GCOV: why sample.gcda and sample.gcno may be different - gcov

Question

At first I take the message sample.gcda:stamp mismatch with graph file
the order of compilation and running is observed
hexdump -e '"%x\n"' -s8 -n4 sample.gcno -> aaa1aaaa
hexdump -e '"%x\n"' -s8 -n4 sample.gcda -> bbb2bbbb

Answer 1

stamp mismatch with graph file
Means that graph file has been compiled again after binaries built.
If the compilation order is correct, you could try to check if there is a compilation of the sample.cpp twice somewhere in building rules.
For example we have something like that:
g++ ... sample.cpp -o sample
g++ ... -shared sample.cpp -o sample2.o
So one file is compiled twice. It will cause that gcno file will be updated by new timestamp that will not match to gcda file anymore.

Answer 2

If you performed your product or application testing thoroughly and manually and spent lot of effort on it. If your objective is to get code coverage report using lcov and gcov but by mistake deleted gcno files. You can regenerate gcno files by recompiling the code but it will be generated with new timestamp and gcov reports error saying "stamp mismatch with graph file" and no code coverage report will be generated. This will result in all your testing effort getting wasted.
There is a shortcut to still generate the code coverage report. This is just a workaround and should not be relied upon all the time. Its recommended to preserve *.gcno files till your testing completes.
Note down your gcc version(gcc -v) and download its source code from one of the mirror sites
Eg - ftp://gd.tuwien.ac.at/gnu/sourceware/gcc/releases/gcc-4.4.6/gcc-4.4.6.tar.bz2
After extracting downloaded file, gcc the folder structure will be as follows
gcc-4.4.6
gcc-4.4.6/gcc
If you directly go inside gcc-4.4.6/gcc and try to do ./configure and compile(make) from there then you will encounter below problem
build/genmodes -h > tmp-modes.h
/bin/sh: build/genmodes: No such file or directory
Solution is do ./configure and make from gcc-4.4.6 and no errors will be shown related to genmodes. This will compile all modules including gcc. You may have to install mpfr and gmp modules which are needed by gcc if any error shown by ./configure
goto gcc-4.4.6/gcc/gcov.c and comment below lines and then recompile with above command
/* if (tag != bbg_stamp)
{
fnotice (stderr, "%s:stamp mismatch with graph file\n", da_file_name);
goto cleanup;
}*/
Example path of new gcov binary after compilation is gcc-4.4.6/host-x86_64-unknown-linux-gnu/gcc/gcov
Place this binary in /usr/bin and regenerate code coverage report with command as shown in below example
lcov --capture --directory ./ --output-file coverage.info ; genhtml coverage.info --output-directory /var/www/html/coverage
Now you should not get "stamp mismatch with graph file" error and you will get code coverage report properly