GCOV(1)
👑 gcov — the GNU coverage testing tool (gcov(1), section 1) 💖
okay bestie, pull up a chair, because this is the manual page for gcov, GNU's coverage testing tool. it lives in section 1 of the manual, it ships with GNU, and it's about to tell you everything about which parts of your code actually run. let's get into it ✨
🌸 NAME 🌸
gcov — coverage testing tool 💅
✨ 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]
fileslike, that's the whole vibe in one block. the files at the end are the ones you're feeding it 🍓
🦋 DESCRIPTION 🦋
okay so gcov is a test coverage program. you use it in concert with GCC to analyze your programs, and it helps you write more efficient, faster-running code and find the untested parts of your program. iconic behavior honestly.
you can use gcov as a profiling tool to help you discover where your optimization efforts will best affect your code. and you can also run gcov alongside the other profiling tool, gprof, to figure out which parts of your code are eating the most computing time. no cap. 🔥
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 go module by module and see which ones should be optimized. gcov helps you figure out where to put in that optimization work. 💪
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. it's giving quality assurance, and we love that 🫶
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. like, it's hiding the tea from you 😭
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. problem solved, slay ✨
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. they're a duo. a duo of icons. 👑
⚠️ important caveat bestie: gcov works only on code compiled with GCC. it is not compatible with any other profiling or test coverage mechanism. periodt. don't try it with anything else, it will not go well 💀
💅 OPTIONS 💅
-a,--all-blocks— write individual execution counts for every basic block. normallygcovoutputs 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. nosy in the best way 🔍-b,--branch-probabilities— write branch frequencies to the output file, and write branch summary info to the standard output. this option lets you see how often each branch in your program was taken. unconditional branches will not be shown, unless the-uoption is given. 🌈-c,--branch-counts— write branch frequencies as the number of branches taken, rather than the percentage of branches taken. same numbers, different outfit 💅-d,--display-progress— display the progress on the standard output. so you can watch it cook 👀-f,--function-summaries— output summaries for each function in addition to the file level summary. more detail, more better 🎀-h,--help— display help about usinggcov(on the standard output), and exit without doing any further processing. short and sweet 🦋-j,--json-format— output the gcov file in an easy-to-parse JSON intermediate format which does not require source code for generation. the JSON file is compressed with the gzip compression algorithm and the files have a.gcov.json.gzextension. ngl this one is my fave, machine-readable queen 👑
the 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": ["$file"]
}fields of the root element have following semantics:
current_working_directory: working directory where a compilation unit was compileddata_file: name of the data file (GCDA)format_version: semantic version of the formatgcc_version: version of the GCC compiler
each file has the following form:
{
"file": "a.c",
"functions": ["$function"],
"lines": ["$line"]
}fields of the file element have following semantics:
file_name: name of the source file
each function has the following form:
{
"blocks": 2,
"blocks_executed": 2,
"demangled_name": "foo",
"end_column": 1,
"end_line": 4,
"execution_count": 1,
"name": "foo",
"start_column": 5,
"start_line": 1
}fields of the function element have following semantics:
blocks: number of blocks that are in the functionblocks_executed: number of executed blocks of the functiondemangled_name: demangled name of the functionend_column: column in the source file where the function endsend_line: line in the source file where the function endsexecution_count: number of executions of the functionname: name of the functionstart_column: column in the source file where the function beginsstart_line: line in the source file where the function begins
📌 note that line numbers and column numbers number from 1. in the current implementation,start_lineandstart_columndo not include any template parameters and the leading return type, but that this is likely to be fixed in the future. so like, heads up, it might change on you 🙈
each line has the following form:
{
"branches": ["$branch"],
"count": 2,
"line_number": 15,
"unexecuted_block": false,
"function_name": "foo",
}branches are present only with the -b option. fields of the line element have following semantics:
count: number of executions of the lineline_number: line numberunexecuted_block: flag whether the line contains an unexecuted block (not all statements on the line are executed)function_name: a name of a function this line belongs to (for a line with an inlined statements can be not set)
each branch has the following form:
{
"count": 11,
"fallthrough": true,
"throw": false
}fields of the branch element have following semantics:
count: number of executions of the branchfallthrough: true when the branch is a fall through branchthrow: true when the branch is an exceptional branch
-H,--human-readable— write counts in human readable format (like24.6k). because24600is not the vibe 🍓-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 the-aoption. aesthetic AND informative 💖-l,--long-file-names— create long file names for included source files. for example, if the header filex.hcontains code, and was included in the filea.c, then runninggcovon the filea.cwill produce an output file calleda.c##x.h.gcovinstead ofx.h.gcov. this can be useful ifx.his included in multiple source files and you want to see the individual contributions. if you use the-poption, both the including and included file names will be complete path names. receipts, fully itemized 🧾-m,--demangled-names— display demangled function names in output. the default is to show mangled function names. pick your fighter ✨-n,--no-output— do not create the gcov output file. just vibes, no 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.gcdadata 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.gcovfiles. 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. chaos organized 🌀-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. hot lines get the glow-up 🔥-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. not our business, not our problem 🚫-sdirectory,--source-prefixdirectory — okay so this is a prefix for source file names that you wanna strip out whengcovmakes the output coverage files. it's lowkey a lifesaver when you're building in a separate directory and you don't want the path to the source dir messing with the output file names. ngl important note: this prefix detection happens before gcov decides whether the source file is absolute. so yeah, order matters bestie 💅-t,--stdout— just yeets the output to standard output instead of writing out files. periodt.-u,--unconditional-branches— when branch probabilities are on the menu, this includes the unconditional branches too. but honestly? unconditional branches are usually not the vibe, so they're skipped by default.-v,--version— shows thegcovversion number (on standard output, bestie) and then dips out immediately. no further processing, no extra tasks, just the version and bye 👋-w,--verbose— spills the tea on basic blocks and arcs — verbose info, the whole lore. 🫖-x,--hash-filenames— when you're using--preserve-paths,gcovuses the full pathname of your source files to build the output filename. and like… that can get long and overflow filesystem limits, which is not the move. this option instead creates names likesource-file##md5.gcov, wheresource-fileis just the final filename part andmd5is calculated from the full mangled name that would've been used otherwise. it's basically the alternative to--preserve-pathson systems that have a filesystem limit. so if your filesystem is giving strict, this is the fix ✨
okay so rule number one: run gcov from the same current directory you were in when you invoked the compiler. if you don't, it literally can't find your source files and it'll be a whole mess 😭. gcov makes files called mangledname.gcov right in the current directory. each one holds the coverage info for the source file it belongs to. you get one .gcov file for every source (or header) file that contains code and was compiled to make the data files. the mangledname part of the output filename is usually just the source file name, but it can get more complicated if you used the -l or -p options. go check those options for the deets, bestie.
if you hit gcov with multiple input files, it sums up the contributions from each one. and yeah, usually you'd run it with the exact same list of files as the final link of your executable. matchy matchy, you know? 💅
the .gcov files have fields separated by colons : alongside your program source code. the format is:
<execution_count>:<line_number>:<source line text>extra block info might come after each line if you asked for it with a command line option. the execution_count is - for lines that have no code. unexecuted lines are marked ##### or =====, depending on whether they're reachable by non-exceptional paths or only exceptional paths like C++ exception handlers. given the -a option, unexecuted blocks get marked $$$$$ or %%%%%, depending on whether a basic block is reachable via non-exceptional or exceptional paths. executed basic blocks that have a statement with zero execution_count end with a * character and get colored magenta when you use the -k option. this functionality is not supported in Ada. sorry Ada girlies 💔
heads up: GCC can completely remove the bodies of functions it doesn't need — like if they're inlined everywhere. those functions get marked with -, which can be confusing ngl. use the -fkeep-inline-functions and -fkeep-static-functions options to keep those functions around so gcov can properly show their execution_count. periodt.
some info lines at the very start have a line_number of zero. these preamble lines look like:
-:0:<tag>:<value>the ordering and number of these preamble lines will grow as gcov development progresses — so do not rely on them staying the same. use tag to find a particular preamble line. fr fr, don't get attached.
the extra block information looks like:
<tag> <information>it's human readable but also simple enough for machines to parse. best of both worlds, iconic.
when it prints percentages, 0% and 100% only show up when the values are exactly 0% and 100%. other values that would normally round to 0% or 100%? they get printed as the nearest non-boundary value instead. no rounding up to the extremes, bestie. it's giving precision 💅
when you're using gcov, you must first compile your program with the special GCC option --coverage. this tells the compiler to generate the extra info gcov needs (basically a flow graph of your program) and also adds extra code in the object files for generating the profiling info gcov wants. those additional files get placed in the directory where the object file is. so don't skip this step, okay? it's mandatory, no cap.
running your program will make profile output appear. for every source file compiled with -fprofile-arcs, there's a matching .gcda file that gets dropped in the object file directory. cute little data trail ✨
running gcov with your program's source file names as arguments will now spit out a listing of the code plus how often each line executed. for example, if your program is called tmp.cpp, here's 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's 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 shows up multiple times in the report. the first occurrence gives the total number of executions of the line, and the next two belong to the instances of class Foo constructors. also, as you can see, line 30 has some unexecuted basic blocks, so its execution count gets an asterisk symbol. it's giving unfinished business 💀
when you use the -a option, you 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:}okay so in this mode, each basic block is only shown on one single line — the last line of the block. 💅 A multi-line block will only contribute 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 lines after it 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 — 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. And yes bestie, 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 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. 🎀
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 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 doesn't match the executable (differing number of basic block counts) it will ignore the contents of the file. It then adds in the new execution counts and finally writes the data to the file. 🦋
💜 Using gcov with GCC Optimization 💜
If you plan to use gcov to help optimize your code, you must first compile your program with 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 isn't separate code for each line. Hence the gcov output looks like this if you compiled the program with optimization:
100: 12:if (a != b)
100: 13: c = 1;
100: 14:else
100: 15: c = 0;The output shows that this block of code, combined by optimization, executed 100 times. In one sense this result is correct, because there was only one instruction representing all four of these lines. However, the output does not indicate how many times the result was 0 and how many times the result was 1. 💀
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. 👑
🌈 SEE ALSO 🌈
gpl(7), gfdl(7), fsf-funding(7), gcc(1) and the Info entry for gcc. 🫶
💖 COPYRIGHT 💖
Copyright (c) 1996-2022 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 comes from the gcc-12 crew, dated 2022-03-19, and it's filed under GCOV(1) — go forth and profile, you absolute icon 👑💖
📦 where this page comes from
- ubuntu jammy: gcc-12 12-20220319-1ubuntu1
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms