lws can be built with gcov-format coverage instrumentation, with gcc or clang, and a script summarizes what the ctest run actually executed.

Locally

cmake .. -DLWS_WITH_GCOV=1 -DLWS_WITH_MINIMAL_EXAMPLES=1 <other options>
make -j8
ctest -j8
make coverage

-DLWS_WITH_GCOV=1 adds -fprofile-arcs -ftest-coverage to every object in the tree. Each test process writes its counters (.gcda files next to the objects) when it exits normally; ctest just runs the tests as usual. Tests that are killed, or time out, flush nothing and so contribute nothing.

make coverage runs scripts/coverage-report.py, which drives gcov (or llvm-cov gcov for a clang-built tree) over every .gcno in the build tree, merges the results per source file and prints, for lib/:

overall line and function coverage
a per-directory table, worst first
every function that was compiled but has under 50% line coverage, worst first, with the file and line it starts on

Counters accumulate across runs until the .gcda files are deleted (find . -name '*.gcda' -delete in the build tree), so a report after running a single test shows only what that test reached.

The script runs from the build directory and takes the compiler and the source directory from its CMakeCache.txt. Options:

--include PREFIX   count sources under this prefix instead of lib/ (repeatable)
--threshold PCT    list functions below this line coverage (default 50)
--limit N          list at most N functions (default: all)
--json             also write the totals and the function list to
                   coverage-summary.json in the build directory

LWS_GCOV="llvm-cov gcov" in the environment overrides the reader matched to the compiler.

It only reports. It exits 0 even when it cannot find a reader or read some of the data files, and says so in the output.

In Sai

The coverage configuration in .sai.json builds distro_recommended instrumented on the rocky9-ca/aarch64-a72a55-rk3588/clang platform, with fault injection, http stream compression (deflate and brotli) and ws permessage-deflate forced on so their tests are in the run, then runs ctest and prints the report into the job log; build/coverage-summary.json is kept as an artifact. A clang-built tree needs llvm-cov on the builder to read the counters; without it the report says so and the job is not affected. The builder also needs the brotli development package.

lws-api-test-http-compression is the transfer check for the compression paths: it serves a generated multi-megabyte text file named after its own sha256 and fetches it back over h1 and h2c with each content-encoding, decoding client-side and checking the digest against the name.