[question] Manually configuring (or disabling) the /MPxx flag for MSVC profiles?

• I’ve read the CONTRIBUTING guide.

Question

With an MSVC profile CMakeToolchain generator creates a conan_toolchain.cmake file that sets the number of compilation processes by setting the /MP flag like so:

string(APPEND CONAN_CXX_FLAGS " /MP12")
string(APPEND CONAN_C_FLAGS " /MP12")
# snip
  string(APPEND CMAKE_CXX_FLAGS_INIT " ${CONAN_CXX_FLAGS}")
# snip
  string(APPEND CMAKE_C_FLAGS_INIT " ${CONAN_C_FLAGS}")

How can I modify/disable this functionality? (Obviously aside from manually editing the lines or wrapping the toolchain in my own toolchain.)

Background

I have a project that relies heavily on PCL, which isn’t yet available as a Conan recipe, so it needs to be built manually. I’m attempting to use Conan to set up the CMake toolchain for the PCL dependencies so I can avoid fragmentation as much as possible when I directly use those third-party deps in my own project. It’s straightforward enough with a conanfile.txt like so:

[requires]
zlib/1.2.12
boost/1.78.0
eigen/3.4.0
flann/1.9.2
qhull/8.0.1

[generators]
CMakeDeps
CMakeToolchain

And using a default profile:

[settings]
os=Windows
os_build=Windows
arch=x86_64
arch_build=x86_64
compiler=Visual Studio
compiler.version=16
build_type=Release
[options]
[conf]
[build_requires]
[env]

This configuration on a 6 core CPU with HT gives me the /MP12 flag, which is sensible, but unfortunately this breaks some of the PCL scripts because they assume that if you’ve set any non-default flags you’ve set every required flag and don’t want any additional added.

I can work around this by wrapping the toolchain and modifying CMAKE_<LANG>_FLAGS_INIT or by manually setting all required flags, but with CUDA/Ninja etc., that might be a maintainability issue, so wanted to figure out if I can simply disable this from the Conan side. Alternatively I’m open to any other obvious solution if there’s something I missed.

[mvhv]

@memsharded Ahh, you’re right… Thanks for that, it seems like I was just thinking too hard about it.

I was looking at conan.tools.build.cpu.build_jobs() and focusing on the defaults for None but I overlooked how this obvious if statement falls through on 0.

Thanks.

[memsharded]

I think that tools.build:jobs=0 will simply not define that flags, please try it and let me know