Missing C23 functions in <tgmath.h> on Linux

[hjl-tools]

On Linux, glibc provides C23 functions in <tgmath.h>. Cang doesn't use glibc <tgmath.h> and <tgmath.h> from clang
doesn't have all newly added C23 functions in glibc. As the result, not all glibc supported C23 functions are available
when clang is used on Linux.

[KHariHaraChaitanya]

@MaskRay I am interested to take this issue, I am new to LLVM. It would be great if you can guide me through the requirements. Thanks.

[hjl-tools]

Ideally, Clang should use <tgmath.h> provided by glibc, instead of its own <tgmath.h>, on Linux.

[aabhinavg1]

Hi @hjl-tools , why should clang use the to math.h provided by the glibc?
The following things come to mind. Correct me if I'm wrong.

1. Consistency with STL: glibc is maintained and tested in conjunction with Linux Standard Libraries
2. Better Compatibility : The version of to math that glibc has is designed to work seamlessly with another glibc component
3. Performance Optimisation: Instead of potentially missing out on optimisation.
4. Last but not least Avoid duplicate efforts

[hjl-tools]

Unless Clang uses its own libm, glibc's <tgmath.h> is needed to access to ALL functions in glibc's libm, especially these
functions are added to glibc after the clang binary was built.

[AngryLoki]

Do you mean LLVM libc? If yes, here is the list of C23 functions with their status: https://libc.llvm.org/c23.html#:~:text=strndup-,tgmath.h,-acospi

[aabhinavg1]

Hi @KHariHaraChaitanya you can try to implement similar patch inside the llvm-project/clang/lib/Headers/lgmath.h

#if defined(__APPLE__) && __STDC_HOSTED__
  // macOS: Ensure the system's tgmath.h is included
  #if __has_include_next(<tgmath.h>)
    // Only include the system's tgmath.h once
    #if !defined(__CLANG_TGMATH_OVERRIDE__)
      #define __CLANG_TGMATH_OVERRIDE__
      #include_next <tgmath.h>  // This will include the macOS system version of tgmath.h
    #endif
  #endif
#elif defined(__GLIBC__) && !defined(_LIBCPP_HAS_NO_GLIBC_TGMATH)
  // Linux with glibc: Prefer the system's tgmath.h (from glibc)
  #include_next <tgmath.h>  // This should include the glibc version of tgmath.h
#else
  // For other systems, include Clang's own tgmath.h
  #include <tgmath.h>  // Default to Clang's version for non-Apple and non-GLIBC systems

Also test in on the linux distribution
You can use the following command

 git:(main) ✗ bin/clang -target x86_64-unknown-linux-gnu -std=c2x -E -v -x c -I/home/compilersutra/llvm/glibc-2.37/new_build/include ../../test/test_23.c -lm | grep tgmath

output
bash

clang version 20.0.0git (https://github.com/llvm/llvm-project.git 56e944bede9654127cc210506a6cccdd43cd96e7)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/compilersutra/llvm/llvm-project/build/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Candidate multilib: .;@m64
Selected multilib: .;@m64
clang: warning: -lm: 'linker' input unused [-Wunused-command-line-argument]
 (in-process)
 "/home/compilersutra/llvm/llvm-project/build/bin/clang-20" -cc1 -triple x86_64-unknown-linux-gnu -E -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name test_23.c -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home/compilersutra/llvm/llvm-project/build -v -fcoverage-compilation-dir=/home/compilersutra/llvm/llvm-project/build -resource-dir /home/compilersutra/llvm/llvm-project/build/lib/clang/20 -I /home/compilersutra/llvm/glibc-2.37/new_build/include -internal-isystem /home/compilersutra/llvm/llvm-project/build/lib/clang/20/include -internal-isystem /usr/local/include -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -std=c2x -ferror-limit 19 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcolor-diagnostics -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o - -x c ../../test/test_23.c
clang -cc1 version 20.0.0git based upon LLVM 20.0.0git default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/include"
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /home/compilersutra/llvm/glibc-2.37/new_build/include
 /home/compilersutra/llvm/llvm-project/build/lib/clang/20/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
# 1 "/home/compilersutra/llvm/glibc-2.37/new_build/include/tgmath.h" 1
# 26 "/home/compilersutra/llvm/glibc-2.37/new_build/include/tgmath.h"
# 27 "/home/compilersutra/llvm/glibc-2.37/new_build/include/tgmath.h" 2
# 31 "/home/compilersutra/llvm/glibc-2.37/new_build/include/tgmath.h" 2
In file included from ../../test/test_23.c:2:
/home/compilersutra/llvm/glibc-2.37/new_build/include/tgmath.h
# 32 "/home/compilersutra/llvm/glibc-2.37/new_build/include/tgmath.h" 2

Changes

1. Conditional inclusion of the header file <tgmath.h>:

   • On Macos (Apple's) prioritizing the <tgmath.h>.
   • modify __GLIBC__ to ensure compatibility with glibc's <tgmath.h>.
   • Fallback to default Clang's if the system doesn't have system version <tgmath.h> .

2. Generating the warnings to highlight which version is being used.
   @KHariHaraChaitanya you can do the testing on

Testing

• Test on macOS, Linux (glibc), and other Linux distributions without glibc.
• Verified that the patch does not affect existing functionality and resolves the reported issue.

Hi @MaskRay Could you suggest what test we can do here for the following patch before releasing the patch for the review.