Thread

1. Validate user-supplied c_args in meson builds

   Bertrand Drouvot <bertranddrouvot.pg@gmail.com> — 2026-03-18T10:01:53Z

   Hi hackers,
   
   While using an invalid c_args by mistake with clang and meson, I observed the
   following:
   
   $ CC="clang" meson setup meson_build -Dc_args="-Wbad"
   
   produces:
   
   meson.build:645:4: ERROR: Problem encountered: C compiler does not support C11
   
   This is misleading: my C compiler does support C11 but the C11 check fails
   due to the invalid arg, as stated in meson-log.txt:
   
   "
   stderr:
   error: unknown warning option '-Wbad' [-Werror,-Wunknown-warning-option]
   "
   
   OTOH, providing an invalid CFLAG with autoconf and using clang currently produces
   warnings like:
   
   "
   warning: unknown warning option '-Wbad' [-Wunknown-warning-option]
   "
   
   That's perfectly fine and not misleading.
   
   If using gcc instead of clang, then:
   
   1/ with autoconf, we get:
   
   "
   checking whether the C compiler works... no
   configure: error: in `/home/postgres/postgresql/postgres':
   configure: error: C compiler cannot create executables
   See `config.log' for more details
   "
   
   and in config.log:
   
   "
   configure:4028: checking whether the C compiler works
   configure:4050: gcc -O0 -Wbad   conftest.c  >&5
   gcc: error: unrecognized command-line option '-Wbad'"
   
   That's not misleading.
   
   2/ with meson, we get:
   
   "
   $ CC="gcc" meson setup meson_build -Dc_args="-Wbad"
   
   meson.build:9:0: ERROR: Compiler gcc cannot compile programs.
   "
   
   That's not misleading.
   
   So it looks like that GCC treats an invalid CFLAG as an error by itself, while
   clang only treats it as a warning (unless -Werror=unknown-warning-option is present).
   
   Also, it looks like that when using clang, meson injects -Werror=unknown-warning-option
   into cc.compiles().
   
   This can be confirmed by creating a simple meson.build as:
   
   "
   project('test', 'c')
   cc = meson.get_compiler('c')
   cc.compiles('int main(void){return 0;}', name: 'test probe')
   "
   
   and running the compilation with a valid arg:
   
   $ CC=gcc meson setup testbuild  -Dc_args="-Wunused-value"
   
   and check:
   
   $ grep -c unknown-warning-option testbuild/meson-logs/meson-log.txt
   0
   
   While (with clang):
   
   $ CC=clang meson setup testbuild  -Dc_args="-Wunused-value"
   $ grep -c unknown-warning-option testbuild/meson-logs/meson-log.txt
   1
   
   So, PFA, a patch that adds a cc.has_multi_arguments() check before the C11 test so
   that :
   
   CC="clang" meson setup meson_build -Dc_args="-Wbad"
   
   now produces:
   
   "
   Compiler for C supports arguments -Wbad: NO
   
   meson.build:625:2: ERROR: Problem encountered: One or more c_args are not supported by clang 21.0.0
   "
   
   and does not report wrongly that the C compiler does not support C11 when using
   clang with meson (and an invalid c_args).
   
   Regards,
   
   -- 
   Bertrand Drouvot
   PostgreSQL Contributors Team
   RDS Open Source Databases
   Amazon Web Services: https://aws.amazon.com
   

2. Re: Validate user-supplied c_args in meson builds

   Tristan Partin <tristan@partin.io> — 2026-04-15T21:36:35Z

   I actually think this is a bug in Meson. I created a PR[0] to propose 
   a fix, but one person's bug is another person's feature. The actual 
   content of the patch seems fine to me, but I would wait to see the 
   result of the pull request fist.
   
   [0]: https://github.com/mesonbuild/meson/pull/15714
   
   -- 
   Tristan Partin
   PostgreSQL Contributors Team
   AWS (https://aws.amazon.com)