C extensions, portability, and alternative compilers

One of my pet-peeves with C projects is that it's so often more or less "works on my machine" when written by Linux users (as a Windows and FreeBSD user it often hits you on both those platforms).

The article highlights a typical piece:

  #if !(defined __GNUC__ || defined __clang__ || defined __TINYC__)
  # define __attribute__(xyz)     /* Ignore */
  #endif

There is no reason that !defined check to not include a check for __attribute__ already being defined (a custom compiler author could then force an define for __attribute__ that translates to an internal __mycompiler__attribute__ replacement by default).

But outside of that, just trying to compile on FreeBSD you often run into systemd dependencies or other non-posix behaviors (Not to mention on Windows but I'm not here to bring on flamewars so I'll leave that part).

Yes, when I implemented ImportC (a C compiler built in to the D compiler), I had to spend a lot of time finding ways to work with all the nutburger nonsense in the various .h files.

https://github.com/dlang/dmd/blob/master/druntime/src/import...

https://github.com/dlang/dmd/blob/master/druntime/src/__impo...

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)

Why would they not do something like?

  + #if !(defined __GLIBC_COMPILER_SUPPORTS_ATTRIBUTES__)
  - #if !(defined __GNUC__ || defined __clang__ || defined __TINYC__)
  # define __attribute__(xyz)     /* Ignore */
  #endif

(or probably a more fine grained for each attribute they try to use)

Considering such checks are fairly conventional in downstream C++ libraries based on compilers (for example checking OS platform or compiler, e.g. [Boost.Config](https://www.boost.org/doc/libs/latest/libs/config/). Modern C++ even went ahead and standardized this somewhat https://en.cppreference.com/cpp/utility/feature_test )

I just implemented a fast small compiler rcc to compete against tcc, and these glibc header quirks were simply fixed in the same way clang does it. By defining all gcc predefines and implementing all gcc extensions. Needed a day. And my headers are clean compared to glibc. The others in this league are slimcc, kefir and cproc. No other compilers can parse glibc headers. tcc has this special exception.

What is the feasible way to test code against the matrix of compilers/oses?