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gcov uses two files for profiling. The names of these files are derived from the original object file by substituting the file suffix with either .gcno, or .gcda. All of these files are placed in the same directory as the object file, and contain data stored in a platform-independent format.
The .gcno file is generated when the source file is compiled with the GCC -ftest-coverage option. It contains information to reconstruct the basic block graphs and assign source line numbers to blocks.
The .gcda file is generated when a program containing object files built with the GCC -fprofile-arcs option is executed. A separate .gcda file is created for each object file compiled with this option. It contains arc transition counts, and some summary information.
The full details of the file format is specified in gcov-io.h, and functions provided in that header file should be used to access the coverage files.