GCOV(1)

✨ gcov(1) ✨

okay bestie, pull up a chair, we're doing GNU gcov(1) — the coverage testing tool 💖

---

🌸 NAME 🌸

gcov — it's literally the coverage testing tool, no cap 🎀

---

✨ SYNOPSIS ✨

gcov [-v|--version] [-h|--help]
     [-a|--all-blocks]
     [-b|--branch-probabilities]
     [-c|--branch-counts]
     [-g|--conditions]
     [-e|--prime-paths]
     [--prime-paths-lines[=type]]
     [--prime-paths-source[=type]]
     [-d|--display-progress]
     [-f|--function-summaries]
     [--include regex]
     [--exclude regex]
     [-j|--json-format]
     [-H|--human-readable]
     [-k|--use-colors]
     [-l|--long-file-names]
     [-m|--demangled-names]
     [-M|--filter-on-demangled]
     [-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

---

💅 DESCRIPTION 💅

gcov is a test coverage program. It's giving main character energy. You use it in concert with GCC to analyze your programs, so you can write code that's more efficient and runs faster, and also find the parts of your program that nobody ever tested. 🫶

You can use gcov as a profiling tool to help you figure out where your optimization efforts are gonna hit the hardest. And you can also use gcov alongside that other profiling tool, gprof, to see which parts of your code are eating the most computing time. 😭

Profiling tools help you analyze your code's performance. Using a profiler like 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 it's compiled, you can look at each module and see which ones should be optimized. gcov helps you determine where to work on optimization. ✨

Software developers also use coverage testing alongside testsuites, to make sure software is actually good enough for a release. Ttestsuites can verify that a program works as expected; a coverage program tests to see how much of the program is exercised by the testsuite. Then developers can figure out what kinds of test cases need to be added to the testsuites, which makes both the testing better and the final product better. 💖

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. Also, 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. periodt 💀

---

🎀 OPTIONS 🎀

• -a, --all-blocks — Write individual execution counts for every basic block. Normally gcov only outputs execution counts for the main blocks of a line. With this option you can clock whether blocks within a single line aren't being executed. 🔍
• -b, --branch-probabilities — Write branch frequencies to the output file, and write branch summary info to the standard output. This lets you 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. 🔢
• -g, --conditions — Write condition coverage to the output file, and write condition summary info to the standard output. This lets you see if the conditions in your program at least once had an independent effect on the outcome of the boolean expression (that's modified condition/decision coverage, iykyk). This requires you to compile the source with -fcondition-coverage. ✅
• -e, --prime-paths — Write path coverage to the output file, and write path summary info to the standard output. This lets you see how many prime paths were taken at least once. A path is a sequence of basic blocks. A path is simple if it has no repeated blocks (no loops) except maybe the first and last block, and prime if it is a simple path of maximal length. For the regular output this option only includes the number of paths covered. For more fine grained info on paths you can use --prime-paths-lines or --prime-paths-source. With --json-format all path details are included in the output. This requires you to compile the source with -fpath-coverage. 🛤️
• --prime-paths-lines[=type] — Write path coverage to the output file, and write path summary info to the standard output. This lets you see how many prime paths were taken at least once, plus a dense report on the covered or uncovered paths and how to cover them. This mode is useful for automated reporting and progress tracking. type may be omitted, or one of:
  • uncovered — Include the uncovered (not taken) paths. This is the default.
  • covered — Include the covered (taken) paths.
  • both — Include all paths. This is equivalent to using both covered and uncovered.

Here's an example of --prime-paths-lines output:

paths covered 12 of 15
path  2 not covered: lines 8 8(false) 11(true) 11 13(true) 13(true) 14 17
path  3 not covered: lines 8 8(false) 11(true) 11 13(true) 13(false) 16 17
path  4 not covered: lines 8 8(false) 11(true) 11 13(false) 16 17

This means to cover path 2 you must run lines 8, 11, 13, 14, and 17, evaluating the decision at 8 false and the decisions at 11 and 13 to "false". 🧠

• --prime-paths-source[=type] — Write path coverage to the output file, and write path summary info to the standard output. This lets you see how many prime paths were taken at least once, plus a detailed report on the uncovered paths and how to cover them. This mode is useful for understanding paths and interactions between sections of your program. type may be omitted, or one of:
  • uncovered — Include the uncovered (not taken) paths. This is the default.
  • covered — Include the covered (taken) paths.
  • both — Include all paths. This is equivalent to using both covered and uncovered.

Here's an example of --prime-paths-source output:

path 10 not covered:
BB  3:           8:  for (i = 0; i < 10; i++)
BB  3:           9:    total += i;
BB  4: (false)   8:  for (i = 0; i < 10; i++)
BB  5: (true)   11:  int v = total > 100 ? 1 : 2;
BB  6:          11:  int v = total > 100 ? 1 : 2;
BB  8: (false)  13:  if (total != 45 && v == 1)
BB 11:          16:    printf ("Success\n");
BB 12:          17:  return 0;

The first (BB) column is the sequence of basic blocks (see -w). The middle column (true/false) is the decision for that line. The third column is the line number. The fourth column is the line itself. These lines must be run in this order to cover path 10. 🌈

• -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. 📝
• --include regex — Include functions matching regex. This option makes gcov only report on functions that match the extended regular expression regex. This flag can be combined with --exclude. If a function matches both includes and excludes, the last include/exclude applies. By default gcov reports on all functions, but if a --include is used then only functions matching the include will be reported. 💖
• --exclude regex — Exclude functions matching regex. This option makes gcov not report on functions that match the extended regular expression regex. This flag can be combined with --include. If a function matches both includes and excludes, the last include/exclude applies. By default gcov reports on all functions, and if --exclude is used then functions matching it will be omitted. 🙈
• -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": "2",
  "gcc_version": "11.1.1 20210510"
  "files": ["$file"]
}

Fields of the root element have following semantics:

• current_working_directory: working directory where a compilation unit was compiled
• data_file: name of the data file (GCDA)
• format_version: semantic version of the format

Changes in version 2:

• calls: information about function calls is added

• gcc_version: version of the GCC compiler

Each file has the following form:

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

Fields of the file element have following semantics:

• file_name: name of the source file

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
}

so like, here's what every single field in that function element is actually telling you 💅

• blocks — how many blocks are chillin' inside this function
• blocks_executed — how many of those blocks actually got executed
• demangled_name — the demangled name of the function, bestie
• end_column — the column in the source file where the function ends
• end_line — the line in the source file where the function ends
• execution_count — how many times the function got executed
• name — the name of the function
• start_column — the column in the source file where the function begins
• start_line — the line in the source file where the function begins

okay important note, no cap: line numbers and column numbers start numbering from 1. And in the current implementation, start_line and start_column do not include any template parameters and the leading return type — but that's likely to be fixed in the future, so don't @ me.

---

Every line looks like this, periodt:

{
  "block_ids": ["$block_id"],
  "branches": ["$branch"],
  "calls": ["$call"],
  "count": 2,
  "conditions": ["$condition"],
  "line_number": 15,
  "unexecuted_block": false,
  "function_name": "foo",
}

✨ Branches and calls only show up when you pass the -b option ✨. And the fields of the line element mean:

• block_ids — the IDs of the basic blocks that belong to this line
• count — how many times the line got executed
• line_number — the line number, obviously
• unexecuted_block — a flag for whether the line contains an unexecuted block (aka not all the statements on the line are executed)
• function_name — the name of the function this line belongs to (for a line with inlined statements, this can be not set 🫠)

Each branch is giving:

{
  "count": 11,
  "destination_block_id": 17,
  "fallthrough": true,
  "source_block_id": 13,
  "throw": false
}

and the branch element's fields mean:

• count — how many times this branch got executed
• fallthrough — true when the branch is a fall through branch
• throw — true when the branch is an exceptional branch
• isource_block_id — the ID of the basic block where this branch happens
• destination_block_id — the ID of the basic block this branch jumps to

Each call is shaped like:

{
  "destination_block_id": 1,
  "returned": 11,
  "source_block_id": 13
}

and the call element's fields mean:

• returned — how many times a function call returned (the call count is equal to line::count, iykyk)
• isource_block_id — the ID of the basic block where this call happens
• destination_block_id — the ID of the basic block this call continues to after it returns

Each condition is shaped like:

{
  "count": 4,
  "covered": 2,
  "not_covered_false": [],
  "not_covered_true": [0, 1],
}

and the condition element's fields mean:

• count — the number of condition outcomes in this expression
• covered — the number of covered condition outcomes in this expression
• not_covered_true — the terms, by index, that were not seen as true in this expression
• not_covered_false — the terms, by index, that were not seen as false in this expression

---

• -H, --human-readable — writes the counts in a human readable format (like 24.6k). cute numbers, we love her 💖
• -k, --use-colors — uses colors for lines of code that have zero coverage. We use red for non-exceptional lines and cyan for exceptional ones. Same colors get used for basic blocks when you pass -a. aesthetic AND informative ✨
• -l, --long-file-names — creates long file names for included source files. Like, say the header file x.h contains code and it was included in the file a.c — then running gcov on a.c will produce an output file called a.c##x.h.gcov instead of x.h.gcov. Super useful if x.h gets included in multiple source files and you wanna see the individual contributions 🫶. And if you use the -p option, both the including and the included file names will be complete path names.
• -m, --demangled-names — displays demangled function names in the output. The default is to show mangled function names (the ugly ones, ngl).
• -M, --filter-on-demangled — makes --include and --exclude match demangled names. This only affects the matching and does not imply --demangled-names, but it can safely be combined with it. no drama, they coexist 💅
• -n, --no-output — do not create the gcov output file. just vibes, no files.
• -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. pick your fighter 🎀
• -p, --preserve-paths — preserves complete path information in the names of generated .gcov files. Without this option, just the filename component is used. With it, 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. slay, no more filename collisions 🔥
• -q, --use-hotness-colors — emits perf-like colored output for hot lines. The legend of the color scale is printed at the very beginning of the output file. hot girl lines get hot colors 💅
• -r, --relative-only — only outputs 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 in there is normally uninteresting. respectfully, we don't care about them 🙈
• -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. order matters, bestie ⭐
• -t, --stdout — output to standard output instead of output files. straight to your terminal, no middleman ✨
• -u, --unconditional-branches — when branch probabilities are given, include those of unconditional branches. Unconditional branches are normally not interesting (fr, they're kinda boring).
• -v, --version — displays the gcov version number (on the standard output), and exits without doing any further processing. bye bestie 👋
• -w, --verbose — prints verbose informations related to basic blocks and arcs. the tea ☕
• -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. short king filenames 👑

---

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 — 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 — 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. ✨ robot-friendly ✨

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. no rounding up for clout, bestie 🎀

🎀 INVOCATION 🎀

okay bestie, listen up, because this is the part where it all comes together. before gcov can do literally anything for you, you have to compile your program with a special GCC option: --coverage. that one tells the compiler to generate the extra info gcov needs (basically a flow graph of the program) and it also drops extra code into the object files so the profiling info gcov wants can actually get generated. and where do those extra files go? into the directory where the object file lives. no cap. 📁✨

running your program is what makes the profile output happen. for every source file compiled with -fprofile-arcs, an accompanying .gcda file gets placed in the object file directory. that's the whole vibe. 💖

then you run gcov with your program's source file names as arguments and boom — you get a listing of the code along with the frequency of execution for each line. so like, if your program is called tmp.cpp, here's what the basic gcov experience looks like:

$ 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 the output from gcov. here's a sample, and yes she's long, we're reading all of it:

                -:    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:}

💀 heads up: line 7 shows up in this report multiple times. the first occurrence presents the total number of executions of the line, and the next two belong to the instances of class Foo's constructors. and see how line 30 contains some unexecuted basic blocks? that's why its execution count has an asterisk symbol on it. iykyk.

---

when you use the -a option you get individual block counts, and the output is giving:

                -:    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 only shows up on one line — the last line of the block. a multi-line block only contributes to the execution count of that last line, and the other lines won't be shown as containing code, unless previous blocks end on those lines. the total execution count of a line is shown, and the subsequent lines show the execution counts for the individual blocks that end on that line. after each block, the branch and call counts of the block get shown too — but only 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. not a single one. 😭

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 gets printed showing how many times the function is called, how many times it returns and what percentage of the function's blocks were executed. so like, the receipts. 🧾✨

okay so for every basic block, there's a lil line printed right after the last line of that basic block, and it tells you about the branch or the call that ends it. 💅 if multiple basic blocks end on the same source line you can get multiple branches and calls listed for one single line — and in that case each branch and call gets its own number. ngl there's no simple way to map those branches and calls back to source constructs, but generally the lowest-numbered branch or call lines up with the leftmost construct on that source line. iykyk. 🦋

For a branch: if it got executed at least once, you get a percentage — number of times the branch was taken divided by number of times it was executed. Otherwise? the message "never executed" gets printed. 💀

For a call: if it was executed at least once, you get a percentage — number of times the call returned divided by number of times the call was executed. Usually that's 100%, but it can be less for functions that call "exit" or "longjmp", because those lil guys don't necessarily return every time they're called. 🙈

---

When you use the -g option, your output looks like this:

       $ gcov -t -m -g tmp
               -:    0:Source:tmp.cpp
               -:    0:Graph:tmp.gcno
               -:    0:Data:tmp.gcda
               -:    0:Runs:1
               -:    1:#include <stdio.h>
               -:    2:
               -:    3:int
               1:    4:main (void)
               -:    5:{
               -:    6:  int i, total;
               1:    7:  total = 0;
               -:    8:
              11:    9:  for (i = 0; i < 10; i++)
       condition outcomes covered 2/2
              10:   10:    total += i;
               -:   11:
              1*:   12:  int v = total > 100 ? 1 : 2;
       condition outcomes covered 1/2
       condition  0 not covered (true)
               -:   13:
              1*:   14:  if (total != 45 && v == 1)
       condition outcomes covered 1/4
       condition  0 not covered (true)
       condition  1 not covered (true false)
           #####:   15:    printf ("Failure\n");
               -:   16:  else
               1:   17:    printf ("Success\n");
               1:   18:  return 0;
               -:   19:}

For every condition, the number of taken and total outcomes get printed. and if there are uncovered outcomes, a line gets printed for each condition showing the uncovered outcome in parentheses. 🎀 Conditions are identified by their index — index 0 is the left-most condition. so in "a || (b && c)", a is condition 0, b is condition 1, and c is condition 2. periodt.

An outcome is considered covered if it has an independent effect on the decision — also known as masking MC/DC (Modified Condition/Decision Coverage). ✨ In this example the decision evaluates to true and a is evaluated, but not covered. why? because a can't affect the decision independently — both a and b must change value for the decision to change. 💅

       $ gcov -t -m -g tmp
               -:    0:Source:tmp.c
               -:    0:Graph:tmp.gcno
               -:    0:Data:tmp.gcda
               -:    0:Runs:1
               -:    1:#include <stdio.h>
               -:    2:
               1:    3:int main()
               -:    4:{
               1:    5:  int a = 1;
               1:    6:  int b = 0;
               -:    7:
               1:    8:  if (a && b)
       condition outcomes covered 1/4
       condition  0 not covered (true false)
       condition  1 not covered (true)
           #####:    9:    printf ("Success!\n");
               -:   10:  else
               1:   11:    printf ("Failure!\n");
               -:   12:}

---

When you compile with --coverage -fpath-coverage and use the option -e, your output looks like this:

       $ gcov -t -e tmp
               -:    0:Source:tmp.cpp
               -:    0:Graph:tmp.gcno
               -:    0:Data:tmp.gcda
               -:    0:Runs:1
               -:    1:#include <stdio.h>
               -:    2:
       paths covered 4 of 15
               1:    3:int main ()
               -:    4:{
               -:    5:  int i, total;
               1:    6:  total = 0;
               -:    7:
              11:    8:  for (i = 0; i < 10; i++)
              10:    9:    total += i;
               -:   10:
              1*:   11:  int v = total > 100 ? 1 : 2;
               -:   12:
              1*:   13:  if (total != 45 && v == 1)
           #####:   14:    printf ("Failure\n");
               -:   15:  else
               1:   16:    printf ("Success\n");
               1:   17:  return 0;
               -:   18:}

This output is lowkey useful for figuring out roughly where coverage is missing, and for testing how different inputs change the coverage. 🔥 and --prime-paths-source is a genuinely useful tool for understanding paths, bestie.

       $ gcov -t --prime-paths-source tmp
               -:    0:Source:tmp.cpp
               -:    0:Graph:tmp.gcno
               -:    0:Data:tmp.gcda
               -:    0:Runs:1
               -:    1:#include <stdio.h>
               -:    2:
       paths covered 4 of 15
       path 1:
       BB  2:           3:int main ()
       BB  2:           6:  total = 0;
       BB  2:           8:  for (i = 0; i < 10; i++)
       BB  4: (false)   8:  for (i = 0; i < 10; i++)
       BB  5: (true)   11:  int v = total > 100 ? 1 : 2;
       BB  6:          11:  int v = total > 100 ? 1 : 2;
       BB  8: (true)   13:  if (total != 45 && v == 1)
       BB  9: (true)   13:  if (total != 45 && v == 1)
       BB 10:          14:    printf ("Failure\n");
       BB 12:          17:  return 0;

In this mode, gcov prints details on the missing paths. The first column lists the sequence of basic blocks (BB). The second column is the decision to take at that line, if there is one. The final columns are the line number and the line itself. this is so good for understanding the paths — especially the ones that are hard to cover or even literally unreachable. 😭 Lines may be repeated (look at that "for" loop) if the same line is part of multiple basic blocks. This mode is intended for humans and it's good at understanding what code is exercised under testing, or for given inputs. Heads up tho: this output is quite verbose, and for focusing on specific functions you can combine it with the filters --include and --exclude. 💖

---

A denser output is available with --prime-paths-lines, which looks like this:

               -:    0:Source:tmp.cpp
               -:    0:Graph:tmp.gcno
               -:    0:Data:tmp.gcda
               -:    0:Runs:1
               -:    1:#include <stdio.h>
               -:    2:
       paths covered 4 of 15
       path  1 not covered: lines 8 8(false) 11(true) 11 13(true) 13(true) 14 17
       path  2 not covered: lines 8 8(false) 11(true) 11 13(true) 13(false) 16 17
       path  3 not covered: lines 8 8(false) 11(true) 11 13(false) 16 17
       path  4 not covered: lines 8 8(false) 11(false) 11 13(true) 13(true) 14 17
       path  5 not covered: lines 8 8(false) 11(false) 11 13(true) 13(false) 16 17
       path  6 not covered: lines 8 8(false) 11(false) 11 13(false) 16 17
       path  8 not covered: lines 9 8(false) 11(true) 11 13(true) 13(true) 14 17
       path  9 not covered: lines 9 8(false) 11(true) 11 13(true) 13(false) 16 17
       path 10 not covered: lines 9 8(false) 11(true) 11 13(false) 16 17
       path 11 not covered: lines 9 8(false) 11(false) 11 13(true) 13(true) 14 17
       path 12 not covered: lines 9 8(false) 11(false) 11 13(true) 13(false) 16 17
               1:    3:int main ()
               -:    4:{

In this mode, every missing path gets expanded using the lines and decisions, just like --prime-paths-source, except it's printed on a single line. 🌸 This mode gives a good overview over the paths, and it's great for tracking how different tests and inputs exercise the code. obsessed, honestly.

---

The execution counts are cumulative. So 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. Then it adds in the new execution counts, and finally writes the data to the file. 💅

---

You can report on a subset of functions by using --include and --exclude. This is super useful when combined with --stdout, like when you're trying to understand behavior and coverage for a particular function by running a test, looking at gcov output, testing another input, and running gcov again. 🔁

       $ gcov -m --stdout --include inc tmp
               -:    0:Source:tmp.cpp
               -:    0:Graph:tmp.gcno
               -:    0:Data:tmp.gcda
               -:    0:Runs:1
              2*:    8:  void inc () { b++; }
       ------------------
       Foo<char>::inc():
           #####:    8:  void inc () { b++; }
       ------------------
       Foo<int>::inc():
               2:    8:  void inc () { b++; }
       ------------------

gcov will match on mangled names by default, and you can control that with the -M flag. 👀 note that matching and reporting are independent — so you can match on mangled names while printing demangled names, and vice versa. bestie, the flexibility?? To report on the "int" instantiation of "Foo" matching on mangled and demangled names:

       $ gcov -t -m -M tmp --include 'Foo<int>'
               -:    0:Source:tmp.cpp
               -:    0:Graph:tmp.gcno
               -:    0:Data:tmp.gcda
               -:    0:Runs:1
               1:    7:  Foo(): b (1000) {}
               2:    8:  void inc () { b++; }

       $ gcov -t -m tmp --include 'FooIi'
               -:    0:Source:tmp.cpp
               -:    0:Graph:tmp.gcno
               -:    0:Data:tmp.gcda
               -:    0:Runs:1
               1:    7:  Foo(): b (1000) {}
               2:    8:  void inc () { b++; }

The arguments you pass to --include and --exclude are extended regular expressions — literally the same flavour as grep -E — so a pattern like "in.?" matches both "inc" and "main". And if you're running with -M? Then all the "int" instantiations of "Foo" match too, bestie. You can stack --include and --exclude as many times as you like, and if one name matches multiple filters, it's the last filter that matched which takes preference. So like, for example, to match "main" and the "int" instantiation of "inc", while omitting the "Foo" constructor:

$ gcov -t -m -M --include in --exclude Foo --include '<int>::inc' tmp
        -:    0:Source:tmp.cpp
        -:    0:Graph:tmp.gcno
        -:    0:Data:tmp.gcda
        -:    0:Runs:1
        2:    8:  void inc () { b++; }
        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;

---

🌸 Using gcov with GCC Optimization 🌸

Okay so if you wanna use gcov to help optimize your code, first things first: you have to compile your program with a special GCC option, --coverage. That's the non-negotiable. Other than that, go off, use any other GCC options you want 💅 — but if you actually want to prove that every single line in your program got executed, then you should not compile with optimization at the same time. Because on some machines the optimizer can straight-up eliminate simple code lines by smooshing them together with other lines. Like, code that looks like this:

if (a != b)
  c = 1;
else
  c = 0;

can get compiled into one single instruction on some machines. And in that case, there is genuinely no way for gcov to calculate separate execution counts for each line, because there isn't separate code for each line. So the gcov output ends up looking 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 is telling you that this whole block of code, combined by optimization, executed 100 times. In one sense this result is correct — there really was only one instruction representing all four of these lines. But the output does not tell you how many times the result was 0 and how many times the result was 1. 😭

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

If the function isn't inlined, the compiler has to emit an out of line copy of the function, in any object file that needs it. So if fileA.o and fileB.o both contain out of line bodies of a particular inlineable function, they'll 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. The unfortunate part? 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. 💀

And 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 gets shown for the same source lines, the line counts themselves might seem inconsistent. It's giving chaos but it's fr fr just how it works.

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. So __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.

The profiling run-time library reports various errors related to profile manipulation and profile saving. Errors get printed into standard error output, or into the GCOV_ERROR_FILE file, if that environment variable is used. And in order to terminate immediately after an error occurs, set the GCOV_EXIT_AT_ERROR environment variable. That one can help you find profile clashing, which is what leads to a misleading profile. 🕵️

---

👀 SEE ALSO 👀

• gpl(7)
• gfdl(7)
• fsf-funding(7)
• gcc(1) — and the Info entry for gcc.

---

💖 COPYRIGHT 💖

Copyright (c) 1996-2026 Free Software Foundation, Inc.

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 included in the gfdl(7) man page.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.

---

and that's a wrap, bestie 🎀✨ this whole thing shipped as part of gcc-16, dated 2026-08-19 — go forth, instrument everything, and may all your line counts be nonzero 💅🌈