For those who are making indie C compilers that don't pretend to be __GNUC__ but want to compile real world projects, slimcc's test script[1] and platform header hacks[2] might save you some time. [1] https://github.com/fuhsnn/slimcc/blob/main/scripts/linux_thi...

[2] https://github.com/fuhsnn/slimcc/blob/main/slimcc_headers/pl...

Some more fun stories:

- Game projects default to using SIMD so for example SDL and STB you always need to pass -DSDL_DISABLE_IMMINTRIN_H and -DSTB_NO_SIMD

- math.h's NAN usually fall back to (0.0f / 0.0f), which will print "-nan" with printf, some projects test suite fail because of it (they expected "nan").

- NetBSD's sys/cdefs.h straight up #error's if you don't pretend to be GCC or PCC.

- Some projects can't compile without __attribute__((always_inline)) because they use it on non-static functions.

- Many projects probe -fvisibility in the build system and pass -fvisibility=hidden to compile, but in the headers they gate __attribute__((visibility(default))) behind __GNUC__ checks, so you'll get missing symbols.

- Some projects use if(0) { undefined_function() } to fake static_assert(), there is even a bug report from QEMU to Clang because it failed to optimize in -O0 a certain `if` written this way.

- Even if you define __STDC_NO_VLA__, projects might fall back to alloca() code path that's untested and broken (python and jemalloc both had this problem, already reported)

- Valkey has broken __builtin_ctzll fallback nobody noticed (reported).

- Zig's C bootstrap path expects the compiler to have GCC/Clang-tier optimization and stack overflows if you don't (reported).

- I contributed stdatomic.h code path for Ruby just to compile it with slimcc, pretty sure it's still the only user of the code path.

- I implemented __has_extension in the hope that projects can use it to query gnu_asm; but SQLite broke because they use __has_extension(c_atomic) to query GNU atomics builtin, but c_atomic actually is meant for C11 _Atomic (IMO they should use __has_builtin)