GCOV(1)
💅 GCOV(1) — the GNU coverage queen 👑
ok so welcome to gcov, bestie. it's a GNU thing, it lives in section 1 of the manual, and it is literally the coverage testing tool. if you've ever been like "hmm is my code even being run tho" — this is the girl for you. 🎀
✨ NAME ✨
gcov — coverage testing tool. that's it, that's the whole vibe. she tests your coverage. 🦋
🌸 SYNOPSIS 🌸
gcov [-v|--version] [-h|--help]
[-a|--all-blocks]
[-b|--branch-probabilities]
[-c|--branch-counts]
[-d|--display-progress]
[-f|--function-summaries]
[-i|--json-format]
[-j|--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]
filesyes there are a lot of flags. she's extra and we love her for it. 💅
💖 DESCRIPTION 💖
so. gcov is a test coverage program. test coverage, meaning it watches your code run and tells you which parts actually got exercised. you use it in concert with GCC — like, they're a duo, a duo that ate — to analyze your programs so you can make more efficient, faster running code and find the untested parts of your program. 🕵️♀️
you can use gcov as a profiling tool to help you figure out where your optimization efforts will best affect your code. you can also run gcov alongside the other profiling tool, gprof, to assess which parts of your code use the greatest amount of computing time. two tools, one slay. ✨
🧚 so what do profiling tools even do
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 compiled, you can look at each module to see which modules should be optimized. gcov helps you determine where to work on optimization. main character energy for your hot spots.
🎀 coverage testing & testsuites (yes this matters)
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. then developers can determine what kinds of test cases need to be added to the testsuites to create both better testing and a better final product. 🌈
lowkey this is the whole "ship it" pipeline. coverage = receipts. 🧾
😭 the compile-it-right warning (do not skip this one)
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. fr fr, turn the optimizer off.
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, periodt. 💅
📓 the logfile situation
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. 🫶
⚠️ and here's the tea: gcov works only on code compiled with GCC. it is not compatible with any other profiling or test coverage mechanism. no crossovers, no collabs, she's exclusive. 💀
👑 OPTIONS 👑
one bullet per flag, bestie, no skipping. she listed them all and so will i.
-a,--all-blocks— write individual execution counts for every basic block. normallygcovonly 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-uoption is given.-c,--branch-counts— write branch frequencies as the number of branches taken, rather than the percentage of branches taken. same tea, different unit. 🍓-d,--display-progress— display the progress on the standard output. she's keeping you posted. ✨-f,--function-summaries— output summaries for each function in addition to the file level summary.-h,--help— display help about usinggcov(on the standard output), and exit without doing any further processing. ask and she'll tell you, then she's out. 🚪-i,--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 gzip compression algorithm and the files have.gcov.json.gzextension.
structure of the JSON is following:
{
"current_working_directory": <current_working_directory>,
"data_file": <data_file>,
"format_version": <format_version>,
"gcc_version": <gcc_version>
"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
- gcc_version: version of the GCC compiler
each file has the following form:
{
"file": <file_name>,
"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": <blocks>,
"blocks_executed": <blocks_executed>,
"demangled_name": "<demangled_name>,
"end_column": <end_column>,
"end_line": <end_line>,
"execution_count": <execution_count>,
"name": <name>,
"start_column": <start_column>
"start_line": <start_line>
}fields of the function element have following semantics:
- blocks: number of blocks that are in the function
- blocks_executed: number of executed blocks of the function
- demangled_name: demangled name of the function
- end_column: column in the source file where the function ends
- end_line: line in the source file where the function ends
- execution_count: number of executions of the function
- name: name of the function
- start_column: column in the source file where the function begins
- start_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. 🔮
each line has the following form:
{
"branches": [<branch>],
"count": <count>,
"line_number": <line_number>,
"unexecuted_block": <unexecuted_block>
"function_name": <function_name>,
}branches are present only with -b option. fields of the line element have following semantics:
- count: number of executions of the line
- line_number: line number
- unexecuted_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": <count>,
"fallthrough": <fallthrough>,
"throw": <throw>
}fields of the branch element have following semantics:
- count: number of executions of the branch
- fallthrough: true when the branch is a fall through branch
- throw: true when the branch is an exceptional branch
-j,--human-readable— write counts in human readable format (like24.6k). no more counting zeros at 2am. 🫶-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.-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. look but don't touch. 🙈-odirectory|file,--object-directorydirectory,--object-filefile — 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. ✨-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. ngl, she's just not that into your system headers. 🚫-sdirectory,--source-prefixdirectory — a prefix you want gcov to ✂️ yeet off source file names while it's generating those output coverage files. Super handy when you're building in a totally separate directory and you do not want the pathname to the source directory showing up in your output file names. heads up though: this prefix detection runs before gcov decides whether the source file is absolute. order matters, bestie 💅-t,--stdout— output straight to standard output instead of writing output files. no files, just vibes ✨-u,--unconditional-branches— when branch probabilities are being given, this includes the ones for unconditional branches too. ngl, unconditional branches are normally not interesting, but if you want them you want them 🫶-v,--version— shows thegcovversion number (on the standard output) and then dips, doing zero further processing. iconic exit behavior 👑-w,--verbose— prints verbose informations related to basic blocks and arcs. yes, the manpage really says "informations", we don't make the rules 🍓-x,--hash-filenames— okay so when you're using--preserve-paths,gcovuses the full pathname of the source files to build an output filename, and that can lead to long filenames that can overflow filesystem limits 😭. this option instead creates names of the formsource-file##md5.gcov, where thesource-filecomponent is just the final filename part, and themd5component is calculated from the full mangled name that would have been used otherwise. it's the alternative to--preserve-pathson systems which have a filesystem limit. slay 💅
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. periodt.
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 it 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. fr fr 🧚
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. best of both worlds 💖
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 crimes here 🔥
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 program's source file names as arguments will now produce a listing of the code along with frequency of execution for each line. for example, if your program is called tmp.cpp, this is what you see when you use the basic gcov facility:
$ g++ --coverage tmp.cpp
$ 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:}okay so in this mode — the branch-probabilities vibe — each basic block only gets shown on one single line, and it's always 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 some previous blocks end on them. The total execution count of a line gets shown, and then the lines right after it show the execution counts for the individual blocks that end on that line. After each block, that block's branch and call counts get shown too — but only if you passed the -b option, bestie. 💅
Because of the way GCC instruments calls, a call count can show up after a line with no individual blocks at all. And look at line 33 in the example — it 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, you get a line printed showing how many times the function was called, how many times it returned, and what percentage of the function's blocks got executed. 🎀
For each basic block, a line gets printed after the last line of the basic block describing the branch or call that ends it. There can be multiple branches and calls listed for a single source line if multiple basic blocks end on that line — in that case, the branches and calls each get their own number. Ngl, there's no simple way to map these branches and calls back to source constructs. In general though? The lowest numbered branch or call corresponds to the leftmost construct on the source line. iykyk. ✨
For a branch: if it was executed at least once, you get a percentage showing the number of times the branch was taken divided by the number of times the branch was executed. If it wasn't executed, you get the message "never executed" instead.
For a call: if it was executed at least once, you get a percentage showing the number of times the call returned divided by the number of times the call was executed. That's usually 100%, but it can be lower for functions that call "exit" or "longjmp" — those might not return every single time they're called. 💀
The execution counts are cumulative, which is lowkey iconic. If the example program ran again without removing the .gcda file, the count for how many times each line in the source was executed would get added to the results of the previous run(s). That's potentially useful in several ways — like accumulating data over a bunch of program runs as part of a test verification suite, or getting more accurate long-term info across a huge number of program runs. Obsessed fr fr.
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 that 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. Slay. 💅
🌸 Using gcov with GCC Optimization 🌸
If you're planning 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 you want — but if you wanna 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. And in that case there's 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 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 that result is correct, because there was only one instruction representing all four of these lines. However, the output doesn't indicate 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 if it isn't 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'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. 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. And 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. Not a bug, just a vibe check fail. 🙈
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 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". And 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 can help users 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) 1996-2020 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 the whole page bestie, you ate it up 💅 the version tag on this one is gcc-10.2.0, dated 2020-07-23, and it lives in section one as GCOV(1). go forth and profile, queen 👑✨
📦 where this page comes from
- ubuntu jammy: gcc-riscv64-unknown-elf 10.2.0-0ubuntu1
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms