Recently I discovered test-driven development and sincerely, I felt like if I had discovered the philosopher stone. Something very useful when approaching development this way are coverage tests in order to check which parts of your code are actually being tested and which are just being skipped.

For this task, one of the available open source options is gcov, that basically collects coverage data from an application execution (ideally a test).

Generating coverage data

First of all, we need to add the --coverage flag to the compiler and the linker so that gcc can generate information for coverage checking ¹.

Once we have our test compiled with these flags, .gcno files will be generated. Then, when we run the test and the test execution ends cleannly, a another set of .gcda files will be generated ².

Reading coverage data

With .gcno and .gcda files generated, we have all the needed information to check for coverage of the executed test. To make a quick check on how much of our code is covered:

gcov cppfile.gcda

This way we get a quick resume of coverage. If we open the generated cppfile.cpp.gcov file, we can see how many times each line was executed. This is specially useful to know which lines have not been executed and hence, improve our test to force those parts. Also, when a line has not been executed, we will see a ##### symbol at the start of the line.

Visualizing coverage data

So now we have generated all needed information, but we can still achieve a better presentation for it. For this task, a few tools are available like lcov and gcovr.

Here, I will show lcov since it is capable of generating a very nice HTML output with all the needed information about the coverage. To use lcov in order to generate a full report:

lcov --base-directory . --directory . --capture --output-file lcovdata.info

• --base-directory: is the directory to use as base for relative paths
• --directory: is the directory containing the .gcda files
• --capture: tells lcov to capture coverage data from previously generated files
• --output-file: the output file with the necessary information for lcov to generate a report

Please note that there is no need to run gcov manually if we are using lcov, basically because lcov runs gcov internally for us. Once we have the lcovdata.info file, we can generate the report.

genhtml -o /tmp/outputfolder lcovdata.info

And finally we can open the generated HTML report, where all the info about coverage is present in a nicer than plain text format.

Hope it helped!

External references

• Michael Stapelberg’s blog, where all of this is much better explained.

• GCC’s official documentation

Footnotes