Using and Porting GNU CC - 6. gcov: a Test Coverage Program

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# 6. `gcov`: a Test Coverage Program

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

This chapter describes version 1.5 of `gcov`.

## 6.1 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` 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 GNU CC. It is not compatible with any other profiling or test coverage mechanism.

## 6.2 Invoking gcov

    gcov [-b] [-v] [-n] [-l] [-f] [-o directory] sourcefile

`-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.
`-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.

When using `gcov`, you must first compile your program with two special GNU CC 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:

                    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.

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.

## 6.3 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.

## 6.4 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 GNU CC ``-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:

            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 `.da` file is generated when a program containing object files built with the GNU CC ``-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 source file name by substituting a `.da` suffix.

The format of the `.da` file is fairly simple. The first 8-byte number is the number of counts in the file, followed by the counts (stored as 8-byte numbers). Each count corresponds to the number of times each arc in the program is executed. The counts are cumulative; each time the program is executed, it attemps to combine the existing `.da` files with the new counts for this invocation of the program. It ignores the contents of any `.da` files whose number of arcs doesn't correspond to the current program, and merely overwrites them instead.

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.

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