gcno

gcno files are part of GCC’s coverage instrumentation workflow used by gcov. They store the static structure of the program for a source file, including the control-flow graph (CFG) with basic blocks and edges and their mapping to source lines. Each source file compiled with coverage options typically yields one corresponding .gcno file. The format is binary and intended for tool consumption rather than manual viewing.

gcno and gcda together enable coverage analysis. The .gcno file is created at compile time, while a .gcda file is produced when the instrumented program runs; the .gcda contains runtime execution counts for the CFG edges and blocks. After running the instrumented program, tools like gcov or gcovr read both files to generate a human-readable coverage report, showing which lines and branches were executed.

Typical workflow: compile with -fprofile-arcs -ftest-coverage (or -coverage) for the source file, run the program to generate .gcda data, and then invoke gcov (or an equivalent tool) to produce a report such as a .gcov text file or an HTML/summary view. The presence and consistency of the .gcno and .gcda files are essential for accurate coverage results; recompilation or source changes can require regenerating the .gcno files.

Notes: .gcno files are architecture- and compiler-version dependent and are not meant to be edited. They reside alongside the corresponding source and object files in the build directory. In practice, coverage analysis unions data from all relevant translation units to produce a complete picture of program coverage.

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