GCOV(1)
💅 gcov(1) — GNU
okay bestie, buckle UP, because we're about to talk about the coverage testing tool, aka gcov, from the GNU fam. this is the little icon that tells you which parts of your code actually ran and which parts are just... sitting there. dead. unbothered. untouched. 🫶
✨ 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 right there. flags and files. slay. 💅
🌸 DESCRIPTION 🌸
okay so gcov is a test coverage program. the way that it works is you use it in concert with GCC, and together you analyze your programs to help you create more efficient, faster-running code, and to sniff out the untested parts of your program. 🕵️♀️
you can also use gcov as a profiling tool — that's how you find out 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 up the most computing time. greedy little sections. 😭
so, profiling tools help you analyze your code's performance. using a profiler like gcov or gprof, you can find out some basic performance stats, like:
- 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 modules should be optimized. gcov is literally there to help you determine 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. because like — testsuites can verify that a program works as expected, sure, but a coverage program tests to see how much of the program is actually exercised by that testsuite. and 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. it's giving glow-up. ✨
you should compile your code without optimization if you plan to usegcov, because the optimization, by combining some lines of code into one function, may not give you as much information as you need to look forhot spotswhere the code is using a great deal of computer time. no cap.
likewise — and this one's important — 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, bestie.
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. team work makes the dream work. 🫶
lowkey a big deal: 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 outputs execution counts only for the main blocks of a line. with this option you can determine if blocks within a single line are not being executed. like, the tea within the line. 🍵
💖 -b, --branch-probabilities
write branch frequencies to the output file, and write branch summary info to the standard output. this option allows you to see how often each branch in your program was taken. unconditional branches will not be shown, unless the -u option is given. 👀
💖 -c, --branch-counts
write branch frequencies as the number of branches taken, rather than the percentage of branches taken. numbers, not percentages. that's the whole thing. 🔢
💖 -d, --display-progress
display the progress on the standard output. so you're not just sitting there wondering. ⏳
💖 -f, --function-summaries
output summaries for each function, in addition to the file level summary. more detail, more better.
💖 -h, --help
display help about using gcov (on the standard output), and exit without doing any further processing. she's just here to explain, not to work. 💅
💖 -j, --json-format
output gcov file in an easy-to-parse JSON intermediate format which does not require source code for generation. the JSON file is compressed with gzip compression algorithm and the files have .gcov.json.gz extension.
structure of the JSON is following:
{
"current_working_directory": "foo/bar",
"data_file": "a.out",
"format_version": "1",
"gcc_version": "11.1.1 20210510"
"files": ["$file"]
}fields of the root element have following semantics:
current_working_directory: working directory where a compilation unit was compileddata_file: name of the data file (GCDA)format_version: semantic version of the formatgcc_version: version of the GCC compiler
each file has the following form:
{
"file": "a.c",
"functions": ["$function"],
"lines": ["$line"]
}fields of the file element have following semantics:
file_name: name of the source file
each function has the following form:
{
"blocks": 2,
"blocks_executed": 2,
"demangled_name": "foo",
"end_column": 1,
"end_line": 4,
"execution_count": 1,
"name": "foo",
"start_column": 5,
"start_line": 1
}fields of the function element have following semantics:
blocks: number of blocks that are in the functionblocks_executed: number of executed blocks of the functiondemangled_name: demangled name of the functionend_column: column in the source file where the function endsend_line: line in the source file where the function endsexecution_count: number of executions of the functionname: name of the functionstart_column: column in the source file where the function beginsstart_line: line in the source file where the function begins
note that line numbers and column numbers number from 1. in the current implementation,start_lineandstart_columndo not include any template parameters and the leading return type but that this is likely to be fixed in the future. so like, don't come for her, she's doing her best. 😭
each line has the following form:
{
"branches": ["$branch"],
"count": 2,
"line_number": 15,
"unexecuted_block": false,
"function_name": "foo",
}branches are present only with -b option. fields of the line element have following semantics:
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 (like 24.6k). because reading raw digits is not the move. 🧚
💖 -k, --use-colors
use colors for lines of code that have zero coverage. we use red color for non-exceptional lines and cyan for exceptional. same colors are used for basic blocks with -a option. aesthetic AND informative. 🌈
💖 -l, --long-file-names
create long file names for included source files. for example, if the header file x.h contains code, and was included in the file a.c, then running gcov on the file a.c will produce an output file called a.c##x.h.gcov instead of x.h.gcov. this can be useful if x.h is included in multiple source files and you want to see the individual contributions. if you use the -p option, both the including and included file names will be complete path names.
💖 -m, --demangled-names
display demangled function names in output. the default is to show mangled function names. so if the names are looking cursed, this is your girl. 💀
💖 -n, --no-output
do not create the gcov output file. she's shy.
💖 -o directory|file, --object-directory directory, --object-file file
specify either the directory containing the gcov data files, or the object path name. the .gcno, and .gcda data files are searched for using this option.
- if a directory is specified, the data files are in that directory and named after the input file name, without its extension.
- if a file is specified here, the data files are named after that file, without its extension.
💖 -p, --preserve-paths
preserve complete path information in the names of generated .gcov files. without this option, just the filename component is used. with this option, all directories are used, with / characters translated to # characters, . directory components removed and unremoveable .. components renamed to ^. this is useful if sourcefiles are in several different directories. 🗂️
💖 -q, --use-hotness-colors
emit perf-like colored output for hot lines. legend of the color scale is printed at the very beginning of the output file. 🔥🔥🔥
💖 -r, --relative-only
only output information about source files with a relative pathname (after source prefix elision). absolute paths are usually system header files and coverage of any inline functions therein is normally uninteresting. so we're skipping them, respectfully. 🙈
-sdirectory,--source-prefixdirectory — okay so this one is a prefix for source file names that gets removed when gcov generates the output coverage files. it's the move when you're building in a separate directory and you do NOT want the pathname to the source directory hanging around when the output file names get decided. ⚠️ but listen: 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. no files, just the terminal, living in the moment ✨-u,--unconditional-branches— when branch probabilities are given, this includes those of unconditional branches too. ngl unconditional branches are normally not interesting, so that's why this is a you-have-to-ask-for-it thing.-v,--version— display thegcovversion number (on the standard output), and exit without doing any further processing. it says hi and then immediately leaves, iconic behavior 💅-w,--verbose— print verbose informations related to basic blocks and arcs. (yes, "informations". we're keeping it. that's what it says 🫶)-x,--hash-filenames— so when you're using--preserve-paths,gcovuses the full pathname of the source files to create an output filename. and that can lead to long filenames that can overflow filesystem limits 😭 not the vibe. this option instead creates names of the formsource-file##md5.gcov, where thesource-filecomponent is 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.
okay so 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, it needs to know where it's standing. 🧭
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. heads up: 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.
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 fake rounding, we're honest 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 -c
$ g++ --coverage tmp.o
$ a.out
$ gcov tmp.cpp -m
File 'tmp.cpp'
Lines executed:92.86% of 14
Creating 'tmp.cpp.gcov'the file tmp.cpp.gcov contains output from gcov. here is a sample:
-: 0:Source:tmp.cpp
-: 0:Working directory:/home/gcc/testcase
-: 0:Graph:tmp.gcno
-: 0:Data:tmp.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <stdio.h>
-: 2:
-: 3:template<class T>
-: 4:class Foo
-: 5:{
-: 6: public:
1*: 7: Foo(): b (1000) {}
------------------
Foo<char>::Foo():
#####: 7: Foo(): b (1000) {}
------------------
Foo<int>::Foo():
1: 7: Foo(): b (1000) {}
------------------
2*: 8: void inc () { b++; }
------------------
Foo<char>::inc():
#####: 8: void inc () { b++; }
------------------
Foo<int>::inc():
2: 8: void inc () { b++; }
------------------
-: 9:
-: 10: private:
-: 11: int b;
-: 12:};
-: 13:
-: 14:template class Foo<int>;
-: 15:template class Foo<char>;
-: 16:
-: 17:int
1: 18:main (void)
-: 19:{
-: 20: int i, total;
1: 21: Foo<int> counter;
-: 22:
1: 23: counter.inc();
1: 24: counter.inc();
1: 25: total = 0;
-: 26:
11: 27: for (i = 0; i < 10; i++)
10: 28: total += i;
-: 29:
1*: 30: int v = total > 100 ? 1 : 2;
-: 31:
1: 32: if (total != 45)
#####: 33: printf ("Failure\n");
-: 34: else
1: 35: printf ("Success\n");
1: 36: return 0;
-: 37:}note that line 7 is shown in the report multiple times. first occurrence presents total number of execution of the line and the next two belong to instances of class Foo constructors. as you can also see, line 30 contains some unexecuted basic blocks and thus execution count has asterisk symbol. ✨
when you use the -a option, you will get individual block counts, and the output looks like this:
-: 0:Source:tmp.cpp
-: 0:Working directory:/home/gcc/testcase
-: 0:Graph:tmp.gcno
-: 0:Data:tmp.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <stdio.h>
-: 2:
-: 3:template<class T>
-: 4:class Foo
-: 5:{
-: 6: public:
1*: 7: Foo(): b (1000) {}
------------------
Foo<char>::Foo():
#####: 7: Foo(): b (1000) {}
------------------
Foo<int>::Foo():
1: 7: Foo(): b (1000) {}
------------------
2*: 8: void inc () { b++; }
------------------
Foo<char>::inc():
#####: 8: void inc () { b++; }
------------------
Foo<int>::inc():
2: 8: void inc () { b++; }
------------------
-: 9:
-: 10: private:
-: 11: int b;
-: 12:};
-: 13:
-: 14:template class Foo<int>;
-: 15:template class Foo<char>;
-: 16:
-: 17:int
1: 18:main (void)
-: 19:{
-: 20: int i, total;
1: 21: Foo<int> counter;
1: 21-block 0
-: 22:
1: 23: counter.inc();
1: 23-block 0
1: 24: counter.inc();
1: 24-block 0
1: 25: total = 0;
-: 26:
11: 27: for (i = 0; i < 10; i++)
1: 27-block 0
11: 27-block 1
10: 28: total += i;
10: 28-block 0
-: 29:
1*: 30: int v = total > 100 ? 1 : 2;
1: 30-block 0
%%%%%: 30-block 1
1: 30-block 2
-: 31:
1: 32: if (total != 45)
1: 32-block 0
#####: 33: printf ("Failure\n");
%%%%%: 33-block 0
-: 34: else
1: 35: printf ("Success\n");
1: 35-block 0
1: 36: return 0;
1: 36-block 0
-: 37:}Okay so in this mode, every basic block gets shown on just one line — the last line of the block. If a block is multi-line, only that last line gets the execution count; the other lines won't be shown as having code, unless some previous blocks ended on them. The total execution count for a line is shown, and then the lines after that show the execution counts for each individual block that ends on that line. And after each block, if you passed the -b option, you'll see the branch and call counts for that block.
Because of how GCC instruments calls, you might see a call count after a line that has no individual blocks. Like, check it — line 33 has 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 every function, you get a line that tells you how many times the function got called, how many times it returned, and what percentage of the function's blocks were actually executed.
For each basic block, a line gets printed right after the last line of that block, describing the branch or call that ends it. If multiple basic blocks end on the same source line, you can get multiple branches and calls listed for that one line. When that happens, each branch and call gets its own number. There's no easy way to map these numbers back to the source code constructs, but generally the lowest-numbered branch or call lines up with the leftmost construct on that source line.
For a branch: if it ran at least once, you'll see a percentage — that's the number of times the branch was taken divided by the number of times it was executed. If it never ran, you'll just see the message "never executed".
For a call: if it ran at least once, you'll get a percentage — the number of times the call returned divided by the number of times it was executed. Usually that's 100%, but it can be lower for functions that call exit or longjmp, because those might not return every time they're called.
The execution counts are cumulative. If you ran the example program again without deleting the .gcda file, the count for how many times each source line executed would get added to the results from the previous run(s). That's lowkey useful for a bunch of things. Like, you could accumulate data over many program runs for a test verification suite, or get more accurate long-term info across a ton of runs.
The data in the .gcda files gets saved right before the program exits. For each source file compiled with -fprofile-arcs, the profiling code first tries to read an existing .gcda file. If that file doesn't match the executable — like if the number of basic block counts is different — it just ignores what's in the file. Then it adds in the new execution counts and finally writes the data to the file.
💅 Using gcov with GCC Optimization 💅
If you're planning to use gcov to help optimize your code, you first gotta compile your program with a special GCC option: --coverage. Other than that, you can use any other GCC options you want. BUT — if you want to prove that every single line in your program actually ran, you should not compile with optimization at the same time. On some machines, the optimizer can eliminate simple code lines by merging them with other lines. For example, code like this:
if (a != b)
c = 1;
else
c = 0;can get compiled into a single instruction on some machines. When that happens, there's literally no way for gcov to calculate separate execution counts for each line, because there's no separate code for each line. So the gcov output ends up looking like this if you compiled with optimization:
100: 12:if (a != b)
100: 13: c = 1;
100: 14:else
100: 15: c = 0;This output shows that the whole block of code — merged together by optimization — executed 100 times. In one way that's correct, since there was only one instruction representing all four of those lines. But the output doesn't tell you how many times the result was 0 and how many times it was 1.
Inlineable functions can create some unexpected line counts. You'll see line counts for the source code of the inlineable function, but what you actually get depends on where the function gets inlined — or if it doesn't get 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 some inlineable function, they'll both also contain coverage counts for that function. When you link fileA.o and fileB.o together, on many systems the linker will pick one of those out-of-line bodies for all calls to that function, and remove or ignore the other one. Sadly, it won't remove the coverage counters for the unused function body. So when instrumented, every use of that function except one will show zero counts.
If the function gets inlined in several places, the block structure at each location might not be identical. For example, a condition might become calculable at compile time in some instances. Since the coverage for all the uses of the inline function gets shown for the same source lines, the line counts themselves can look kinda inconsistent.
Long-running applications can use the __gcov_reset and __gcov_dump facilities to restrict profile collection to just the part of the program you care about. Calling __gcov_reset(void) clears all run-time profile counters to zero. Calling __gcov_dump(void) dumps the profile info collected at that point to the .gcda output files. Instrumented applications use a static destructor with priority 99 to invoke the __gcov_dump function. So __gcov_dump runs after all user-defined static destructors, and after handlers registered with atexit.
If an executable loads a dynamic shared object via dlopen, you need -Wl,--dynamic-list-data to dump all the profile data.
The profiling run-time library reports various errors related to profile manipulation and profile saving. Errors get printed to standard error output, or to the GCOV_ERROR_FILE file if that environment variable is set. If you want to terminate immediately after an error occurs, set the GCOV_EXIT_AT_ERROR environment variable. That can help you find profile clashing, which leads to a misleading profile.
🌈 SEE ALSO 🌈
gpl(7)gfdl(7)fsf-funding(7)gcc(1)and the Info entry forgcc.
👑 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.Source/version: gcc-12.4.0 · Date: 2024-06-20 · Manual: GCOV(1). Byeee, bestie! 💖✨
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