gcov---a Test Coverage Program
******************************

gcov is a tool you can use in conjunction with GCC to test code coverage in your programs.

Introduction to gcov
====================

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:

  * 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 mechanism.

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.

-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.

-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.

-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 .bb, .bbg, and .da 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.

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 .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 .da 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:Object:tmp.bb
             -:    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;
             1:   17:}

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:Object:tmp.bb
             -:    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++)
     branch  0: taken 90%
     branch  1: taken 100%
     branch  2: taken 100%
            10:   10:    total += i;
             -:   11:
             1:   12:  if (total != 45)
     branch  0: taken 100%
         #####:   13:    printf ("Failure\n");
     call    0: never executed
     branch  1: never executed
             -:   14:  else
             1:   15:    printf ("Success\n");
     call    0: returns 100%
             1:   16:  return 0;
             1:   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 .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;
     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:   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.

Brief description of gcov data files
====================================

gcov uses three files for doing profiling.  The names of these files are derived from the original source file by substituting the file suffix with either .bb, .bbg, or .da.  All of these files are placed in the same directory as the source file, and contain data stored in a platform-independent method.

The .bb and .bbg files are generated when the source file is compiled with the GCC -ftest-coverage option.  The .bb file contains a list of source files (including headers), functions within those files, and line numbers corresponding to each basic block in the source file.

The .bb file format consists of several lists of 4-byte integers which correspond to the line numbers of each basic block in the file. Each list is terminated by a line number of 0.  A line number of -1 is used to designate that the source file name (padded to a 4-byte boundary and followed by another -1) follows.  In addition, a line number of -2 is used to designate that the name of a function (also padded to a 4-byte boundary and followed by a -2) follows.

The .bbg file is used to reconstruct the program flow graph for the source file.  It contains a list of the program flow arcs (possible branches taken from one basic block to another) for each function which, in combination with the .bb file, enables gcov to reconstruct the program flow.

In the .bbg file, the format is:

             name of function #0
             checksum of function #0
             number of basic blocks for function #0 (4-byte number)
             total number of arcs for function #0 (4-byte number)
             count of arcs in basic block #0 (4-byte number)
             destination basic block of arc #0 (4-byte number)
             flag bits (4-byte number)
             destination basic block of arc #1 (4-byte number)
             flag bits (4-byte number)
             ...
             destination basic block of arc #N (4-byte number)
             flag bits (4-byte number)
             count of arcs in basic block #1 (4-byte number)
             destination basic block of arc #0 (4-byte number)
             flag bits (4-byte number)
             ...

A -1 (stored as a 4-byte number) is used to separate each function's list of basic blocks, and to verify that the file has been read correctly.

The function name is stored as a -1 (4 bytes), the length (4 bytes), the name itself (padded to 4-byte boundary) followed by a -1 (4 bytes).

The flags are defined as follows:

  * bit0 On function spanning tree

  * bit1 Is a fake edge

  * bit2 Is the fall through edge from one block to its immediate successor.

  * bit3-bit31 For future expansion

The .da file is generated when a program containing object files built with the GCC -fprofile-arcs option is executed.  A separate .da file is created for each source file compiled with this option, and the name of the .da file is stored as an absolute pathname in the resulting object file.  This path name is derived from the object file name by substituting a .da suffix.

The .da consists of one or more blocks with the following structure:

             "magic" number -123 (4-byte number)
     	number of functions (4-byte number)
             length of the "extension block" in bytes
     	extension block (variable length)
       	name of function #0 (the same format as in .bbg file)
             checksum of function #0
     	number of instrumented arcs (4-byte number)
             count of arc #0 (8-byte number)
             count of arc #1 (8-byte number)
             ...
     	count of arc #M_0 (8-byte number)
       	name of function #1 (the same format as in .bbg file)
             checksum of function #1
     	...

Multiple program runs might merge data into a single block, or might append a new block. The current structure of the extension block is as follows:

             number of instrumented arcs in whole program (4-byte number)
             sum all of instrumented arcs in whole program (8-byte number)
             maximal value of counter in whole program (8-byte number)
             number of instrumented arcs in the object file (4-byte number)
             sum all of instrumented arcs in the object file (8-byte number)
             maximal value of counter in the object file (8-byte number)

All three of these files use the functions in gcov-io.h to store integers; the functions in this header provide a machine-independent mechanism for storing and retrieving data from a stream.