Re: C11 / VS 2019 - Mailing list pgsql-hackers

On Fri, 18 Jul 2025 at 23:12, Peter Eisentraut <peter@eisentraut.org> wrote:
> Note that gcc and clang switched to C11 by default a long time ago
> (gcc-5 and clang-3.6), so for most users all these tests won't need to
> do anything.  If you want to test it, you could simulate an older
> default like
>
> ./configure CC='gcc -std=c99'
>
> and then the test should decide that it needs to add another option to
> override the C mode.

This isn't quite my area of expertise, but I've been reviewing the
patch and testing. I don't see any issues with VS2022.

On Linux, doing a bit more testing using meson and clang, I tried with
CC=clang CFLAGS=-std=c99 . I get:

Checking if "C11" compiles: NO
Checking if "C11 with -std=gnu11" compiles: YES

But later:

FAILED: src/backend/parser/parser.a.p/meson-generated_.._gram.c.o
clang -Isrc/backend/parser/parser.a.p -Isrc/backend/parser
-I../src/backend/parser -Isrc/include -I../src/include
-I/usr/include/libxml2 -fdiagnostics-color=always
-D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -O2 -g -std=gnu11
-fno-strict-aliasing -fwrapv -fexcess-precision=standard -D_GNU_SOURCE
-Wmissing-prototypes -Wpointer-arith -Werror=vla
-Werror=unguarded-availability-new -Wendif-labels
-Wmissing-format-attribute -Wcast-function-type -Wformat-security
-Wdeclaration-after-statement -Wmissing-variable-declarations
-Wno-unused-command-line-argument -Wno-compound-token-split-by-macro
-Wno-format-truncation -Wno-cast-function-type-strict -std=c99 -fPIC
-pthread -DBUILDING_DLL -MD -MQ
src/backend/parser/parser.a.p/meson-generated_.._gram.c.o -MF
src/backend/parser/parser.a.p/meson-generated_.._gram.c.o.d -o
src/backend/parser/parser.a.p/meson-generated_.._gram.c.o -c
src/backend/parser/gram.c
../src/backend/parser/gram.y:19362:12: error: call to undeclared
function 'typeof'; ISO C99 and later do not support implicit function
declarations [-Wimplicit-function-declaration]
 19362 |                 result = copyObject(p->argType);

It's not quite clear to me why the C11 test passes but then the build
fails. I assumed the mixed -std=gnu11 then -std=c99 are passed in the
test as well. I don't know how to check.

Strangely, if I swap the order of the c11_test_args so the test tries
and uses -std=c11 first, it works ok.

@@ -558,7 +558,7 @@ if not cc.compiles(c11_test, name: 'C11')
   if cc.get_id() == 'msvc'
     c11_test_args = ['/std:c11']
   else
-    c11_test_args = ['-std=gnu11', '-std=c11']
+    c11_test_args = ['-std=c11', '-std=gnu11']
   endif

Perhaps this isn't an issue. I just did this to try to test what would
happen in a compiler that defaulted to c99. I'm unsure how clang
processes conflicting compiler flags.

David