GCOV(1)
✨ GCOV(1) ✨
okay so bestie, listen — this is gcov, section 1 of the GNU manual, and it's literally the coverage testing tool that's gonna tell you exactly which parts of your code are slaying and which parts are just... sitting there, unexercised, doing nothing. 💅
💖 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🎀 NAME 🎀
gcov — the coverage testing tool. that's her. that's the whole vibe. 💅
🌸 DESCRIPTION 🌸
gcov is a test coverage program. you run it in concert with GCC to analyze your programs so you can write more efficient, faster-running code and discover the untested parts of your program. no cap, it's giving detective energy. 🔍
you can also use gcov as a profiling tool to help discover where your optimization efforts will best affect your code. and you can team it up with the other profiling tool, gprof, to assess which parts of your code use the greatest amount of computing time. dream team fr fr. 🫶
Profiling tools help you analyze your code's performance. Using a profiler such as 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 and see which modules should be optimized. gcov helps you determine where to work on optimization. main character energy honestly. 👑
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 a whole glow-up cycle. ✨
okay this next bit is important — 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 for'hot spots'where the code is using a great deal of computer time. 🙈
Likewise, because gcov accumulates statistics by line (at the lowest resolution), it works best with a programming style that places only one statement on each line. If you use complicated macros that expand to loops or to other control structures, the statistics are less 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. girl math solved. 🧮
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. that's the boundary, bestie, we respect it. 🚫
💅 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. like zooming in on the group chat to see who actually replied. 👀-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. receipts not ratios. 🧾-g,--conditions— Write condition coverage to the output file, and write condition summary info to the standard output. This option lets you see if the conditions in your program at least once had an independent effect on the outcome of the boolean expression (modified condition/decision coverage). 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 option 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-linesor--prime-paths-source. With--json-formatall 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 option lets you see how many prime paths were taken at least once, and dense report on the covered or uncovered paths and how to cover them. This mode is useful for automated reporting and progress tracking.typemay 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 bothcoveredanduncovered.
This is 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 17This means to cover path 2 you must run lines 8, 11, 13, 14, and 17, evaluting the decision at 8 false and the decisions at 11 and 13 to "false". iykyk 🧠
--prime-paths-source [=type]— Write path coverage to the output file, and write path summary info to the standard output. This option lets you see how many prime paths were taken at least once, and detailed report on the uncovered paths an how to cover them. This mode is useful for understanding paths and interactions between sections of your program.typemay 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 bothcoveredanduncovered.
This is 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. iconic layout honestly 🌈
-d,--display-progress— Display the progress on the standard output. little loading bar moment. ⏳-f,--function-summaries— Output summaries for each function in addition to the file level summary.--include regex— Include functions matching regex. This option makesgcovonly 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 defaultgcovreports on all functions, but if a--includeis used then only functions matching the include will be reported. 🎯--exclude regex— Exclude functions matching regex. This option makesgcovnot 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 defaultgcovreports on all functions, and if--excludeis used then functions matching it will be omitted. bye bestie 👋-h,--help— Display help about usinggcov(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.gzextension. 🗜️
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": ["$file"]
}Fields of the root element have following semantics:
current_working_directory: working directory where a compilation unit was compileddata_file: name of the data file (GCDA)format_version: semantic version of the format
Changes in version 2:
calls: information about function calls is added
gcc_version: version of the GCC compiler
Each file has the following form:
{
"file": "a.c",
"functions": ["$function"],
"lines": ["$line"]
}Fields of the file element have following semantics:
file_name: name of the source file
Each function has the following form:
{
"blocks": 2,
"blocks_executed": 2,
"demangled_name": "foo",
"end_column": 1,
"end_line": 4,
"execution_count": 1,
"name": "foo",
"start_column": 5,
"start_line": 1
}okay so the function element's fields mean this, bestie, write it down 📝:
blocks— how many blocks live in the functionblocks_executed— how many of those blocks actually got executeddemangled_name— the demangled name of the functionend_column— the column in the source file where the function endsend_line— the line in the source file where the function endsexecution_count— number of times the function ranname— the function's namestart_column— the column in the source file where the function beginsstart_line— the line in the source file where the function begins
lowkey important: line numbers and column numbers count from 1. and in the current implementation,start_lineandstart_columndo not include any template parameters and the leading return type — but that's likely to be fixed in the future, so don't @ me 💅
Every line looks like this:
{
"block_ids": ["$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, fyi. The line element's fields:
block_ids— the IDs of the basic blocks that belong to this linecount— number of executions of the lineline_number— the line number, obviunexecuted_block— a flag for whether the line has an unexecuted block in it (aka not all statements on the line got executed)function_name— the name of the function this line belongs to (for a line with inlined statements, it can be not set)
Every branch goes like:
{
"count": 11,
"destination_block_id": 17,
"fallthrough": true,
"source_block_id": 13,
"throw": false
}Branch fields, one by one:
count— number of executions of the branchfallthrough—truewhen the branch is a fall through branchthrow—truewhen the branch is an exceptional branchisource_block_id— ID of the basic block where this branch happensdestination_block_id— ID of the basic block this branch jumps to
Every call is giving:
{
"destination_block_id": 1,
"returned": 11,
"source_block_id": 13
}Call fields:
returned— number of times a function call returned (the call count is equal toline::count)isource_block_id— ID of the basic block where this call happensdestination_block_id— ID of the basic block this call continues in after return
And every condition is shaped like:
{
"count": 4,
"covered": 2,
"not_covered_false": [],
"not_covered_true": [0, 1],
}Condition fields:
count— number of condition outcomes in this expressioncovered— number of covered condition outcomes in this expressionnot_covered_true— terms, by index, not seen as true in this expressionnot_covered_false— terms, by index, not seen as false in this expression
-H,--human-readable— write those counts in human readable format (like24.6k). cute and compact ✨-k,--use-colors— colors for lines of code that have zero coverage. red for non-exceptional lines, cyan for exceptional ones. same colors get used for basic blocks when you pass-a🌈-l,--long-file-names— long file names for included source files. like, if the header filex.hcontains code and it was included in the filea.c, then running gcov ona.cgives you an output file calleda.c##x.h.gcovinstead ofx.h.gcov. super useful whenx.his included in multiple source files and you wanna see each individual contribution 🫶 and if you use-p, both the including and the included file names become complete path names.-m,--demangled-names— show demangled function names in the output. the default is to show mangled function names (sorry).-M,--filter-on-demangled— makes--includeand--excludematch demangled names. this only affects the matching and does not imply--demangled-names, but you can safely combine it with it 👑-n,--no-output— don't create thegcovoutput file. that's it. that's the whole vibe.- *
-o*directory|file,--object-directorydirectory,--object-filefile — point at either the directory that holds the gcov data files, or the object path name. the.gcnoand.gcdadata files get searched for using this option. if you give a directory, the data files are in that directory and named after the input file name, minus its extension. if you give a file, the data files are named after that file, minus its extension 📁 -p,--preserve-paths— keep the complete path info in the names of the generated.gcovfiles. without it, only the filename component is used. with it, all directories are used, with/characters translated to#characters,.directory components removed, and unremoveable..components renamed to^. literally so useful when your source files are in several different directories 🗂️-q,--use-hotness-colors— emit perf-like colored output for hot lines 🔥 the legend of the color scale gets printed at the very beginning of the output file.-r,--relative-only— only output info 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, ngl.-sdirectory,--source-prefixdirectory — a prefix for source file names that gets removed when generating the output coverage files. this one's a lifesaver when you're building in a separate directory and you don't want the pathname to the source directory showing up when it decides the output file names. heads up: this prefix detection is applied **before determining whether the source file is absolute ⚠️-t,--stdout— output to standard output instead of output files. straight to the terminal, no crumbs 💅-u,--unconditional-branches— when branch probabilities are given, include the ones from unconditional branches too. unconditional branches are normally not interesting, but sometimes you just gotta know.-v,--version— display the gcov version number (on the standard output) and exit without doing any further processing. bye ✌️-w,--verbose— print verbose info related to basic blocks and arcs.-x,--hash-filenames— when you're using--preserve-paths, gcov uses the full pathname of the source files to make an output filename. that can lead to long filenames that overflow filesystem limits 😭 this option 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 that have a filesystem limit.
okay so real talk: gcov should be run with the current directory the same as the one you had 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 info of the source file they correspond to. one .gcov file is produced for each source (or header) file containing code that was compiled to produce the data files. the mangledname part of the output file name is usually just the source file name, but it can be something more complicated if you pass -l or -p — go check those options for the details, i covered them above 🫡
if you invoke gcov with multiple input files, the contributions from each input file get summed. typically you'd invoke it with the same list of files as the final link of your executable. makes sense, bestie.
the .gcov files contain the : separated fields along with the program source code. the format is:
<execution_count>:<line_number>:<source line text>additional block info may succeed each line when requested by a command line option. the execution_count is - for lines containing no code. unexecuted lines are marked ##### or =====, depending on whether they're 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 that have a statement with zero execution_count end with a * character and get colored magenta with the -k option 💜
heads up: this functionality is not supported in Ada. 🙈
so like, GCC can completely remove the bodies of functions that aren't needed — for instance if they're inlined everywhere. those functions get marked with -, which is genuinely confusing ngl. use the -fkeep-inline-functions and -fkeep-static-functions options to retain these functions and let gcov properly show their execution_count ✨
some lines of info at the start have a line_number of zero. these preamble lines look like:
-:0:<tag>:<value>the ordering and number of these preamble lines will be augmented as gcov development progresses — so do not rely on them staying unchanged, fr fr. use tag to locate a particular preamble line 🔍
the additional block info is of the form:
<tag> <information>that information is human readable, but designed to be simple enough for machine parsing too. best of both worlds, we love to see it 💖
when it's printing percentages, 0% and 100% are only printed when the values are exactly 0% and 100% respectively. other values that would conventionally get rounded to 0% or 100% are instead printed as the nearest non-boundary value. that's just how it is 🎀
Okay bestie, so before any of this magic happens, you gotta compile your program with a special GCC option: --coverage. That's the whole entry fee 💅. What it does is tell the compiler to generate extra info that gcov needs (lowkey it's just a flow graph of your program), and it also stuffs extra code into the object files so the profiling info can actually get generated later. Those extra files? They land in whatever directory your object file is chilling in. 📁
Then you just run your program, and boom — profile output happens. For every source file compiled with -fprofile-arcs, a matching .gcda file gets dropped into the object file directory. ✨
Now run gcov with your program's source file names as arguments and it'll hand you a listing of the code plus how many times each line ran. Fr fr. 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 is where gcov put all its output. Sample time, bestie — here's what's inside:
-: 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:}So notice how line 7 shows up in the report multiple times? Not a glitch, bestie 👑. The first occurrence gives you the total number of times that line executed, and the next two belong to the instances of the class Foo constructors. And as you can also see, line 30 has some unexecuted basic blocks, so its execution count gets that asterisk symbol. ⭐
Then 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:}In this mode, each basic block only shows up on one line — the last line of the block. So 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 first, and then 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 for that block get shown — but only if you gave the -b option. 🫶
Because of the way GCC instruments calls, a call count can show up after a line that has no individual blocks. And yeah — like you can see, line 33 contains a basic block that never executed. 💀
Now 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:}And for each function, there's a line printed showing how many times the function was called, how many times it returned, and what percentage of that function's blocks were actually executed. Slay. ✨
okay so for every basic block, there's a line printed right after the last line of that block, and it tells you about the branch or call that ends it 💅. and listen—if multiple basic blocks end on the same source line, you can get multiple branches and calls listed for that one line. in that case they each get a number. ngl there's no simple way to map these numbered branches/calls back to source constructs, but generally the lowest-numbered branch or call corresponds to the leftmost construct on that 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 it was executed. otherwise you get the message "never executed". periodt 💀
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. this will usually be 100%, but it may be less for functions that call "exit" or "longjmp", because those might not return every time they're called. lowkey important 🙈
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 are 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. in "a || (b && c)", a is condition 0, b is condition 1, and c is condition 2. slay 🌸
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 cannot affect the decision independently—both a and b must change value for the decision to change. no cap 😭
$ 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 useful to figure out roughly where coverage is missing and for testing how different inputs change the coverage 🔥. the --prime-paths-source is a useful tool for understanding paths. bestie, look:
$ 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 will print 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 useful for understanding the paths, in particular those that are hard to cover or even unreachable. lines may be repeated—for example the "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. this output is quite verbose, and for focusing on specific functions it can be combined with the filters --include and --exclude. iconic 🌈
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 is expanded using the lines and decisions like --prime-paths-source but printed on a single line 💅. this mode provides a good overview over the paths and for tracking how different tests and inputs exercise the code. chef's kiss 👑
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. literally so handy ✨
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. no cap 🔥
you can report on a subset of functions by using --include and --exclude. this is very useful when combined with --stdout 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, which you can control 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. 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++; }okay so real talk about those --include and --exclude filters, bestie — they take extended regular expressions (yes, like grep -E), which means the pattern "in.?" matches both "inc" and "main". 💅 and if you use them with -M, then all "int" instantiations of "Foo" match too. no cap.
You can use --include and --exclude multiple times, and if a name matches multiple filters, the last one to match is the one that takes preference. periodt. So if you wanna match "main" and the "int" instantiation of "inc", while omitting the "Foo" constructor, it's giving:
$ 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 🌈
If you're planning to use gcov to help optimize your code, first things first: compile your program with the special GCC option --coverage. ✨ Aside from that, you can use any other GCC options. But — and this is the important part — 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 error occurs, set GCOV_EXIT_AT_ERROR environment variable. That can help users to find profile clashing which leads to a misleading profile. 🕵️♀️
👀 SEE ALSO 👀
go check out gpl(7), gfdl(7), fsf-funding(7), gcc(1), and the Info entry for gcc. 💖
📜 COPYRIGHT 📜
Copyright (c) 1996-2026 Free Software Foundation, Inc.
okay legal besties, here's the whole thing, no skipping:
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 on GCOV(1), bestie — this page is from gcc-16, dated 2026-08-07. slay. 💅✨
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