CMake: CMAKE_C_EXTENSIONS=ON seems mandatory (with clang)

[Mizux]

TLDR: It seems the SCIP source depends on M_PI which is a GNU extension, i.e. compiling using -std=c99 won't work.

Protocol

If here:

scip/src/CMakeLists.txt

Lines 10 to 12 in a6142b7

	set(CMAKE_C_STANDARD 99)
	set(CMAKE_C_STANDARD_REQUIRED on)

I'm adding the line

set(CMAKE_C_EXTENSIONS OFF)

Then CMake will generate compile line using -std=c99 instead of the "default" -std=gnu99

ref: https://cmake.org/cmake/help/latest/variable/CMAKE_CXX_EXTENSIONS.html
note: You can also pass it on CMake configure line or could be given by a super project integrating SCIP from sources...
note2: Usually CMake default enable extensions seems to have change 17months ago see
Kitware/CMake@f034b0f

Observed

note: I'm doing some integration tests of multithread SCIP so technically I building SCIP using through FetchContent()
full project here: https://github.com/Mizux/scip-multithread

cmake --build build -v
...
[ 17%] Building C object _deps/scip-build/src/CMakeFiles/libscip.dir/scip/cons_soc.c.o
cd /home/mizux/dev/scip-multithread/build/_deps/scip-build/src && /usr/bin/clang
 -I/home/mizux/dev/scip-multithread/build/_deps/scip-build
 -I/home/mizux/dev/scip-multithread/build/_deps/scip-src/src
 -I/home/mizux/dev/scip-multithread/build/_deps/zlib-build
 -I/home/mizux/dev/scip-multithread/build/_deps/zlib-src
 -fsanitize=thread -g -fPIC -fvisibility=hidden -std=c99 # SEE HERE the use of -std=c99
 -MD -MT
 _deps/scip-build/src/CMakeFiles/libscip.dir/scip/cons_soc.c.o
 -MF CMakeFiles/libscip.dir/scip/cons_soc.c.o.d
 -o CMakeFiles/libscip.dir/scip/cons_soc.c.o
 -c /home/mizux/dev/scip-multithread/build/_deps/scip-src/src/scip/cons_soc.c
/home/mizux/dev/scip-multithread/build/_deps/scip-src/src/scip/cons_soc.c:1996:10: error: use of undeclared identifier 'M_PI'
   val = M_PI;
         ^
/home/mizux/dev/scip-multithread/build/_deps/scip-src/src/scip/cons_soc.c:2238:13: error: use of undeclared identifier 'M_PI'
      val = M_PI / pow(2.0, (double) (i+1));
            ^
/home/mizux/dev/scip-multithread/build/_deps/scip-src/src/scip/cons_soc.c:2313:19: error: use of undeclared identifier 'M_PI'
   vals[0] = tan( M_PI / pow(2.0, (double) (N+1)) );
                  ^
3 errors generated.
make[2]: *** [_deps/scip-build/src/CMakeFiles/libscip.dir/build.make:776: _deps/scip-build/src/CMakeFiles/libscip.dir/scip/cons_soc.c.o] Error 1
make[2]: Leaving directory '/home/mizux/dev/scip-multithread/build'
make[1]: *** [CMakeFiles/Makefile2:1057: _deps/scip-build/src/CMakeFiles/libscip.dir/all] Error 2
make[1]: Leaving directory '/home/mizux/dev/scip-multithread/build'
make: *** [Makefile:166: all] Error 2

note: I tweaked the indent of the trace
fun fact: PI only officially exist since C++20 (https://en.cppreference.com/w/cpp/numeric/constants)

Proposal

If i were you:

1. I would be explicit, because in case of a super project build integrating scip with this option to OFF it will fail.
   Ie since it seems mandatory you should force it -> set(CMAKE_C_EXTENSIONS ON)
2. add a define in case...

   #ifndef M_PI
   #define M_PI 3.14159265358979323846
   #endif

Platform

FYI

%uname -a
Linux nuc10i7 5.13.5-arch1-1 #1 SMP PREEMPT Sun, 25 Jul 2021 18:02:58 +0000 x86_64 GNU/Linux
%gcc --version
gcc (GCC) 11.1.0
% clang --version
clang version 12.0.1
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[svigerske]

It is supposed to be -std=c99 -D_XOPEN_SOURCE=600: https://github.com/scipopt/scip/blob/master/make/make.linux.gnu#L12

[Mizux]

Not sure this makefile config file will impact the CMake build unless you do some magic...

note: Update the title

[svigerske]

No, it's not. It was meant as a hint to the people who take care of the cmake system, since, as far as I rememeber, the goal is to have the cmake system behave equivalently to the normal makefiles at some point.

[fschloesser]

This should be resolved with 5d5bd88