GCOV

NAME

gcov − coverage testing tool

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

SYNOPSIS

gcov [ −v | −−version ] [ −h | −−help ]

[ −a | −−all−blocks ] [ −b | −−branch−probabilities ] [ −c | −−branch−counts ] [ −d | −−display−progress ] [ −f | −−function−summaries ] [ −j | −−json−format ] [ −H | −−human−readable ] [ −k | −−use−colors ] [ −l | −−long−file−names ] [ −m | −−demangled−names ] [ −n | −−no−output ] [ −o | −−object−directory directory|file ] [ −p | −−preserve−paths ] [ −q | −−use−hotness−colors ] [ −r | −−relative−only ] [ −s | −−source−prefix directory ] [ −t | −−stdout ] [ −u | −−unconditional−branches ] [ −x | −−hash−filenames ] files

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 codeâs performance. Using a profiler such as gcov or gprof, you can find out some basic performance statistics, such as:

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 âhot spotsâ 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 helpfulâthey 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] files

gcov accepts the following options:

Options
−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.

−d, −−display−progress

Display the progress on the standard output.

−f, −−function−summaries

Output summaries for each function in addition to the file level summary.

−h, −−help

Display help about using gcov (on the standard output), and exit without doing any further processing.

−j, −−json−format

Output gcov file in an easy−to−parse JSON intermediate format which does not require source code for generation. The JSON file is compressed with gzip compression algorithm and the files have .gcov.json.gz extension.

Structure of the JSON is following:

{
  "current_working_directory": "foo/bar",
  "data_file": "a.out",
  "format_version": "1",
  "gcc_version": "11.1.1 20210510"
  "files": ["$file"]
}

Fields of the root element have following semantics:

Each file has the following form:

{
  "file": "a.c",
  "functions": ["$function"],
  "lines": ["$line"]
}

Fields of the file element have following semantics:

Each function has the following form:

{
  "blocks": 2,
  "blocks_executed": 2,
  "demangled_name": "foo",
  "end_column": 1,
  "end_line": 4,
  "execution_count": 1,
  "name": "foo",
  "start_column": 5,
  "start_line": 1
}

Fields of the function element have following semantics:

Note that line numbers and column numbers number from 1. In the current implementation, start_line and start_column do not include any template parameters and the leading return type but that this is likely to be fixed in the future.

Each line has the following form:

{
  "branches": ["$branch"],
  "count": 2,
  "line_number": 15,
  "unexecuted_block": false,
  "function_name": "foo",
}

Branches are present only with −b option. Fields of the line element have following semantics:

Each branch has the following form:

{
  "count": 11,
  "fallthrough": true,
  "throw": false
}

Fields of the branch element have following semantics:

−H, −−human−readable

Write counts in human readable format (like 24.6k).

−k, −−use−colors

Use colors for lines of code that have zero coverage. We use red color for non−exceptional lines and cyan for exceptional. Same colors are used for basic blocks with −a option.

−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 and you want to see the individual contributions. If you use the −p option, both the including and included file names will be complete path names.

−m, −−demangled−names

Display demangled function names in output. The default is to show mangled function names.

−n, −−no−output

Do not create the gcov output file.

−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 input file name, without its extension. If a file is specified here, the data files are named after that file, without its extension.

−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 unremoveable .. components renamed to ^. This is useful if sourcefiles are in several different directories.

−q, −−use−hotness−colors

Emit perf−like colored output for hot lines. Legend of the color scale is printed at the very beginning of the output file.

−r, −−relative−only

Only output information about source files with a relative pathname (after source prefix elision). Absolute paths are usually system header files and coverage of any inline functions therein is normally uninteresting.

−s directory, −−source−prefix directory

A prefix for source file names to remove when generating the output coverage files. This option is useful when building in a separate directory, and the pathname to the source directory is not wanted when determining the output file names. Note that this prefix detection is applied before determining whether the source file is absolute.

−t, −−stdout

Output to standard output instead of output files.

−u, −−unconditional−branches

When branch probabilities are given, include those of unconditional branches. Unconditional branches are normally not interesting.

−v, −−version

Display the gcov version number (on the standard output), and exit without doing any further processing.

−w, −−verbose

Print verbose informations related to basic blocks and arcs.

−x, −−hash−filenames

When using −−preserve−paths, gcov uses the full pathname of the source files to create an output filename. This can lead to long filenames that can overflow filesystem limits. This option creates names of the form source−file##md5.gcov, where the source−file component is the final filename part and the md5 component is calculated from the full mangled name that would have been used otherwise. The option is an alternative to the −−preserve−paths on systems which have a filesystem limit.

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 (or header) 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.

If you invoke gcov with multiple input files, the contributions from each input file are summed. Typically you would invoke it with the same list of files as the final link of your executable.

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. Unexecuted lines are marked ##### or =====, depending on whether they are reachable by non−exceptional paths or only exceptional paths such as C++ exception handlers, respectively. Given the −a option, unexecuted blocks are marked $$$$$ or %%%%%, depending on whether a basic block is reachable via non−exceptional or exceptional paths. Executed basic blocks having a statement with zero execution_count end with * character and are colored with magenta color with the −k option. This functionality is not supported in Ada.

Note that GCC can completely remove the bodies of functions that are not needed â for instance if they are inlined everywhere. Such functions are marked with −, which can be confusing. Use the −fkeep−inline−functions and −fkeep−static−functions options to retain these functions and allow gcov to properly show their execution_count.

Some lines of information at the start have line_number of zero. These preamble lines are of the form

−:0:tag : value The ordering and number of these preamble lines will be augmented as gcov development progresses â do not rely on them remaining unchanged. Use tag to locate a particular preamble line.

The additional block information is of the form

tag information

The information is human readable, but designed to be simple enough for machine parsing too.

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 a special GCC option −−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 programâs 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.cpp, this is what you see when you use the basic gcov facility:

$ g++ −−coverage tmp.cpp −c
$ g++ −−coverage tmp.o
$ a.out
$ gcov tmp.cpp −m
File 'tmp.cpp'
Lines executed:92.86% of 14
Creating 'tmp.cpp.gcov'

The file tmp.cpp.gcov contains output from gcov. Here is a sample:

        −:    0:Source:tmp.cpp
        −:    0:Working directory:/home/gcc/testcase
        −:    0:Graph:tmp.gcno
        −:    0:Data:tmp.gcda
        −:    0:Runs:1
        −:    0:Programs:1
        −:    1:#include <stdio.h>
        −:    2:
        −:    3:template<class T>
        −:    4:class Foo
        −:    5:{
        −:    6:  public:
       1*:    7:  Foo(): b (1000) {}
−−−−−−−−−−−−−−−−−−
Foo<char>::Foo():
    #####:    7:  Foo(): b (1000) {}
−−−−−−−−−−−−−−−−−−
Foo<int>::Foo():
        1:    7:  Foo(): b (1000) {}
−−−−−−−−−−−−−−−−−−
       2*:    8:  void inc () { b++; }
−−−−−−−−−−−−−−−−−−
Foo<char>::inc():
    #####:    8:  void inc () { b++; }
−−−−−−−−−−−−−−−−−−
Foo<int>::inc():
        2:    8:  void inc () { b++; }
−−−−−−−−−−−−−−−−−−
        −:    9:
        −:   10:  private:
        −:   11:  int b;
        −:   12:};
        −:   13:
        −:   14:template class Foo<int>;
        −:   15:template class Foo<char>;
        −:   16:
        −:   17:int
        1:   18:main (void)
        −:   19:{
        −:   20:  int i, total;
        1:   21:  Foo<int> counter;
        −:   22:
        1:   23:  counter.inc();
        1:   24:  counter.inc();
        1:   25:  total = 0;
        −:   26:
       11:   27:  for (i = 0; i < 10; i++)
       10:   28:    total += i;
        −:   29:
       1*:   30:  int v = total > 100 ? 1 : 2;
        −:   31:
        1:   32:  if (total != 45)
    #####:   33:    printf ("Failure\n");
        −:   34:  else
        1:   35:    printf ("Success\n");
        1:   36:  return 0;
        −:   37:}

Note that line 7 is shown in the report multiple times. First occurrence presents total number of execution of the line and the next two belong to instances of class Foo constructors. As you can also see, line 30 contains some unexecuted basic blocks and thus execution count has asterisk symbol.

When you use the −a option, you will get individual block counts, and the output looks like this:

        −:    0:Source:tmp.cpp
        −:    0:Working directory:/home/gcc/testcase
        −:    0:Graph:tmp.gcno
        −:    0:Data:tmp.gcda
        −:    0:Runs:1
        −:    0:Programs:1
        −:    1:#include <stdio.h>
        −:    2:
        −:    3:template<class T>
        −:    4:class Foo
        −:    5:{
        −:    6:  public:
       1*:    7:  Foo(): b (1000) {}
−−−−−−−−−−−−−−−−−−
Foo<char>::Foo():
    #####:    7:  Foo(): b (1000) {}
−−−−−−−−−−−−−−−−−−
Foo<int>::Foo():
        1:    7:  Foo(): b (1000) {}
−−−−−−−−−−−−−−−−−−
       2*:    8:  void inc () { b++; }
−−−−−−−−−−−−−−−−−−
Foo<char>::inc():
    #####:    8:  void inc () { b++; }
−−−−−−−−−−−−−−−−−−
Foo<int>::inc():
        2:    8:  void inc () { b++; }
−−−−−−−−−−−−−−−−−−
        −:    9:
        −:   10:  private:
        −:   11:  int b;
        −:   12:};
        −:   13:
        −:   14:template class Foo<int>;
        −:   15:template class Foo<char>;
        −:   16:
        −:   17:int
        1:   18:main (void)
        −:   19:{
        −:   20:  int i, total;
        1:   21:  Foo<int> counter;
        1:   21−block  0
        −:   22:
        1:   23:  counter.inc();
        1:   23−block  0
        1:   24:  counter.inc();
        1:   24−block  0
        1:   25:  total = 0;
        −:   26:
       11:   27:  for (i = 0; i < 10; i++)
        1:   27−block  0
       11:   27−block  1
       10:   28:    total += i;
       10:   28−block  0
        −:   29:
       1*:   30:  int v = total > 100 ? 1 : 2;
        1:   30−block  0
    %%%%%:   30−block  1
        1:   30−block  2
        −:   31:
        1:   32:  if (total != 45)
        1:   32−block  0
    #####:   33:    printf ("Failure\n");
    %%%%%:   33−block  0
        −:   34:  else
        1:   35:    printf ("Success\n");
        1:   35−block  0
        1:   36:  return 0;
        1:   36−block  0
        −:   37:}

In this mode, each basic block is only shown on one line â 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 33 contains a basic block that was not executed.

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

        −:    0:Source:tmp.cpp
        −:    0:Working directory:/home/gcc/testcase
        −:    0:Graph:tmp.gcno
        −:    0:Data:tmp.gcda
        −:    0:Runs:1
        −:    0:Programs:1
        −:    1:#include <stdio.h>
        −:    2:
        −:    3:template<class T>
        −:    4:class Foo
        −:    5:{
        −:    6:  public:
       1*:    7:  Foo(): b (1000) {}
−−−−−−−−−−−−−−−−−−
Foo<char>::Foo():
function Foo<char>::Foo() called 0 returned 0% blocks executed 0%
    #####:    7:  Foo(): b (1000) {}
−−−−−−−−−−−−−−−−−−
Foo<int>::Foo():
function Foo<int>::Foo() called 1 returned 100% blocks executed 100%
        1:    7:  Foo(): b (1000) {}
−−−−−−−−−−−−−−−−−−
       2*:    8:  void inc () { b++; }
−−−−−−−−−−−−−−−−−−
Foo<char>::inc():
function Foo<char>::inc() called 0 returned 0% blocks executed 0%
    #####:    8:  void inc () { b++; }
−−−−−−−−−−−−−−−−−−
Foo<int>::inc():
function Foo<int>::inc() called 2 returned 100% blocks executed 100%
        2:    8:  void inc () { b++; }
−−−−−−−−−−−−−−−−−−
        −:    9:
        −:   10:  private:
        −:   11:  int b;
        −:   12:};
        −:   13:
        −:   14:template class Foo<int>;
        −:   15:template class Foo<char>;
        −:   16:
        −:   17:int
function main called 1 returned 100% blocks executed 81%
        1:   18:main (void)
        −:   19:{
        −:   20:  int i, total;
        1:   21:  Foo<int> counter;
call    0 returned 100%
branch  1 taken 100% (fallthrough)
branch  2 taken 0% (throw)
        −:   22:
        1:   23:  counter.inc();
call    0 returned 100%
branch  1 taken 100% (fallthrough)
branch  2 taken 0% (throw)
        1:   24:  counter.inc();
call    0 returned 100%
branch  1 taken 100% (fallthrough)
branch  2 taken 0% (throw)
        1:   25:  total = 0;
        −:   26:
       11:   27:  for (i = 0; i < 10; i++)
branch  0 taken 91% (fallthrough)
branch  1 taken 9%
       10:   28:    total += i;
        −:   29:
       1*:   30:  int v = total > 100 ? 1 : 2;
branch  0 taken 0% (fallthrough)
branch  1 taken 100%
        −:   31:
        1:   32:  if (total != 45)
branch  0 taken 0% (fallthrough)
branch  1 taken 100%
    #####:   33:    printf ("Failure\n");
call    0 never executed
branch  1 never executed
branch  2 never executed
        −:   34:  else
        1:   35:    printf ("Success\n");
call    0 returned 100%
branch  1 taken 100% (fallthrough)
branch  2 taken 0% (throw)
        1:   36:  return 0;
        −:   37:}

For each function, a line is printed showing how many times the function is called, how many times it returns and what percentage of the functionâs blocks were executed.

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 ânever executedâ 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 that 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 doesnât 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 a special GCC option −−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 isnât 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.

Inlineable functions can create unexpected line counts. Line counts are shown for the source code of the inlineable function, but what is shown depends on where the function is inlined, or if it is not inlined at all.

If the function is not inlined, the compiler must emit an out of line copy of the function, in any object file that needs it. If fileA.o and fileB.o both contain out of line bodies of a particular inlineable function, they will also both contain coverage counts for that function. When fileA.o and fileB.o are linked together, the linker will, on many systems, select one of those out of line bodies for all calls to that function, and remove or ignore the other. Unfortunately, it will not remove the coverage counters for the unused function body. Hence when instrumented, all but one use of that function will show zero counts.

If the function is inlined in several places, the block structure in each location might not be the same. For instance, a condition might now be calculable at compile time in some instances. Because the coverage of all the uses of the inline function will be shown for the same source lines, the line counts themselves might seem inconsistent.

Long−running applications can use the __gcov_reset and __gcov_dump facilities to restrict profile collection to the program region of interest. Calling __gcov_reset(void) will clear all run−time profile counters to zero, and calling __gcov_dump(void) will cause the profile information collected at that point to be dumped to .gcda output files. Instrumented applications use a static destructor with priority 99 to invoke the __gcov_dump function. Thus __gcov_dump is executed after all user defined static destructors, as well as handlers registered with atexit.

If an executable loads a dynamic shared object via dlopen functionality, −Wl,−−dynamic−list−data is needed to dump all profile data.

Profiling run−time library reports various errors related to profile manipulation and profile saving. Errors are printed into standard error output or GCOV_ERROR_FILE file, if environment variable is used. In order to terminate immediately after an errors occurs set GCOV_EXIT_AT_ERROR environment variable. That can help users to find profile clashing which leads to a misleading profile.

BRIEF DESCRIPTION OF GCOV DATA FILES

gcov uses two files for profiling. The names of these files are derived from the original object file by substituting the file suffix with either .gcno, or .gcda. The files contain coverage and profile data stored in a platform−independent format. The .gcno files are placed in the same directory as the object file. By default, the .gcda files are also stored in the same directory as the object file, but the GCC −fprofile−dir option may be used to store the .gcda files in a separate directory.

The .gcno notes file is generated when the source file is compiled with the GCC −ftest−coverage option. It contains information to reconstruct the basic block graphs and assign source line numbers to blocks.

The .gcda count data file is generated when a program containing object files built with the GCC −fprofile−arcs option is executed. A separate .gcda file is created for each object file compiled with this option. It contains arc transition counts, value profile counts, and some summary information.

It is not recommended to access the coverage files directly. Consumers should use the intermediate format that is provided by gcov tool via −−json−format option.

DATA FILE RELOCATION TO SUPPORT CROSS-PROFILING

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. That implicitly requires running the program on the same system as it was built or having the same absolute directory structure on the target system. The program will try to create the needed directory structure, if it is not already present.

To support cross−profiling, a program compiled with −fprofile−arcs can relocate the data files based on two environment variables:

NOTE:

If GCOV_PREFIX_STRIP is set without GCOV_PREFIX is undefined, then a relative path is made out of the hardwired absolute paths.

For example, if the object file /user/build/foo.o was built with −fprofile−arcs, the final executable will try to create the data file /user/build/foo.gcda when running on the target system. This will fail if the corresponding directory does not exist and it is unable to create it. This can be overcome by, for example, setting the environment as GCOV_PREFIX=/target/run and GCOV_PREFIX_STRIP=1. Such a setting will name the data file /target/run/build/foo.gcda.

You must move the data files to the expected directory tree in order to use them for profile directed optimizations (−fprofile−use), or to use the gcov tool.

PROFILING AND TEST COVERAGE IN FREESTANDING ENVIRONMENTS

In case your application runs in a hosted environment such as GNU/Linux, then this section is likely not relevant to you. This section is intended for application developers targeting freestanding environments (for example embedded systems) with limited resources. In particular, systems or test cases which do not support constructors/destructors or the C library file I/O. In this section, the target system runs your application instrumented for profiling or test coverage. You develop and analyze your application on the host system. We now provide an overview how profiling and test coverage can be obtained in this scenario followed by a tutorial which can be exercised on the host system. Finally, some system initialization caveats are listed.

Overview
For an application instrumented for profiling or test coverage, the compiler generates some global data structures which are updated by instrumentation code while the application runs. These data structures are called the gcov information. Normally, when the application exits, the gcov information is stored to .gcda files. There is one file per translation unit instrumented for profiling or test coverage. The function __gcov_exit(), which stores the gcov information to a file, is called by a global destructor function for each translation unit instrumented for profiling or test coverage. It runs at process exit. In a global constructor function, the __gcov_init() function is called to register the gcov information of a translation unit in a global list. In some situations, this procedure does not work. Firstly, if you want to profile the global constructor or exit processing of an operating system, the compiler generated functions may conflict with the test objectives. Secondly, you may want to test early parts of the system initialization or abnormal program behaviour which do not allow a global constructor or exit processing. Thirdly, you need a filesystem to store the files.

The −fprofile−info−section GCC option enables you to use profiling and test coverage in freestanding environments. This option disables the use of global constructors and destructors for the gcov information. Instead, a pointer to the gcov information is stored in a special linker input section for each translation unit which is compiled with this option. By default, the section name is .gcov_info. The gcov information is statically initialized. The pointers to the gcov information from all translation units of an executable can be collected by the linker in a contiguous memory block. For the GNU linker, the below linker script output section definition can be used to achieve this:

.gcov_info      :
{
  PROVIDE (__gcov_info_start = .);
  KEEP (*(.gcov_info))
  PROVIDE (__gcov_info_end = .);
}

The linker will provide two global symbols, __gcov_info_start and __gcov_info_end, which define the start and end of the array of pointers to gcov information blocks, respectively. The KEEP () directive is required to prevent a garbage collection of the pointers. They are not directly referenced by anything in the executable. The section may be placed in a read−only memory area.

In order to transfer the profiling and test coverage data from the target to the host system, the application has to provide a function to produce a reliable in order byte stream from the target to the host. The byte stream may be compressed and encoded using error detection and correction codes to meet application−specific requirements. The GCC provided libgcov target library provides two functions, __gcov_info_to_gcda() and __gcov_filename_to_gcfn(), to generate a byte stream from a gcov information bock. The functions are declared in #include <gcov.h>. The byte stream can be deserialized by the merge−stream subcommand of the gcov−tool to create or update .gcda files in the host filesystem for the instrumented application.

Tutorial
This tutorial should be exercised on the host system. We will build a program instrumented for test coverage. The program runs an application and dumps the gcov information to stderr encoded as a printable character stream. The application simply decodes such character streams from stdin and writes the decoded character stream to stdout (warning: this is binary data). The decoded character stream is consumed by the merge−stream subcommand of the gcov−tool to create or update the .gcda files.

To get started, create an empty directory. Change into the new directory. Then you will create the following three files in this directory

Firstly, create the header file app.h with the following content:

static const unsigned char a = 'a';

static inline unsigned char *
encode (unsigned char c, unsigned char buf[2])
{
  buf[0] = c % 16 + a;
  buf[1] = (c / 16) % 16 + a;
  return buf;
}

extern void application (void);

Secondly, create the source file app.c with the following content:

#include "app.h"

#include <stdio.h>

/* The application reads a character stream encoded by encode() from stdin,
   decodes it, and writes the decoded characters to stdout.  Characters other
   than the 16 characters 'a' to 'p' are ignored.  */

static int can_decode (unsigned char c)
{
  return (unsigned char)(c − a) < 16;
}

void
application (void)
{
  int first = 1;
  int i;
  unsigned char c;

  while ((i = fgetc (stdin)) != EOF)
    {
      unsigned char x = (unsigned char)i;


      if (can_decode (x))
        {
          if (first)
            c = x − a;
          else
            fputc (c + 16 * (x − a), stdout);
          first = !first;
        }
      else
        first = 1;
    }
}

Thirdly, create the source file main.c with the following content:

#include "app.h"

#include <gcov.h>
#include <stdio.h>
#include <stdlib.h>

/* The start and end symbols are provided by the linker script.  We use the
   array notation to avoid issues with a potential small−data area.  */

extern const struct gcov_info *const __gcov_info_start[];
extern const struct gcov_info *const __gcov_info_end[];

/* This function shall produce a reliable in order byte stream to transfer the
   gcov information from the target to the host system.  */

static void
dump (const void *d, unsigned n, void *arg)
{
  (void)arg;
  const unsigned char *c = d;
  unsigned char buf[2];

  for (unsigned i = 0; i < n; ++i)
    fwrite (encode (c[i], buf), sizeof (buf), 1, stderr);
}

/* The filename is serialized to a gcfn data stream by the
   __gcov_filename_to_gcfn() function.  The gcfn data is used by the
   "merge−stream" subcommand of the "gcov−tool" to figure out the filename
   associated with the gcov information. */

static void
filename (const char *f, void *arg)
{
  __gcov_filename_to_gcfn (f, dump, arg);
}

/* The __gcov_info_to_gcda() function may have to allocate memory under
   certain conditions.  Simply try it out if it is needed for your application
   or not.  */

static void *
allocate (unsigned length, void *arg)
{
  (void)arg;
  return malloc (length);
}

/* Dump the gcov information of all translation units.  */

static void
dump_gcov_info (void)
{
  const struct gcov_info *const *info = __gcov_info_start;
  const struct gcov_info *const *end = __gcov_info_end;

  /* Obfuscate variable to prevent compiler optimizations.  */
  __asm__ ("" : "+r" (info));

  while (info != end)
  {
    void *arg = NULL;
    __gcov_info_to_gcda (*info, filename, dump, allocate, arg);
    fputc ('\n', stderr);
    ++info;
  }
}

/* The main() function just runs the application and then dumps the gcov
   information to stderr.  */


int
main (void)
{
  application ();
  dump_gcov_info ();
  return 0;
}

If we compile app.c with test coverage and no extra profiling options, then a global constructor (_sub_I_00100_0 here, it may have a different name in your environment) and destructor (_sub_D_00100_1) is used to register and dump the gcov information, respectively. We also see undefined references to __gcov_init and __gcov_exit :

$ gcc −−coverage −c app.c
$ nm app.o
0000000000000000 r a
0000000000000030 T application
0000000000000000 t can_decode
                 U fgetc
                 U fputc
0000000000000000 b __gcov0.application
0000000000000038 b __gcov0.can_decode
0000000000000000 d __gcov_.application
00000000000000c0 d __gcov_.can_decode
                 U __gcov_exit
                 U __gcov_init
                 U __gcov_merge_add
                 U stdin
                 U stdout
0000000000000161 t _sub_D_00100_1
0000000000000151 t _sub_I_00100_0

Compile app.c and main.c with test coverage and −fprofile−info−section. Now, a read−only pointer size object is present in the .gcov_info section and there are no undefined references to __gcov_init and __gcov_exit :

$ gcc −−coverage −fprofile−info−section −c main.c
$ gcc −−coverage −fprofile−info−section −c app.c
$ objdump −h app.o

app.o:     file format elf64−x86−64


Sections:
Idx Name          Size      VMA               LMA               File off  Algn
  0 .text         00000151  0000000000000000  0000000000000000  00000040  2**0
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .data         00000100  0000000000000000  0000000000000000  000001a0  2**5
                  CONTENTS, ALLOC, LOAD, RELOC, DATA
  2 .bss          00000040  0000000000000000  0000000000000000  000002a0  2**5
                  ALLOC
  3 .rodata       0000003c  0000000000000000  0000000000000000  000002a0  2**3
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  4 .gcov_info    00000008  0000000000000000  0000000000000000  000002e0  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
  5 .comment      0000004e  0000000000000000  0000000000000000  000002e8  2**0
                  CONTENTS, READONLY
  6 .note.GNU−stack 00000000  0000000000000000  0000000000000000  00000336  2**0
                  CONTENTS, READONLY
  7 .eh_frame     00000058  0000000000000000  0000000000000000  00000338  2**3
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA

We have to customize the program link procedure so that all the .gcov_info linker input sections are placed in a contiguous memory block with a begin and end symbol. Firstly, get the default linker script using the following commands (we assume a GNU linker):

$ ld −−verbose | sed '1,/^===/d' | sed '/^===/d' > linkcmds

Secondly, open the file linkcmds with a text editor and place the linker output section definition from the overview after the .rodata section definition. Link the program executable using the customized linker script:

$ gcc −−coverage main.o app.o −T linkcmds −Wl,−Map,app.map

In the linker map file app.map, we see that the linker placed the read−only pointer size objects of our objects files main.o and app.o into a contiguous memory block and provided the symbols __gcov_info_start and __gcov_info_end :

$ grep −C 1 "\.gcov_info" app.map


.gcov_info      0x0000000000403ac0       0x10
                0x0000000000403ac0                PROVIDE (__gcov_info_start = .)
 *(.gcov_info)
 .gcov_info     0x0000000000403ac0        0x8 main.o
 .gcov_info     0x0000000000403ac8        0x8 app.o
                0x0000000000403ad0                PROVIDE (__gcov_info_end = .)

Make sure no .gcda files are present. Run the program with nothing to decode and dump stderr to the file gcda−0.txt (first run). Run the program to decode gcda−0.txt and send it to the gcov−tool using the merge−stream subcommand to create the .gcda files (second run). Run gcov to produce a report for app.c. We see that the first run with nothing to decode results in a partially covered application:

$ rm −f app.gcda main.gcda
$ echo "" | ./a.out 2>gcda−0.txt
$ ./a.out <gcda−0.txt 2>gcda−1.txt | gcov−tool merge−stream
$ gcov −bc app.c
File 'app.c'
Lines executed:69.23% of 13
Branches executed:66.67% of 6
Taken at least once:50.00% of 6
Calls executed:66.67% of 3
Creating 'app.c.gcov'

Lines executed:69.23% of 13

Run the program to decode gcda−1.txt and send it to the gcov−tool using the merge−stream subcommand to update the .gcda files. Run gcov to produce a report for app.c. Since the second run decoded the gcov information of the first run, we have now a fully covered application:

$ ./a.out <gcda−1.txt 2>gcda−2.txt | gcov−tool merge−stream
$ gcov −bc app.c
File 'app.c'
Lines executed:100.00% of 13
Branches executed:100.00% of 6
Taken at least once:100.00% of 6
Calls executed:100.00% of 3
Creating 'app.c.gcov'

Lines executed:100.00% of 13

System Initialization Caveats
The gcov information of a translation unit consists of several global data structures. For example, the instrumented code may update program flow graph edge counters in a zero−initialized data structure. It is safe to run instrumented code before the zero−initialized data is cleared to zero. The coverage information obtained before the zero−initialized data is cleared to zero is unusable. Dumping the gcov information using __gcov_info_to_gcda() before the zero−initialized data is cleared to zero or the initialized data is loaded, is undefined behaviour. Clearing the zero−initialized data to zero through a function instrumented for profiling or test coverage is undefined behaviour, since it may produce inconsistent program flow graph edge counters for example.

COPYRIGHT

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being GNU General Public License and Funding Free Software, the Front−Cover texts being (a) (see below), and with the Back−Cover Texts being (b) (see below). A copy of the license is in the GNU Free Documentation License.

A GNU Manual

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AUTHOR

Richard M. Stallman and the GCC Developer Community

COPYRIGHT

1988-2022 Free Software Foundation, Inc.