If a file is specified here, the data files are named after that file, without its extension. If this option is not supplied, it defaults to the current directory.
-u
--unconditional-branches: When branch counts are given, include those of unconditional branches. Unconditional branches are normally not interesting.

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 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.

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 and ##### for lines which were never executed. Some lines of information at the start have line_number of zero.

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 sample:

        -:    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++)
       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)
function main called 1 returned 1 blocks executed 75%
        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
        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");
call    0 never executed
        -:   14:  else
        1:   15:    printf ("Success\n");
call    0 called 1 returned 100%
        1:   16:  return 0;
        -:   17:}

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 .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.