Compiler selection and customizing a build#

Selecting a specific compiler#

Meson supports the standard environment variables CC, CXX and FC to select specific C, C++ and/or Fortran compilers. These environment variables are documented in the reference tables in the Meson docs.

Note that environment variables only get applied from a clean build, because they affect the configure stage (i.e., meson setup). An incremental rebuild does not react to changes in environment variables - you have to run git clean -xdf and do a full rebuild, or run meson setup --reconfigure.

Adding a custom compiler or linker flag#

Meson by design prefers builds being configured through command-line options passed to meson setup. It provides many built-in options:

  • For enabling a debug build and the optimization level, see the next section on “build types”,

  • Enabling -Werror in a portable manner is done via -Dwerror=true,

  • Enabling warning levels is done via -Dwarning_level=<val>, with <val> one of {0, 1, 2, 3, everything},

  • There are many other builtin options, from activating Visual Studio (-Dvsenv=true) and building with link time optimization (-Db_lto) to changing the default C++ language level (-Dcpp_std='c++17') or linker flags (-Dcpp_link_args='-Wl,-z,defs').

For a comprehensive overview of options, see Meson’s builtin options docs page.

Meson also supports the standard environment variables CFLAGS, CXXFLAGS, FFLAGS and LDFLAGS to inject extra flags - with the same caveat as in the previous section about those environment variables being picked up only for a clean build and not an incremental build.

Using different build types with Meson#

Meson provides different build types while configuring the project. You can see the available options for build types in the “core options” section of the Meson documentation.

Assuming that you are building from scratch (do git clean -xdf if needed), you can configure the build as following to use the debug build type:

meson setup build --buildtype debug  --prefix=$PWD/build-install

Now, you can use the interface for further building, installing and testing SciPy:

python -s linalg

This will work because after initial configuration, Meson will remember the config options.

Controlling build parallelism#

By default, ninja will launch 2*n_cpu + 2, with n_cpu the number of physical CPU cores, parallel build jobs. This is fine in the vast majority of cases, and results in close to optimal build times. In some cases, on machines with a small amount of RAM relative to the number of CPU cores, this leads to a job running out of memory. In case that happens, lower the number of jobs N such that you have at least 2 GB RAM per job. For example, to launch 6 jobs:

python -m pip install . -Ccompile-args="-j6"


python build -j6

Use GCC and Clang builds in parallel#

It may be useful to have several builds of SciPy in the same repo, for example to compare the differences between two compilers for diagnosing an issue. As discussed, Meson is fully out-of-place, so different builds will not interfere with each other. We assume in the rest of this section that GCC is the default. For example, let us build using GCC and Clang.

  1. Build with GCC:

    python build

Using the above command, meson will build with the (default) GCC compilers in the build directory, and install to the build-install directory.

  1. Build with Clang:

    CC=clang CXX=clang++ FC=gfortran python --build-dir=build-clang build

Using the above commands, Meson will build with the Clang, Clang++ and Gfortran compilers in the build-clang directory, and then install SciPy into build-clang-install.

Meson will remember the compiler selection for the build-clang directory and it cannot be changed, so each future invocation of python --build-dir=build-clang <command> it will automatically use Clang.

Tip: use an alias to make this easier to use, e.g., alias dev-clang="python --build-dir=build-clang" and then dev-clang build.

A common reason to have two builds is to compare between them. For example, to run the scipy.linalg tests for builds with both compilers, do:

python -s linalg                          # run tests for the GCC build
python --build-dir build-clang -s linalg  # run tests for the Clang build