Coverage Measurement and Profiling

“It is easier to optimize correct code, than correct optimized code.”—Yves Deville
Using gprof for Profiling

Like gcov, gprof requires that your programs be compiled with particular options so GCC can instrument your code properly. To use gprof, you must use the -pg option. By default, this profiles each function executed. To profile line by line, also use the standard -g debugging option to insert debugging symbols that gprof can use to create line-by-line reports.

When your program is run, information about function calls and time spent is kept in memory. At program exit, this information is written to a file called gmon.out. gprof uses this file to create profiling reports. Unlike coverage monitoring, which saves data in separate files for each source file, one data file is created for the entire program. Instead of saving data in the same directory as the source file, gmon.out is written to the correct working directory at the time the program exits. So beware of chdir() calls.

By default, the profiling data counts the number of times each function is called, along with the function from which it was called. Runtime data is collected by a sampling process, where the program is examined every sample period, and the function currently executing is recorded. gprof interprets this sampling to report how much time was spent on each area of the code.

Because this is a statistical sampling, it is subject to statistical error. See the gprof manual for a detailed discussion of the error calculation. In short, the times reported by gprof may not be exact. Furthermore, if a function is executed for a relatively small amount of time, it may be missed during the sampling and show 0.0 execution time. This is not a problem for performance profiling. The information that a particular function or block of code did not run for enough time to be sampled tells you that you probably don't want to spend much time optimizing it.

Statistical errors are a problem, however, if you also want to use gprof to show statement or branch coverage in addition to performance data. For exact counts of how many times each line of code has been executed, enable basic block counting by using the -a option at compile time. This adds instrumentation to the code to count the execution of each block of code. It also allows gprof to show exactly how many times each line of code was executed, even if zero time is shown by way of sampling.

It is possible to combine multiple gmon.out files in order to build up measurements over many runs of a program. This is done with the gprof -s command, which takes any number of gmon.out files and creates a new gmon.sum with the combined data. It is possible to add profiling data incrementally by specifying gmon.sum as an input file.

I originally intended to reuse my glib example with gprof, but the different behavior of gprof and gcov made this prospect difficult. The glib test programs are run one after another by the make check command, so each test's gmon.out was overwritten by the next test. Also, the test programs actually are shell scripts, which set some reasonably complex environment variables to run the real test programs, which are kept in a hidden subdirectory. Moral of the story: gcov is the right tool for doing coverage testing for glib.

Instead, I created a simple program to calculate the Fibonacci numbers from zero to 42. The Fibonacci numbers are a series of numbers beginning with 0,1 where the next number in the series is found by adding the previous two numbers (so, 0,1,1,2,3,5,8,13,21,...) A good Web site on the Fibonacci numbers and related mathematical concepts can be found at The number 42 was chosen, not merely for its literary significance to Douglas Adams fans, but because the Fibonnacci numbers begin overflowing signed 32-bit integer arithmetic at 47, so a lower limit was useful.

The calculation of a Fibonacci number can make a good example of a recursive function. Because each Fibonacci number is the sum of the previous two, the simplest approach is to create a function int fibonacci(int n), which simply calls itself with n - 1 and with n - 2, adds the results and returns the value (Listing 3).



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Is there any coverage tool

Neeraj's picture

Is there any coverage tool available that only works on source files and not on executables. I mean I dont want to compile the source files even once still I want to find out the coverage in the code.
Is it possible?

Please help. I'll really appreciate your effort if you get me any information regarding the matter.


help needed in installing ggcov

savita's picture

We have tried installing ggcov

we had followed following steps.

./configure (it satisfies our pc configuration)

make install

It is not getting installed properly...
How can we check it is properly installed or not....?
Is there any other steps to covered...

please kindly help


hai, i understood your

Raja's picture

i understood your problem.....
me also struck with that problem but what i did was,
i just installed ggcov-06 instead of ggcov-08.....
in ggcov ,it is asking someother file which we could not get fro internet...
so better to go to install ggcov-06
thanks and regards,


Anonymous's picture

when i try to run ggcov in SUSE LINUX 10.1 using the command line ggcov ,i do not get any error message.but the results are not displayed.
the installation had no problems.
If anybody who is familiar with ggcov can give me a detailed idea about how to use this tool,please reply.


Anonymous's picture

anybody kindly post a reply regarding how to implement ggcov

I could run ggcov successfully on the command prompt but I can't view any GUI.So please tell me the syntax/procedures of using ggcov.My linux version is SUSE 9.0

error on running kprof

lalitha's picture

when i try to make the kprof i am getting this errors..
Kindly help me to correct and run the application

parseprofile_pose.h:30: error: expected `)' before â&â token
parseprofile_pose.cpp:29: error: prototype for âCParseProfile_pose::CParseProfile_pose(QTextStream&, QPtrVector&)â does not match any in class âCParseProfile_poseâ
parseprofile_pose.h:28: error: candidates are: CParseProfile_pose::CParseProfile_pose(const CParseProfile_pose&)
parseprofile_pose.h:36: error: CParseProfile_pose::CParseProfile_pose()

need help for GCOV

Anonymous's picture

I have a problem:
I'm not able to gcc -fprofile-arcs -ftest-coverage file.c
Could you let me know how I can set environemt CFLAGS variable?
I have problem when I gcc on glib files i.e garray.c

please hel--

valgrind coverage

John Pye's picture

I thought I'd add a comment to say that some people are using Valgrind to do coverage testing using the 'cachegrind' tool.

Great article, a nice clear introduction to the topic.


Anonymous's picture

Thanks very much for this informative example. I was surprised that 50% is about average for code coverage!

Re: Coverage Measurement and Profiling

Anonymous's picture

Very nice article, I enjoyed it and learned new (for me) methods

thanks a lot

Don't forget lcov!

Anonymous's picture

lcov is a nice addition to gcof; like ggcov, it shows summaries
for multiple files. It goes beyond ggcov, I think, in that it
handles entire directory hierarchies. It can handle the Linux
kernel and Wine, for instance. It outputs HTML rather than
presenting a GUI, but that's kind of a plus in my book.
It's at

Re: Don't forget lcov!

ZachFrey's picture

Lcov does look very nice. My intent wasn't to do a comprehensive survey of tools, but to introduce the basics of gcov and gprof and give an example of a representative "add-on" for each.

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