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# Default build options

Meson offers many built-in options to control how the project is built and installed. In the vast majority of cases those have good defaults. There are however a few options that `meson-python` overrides with its own defaults to adjust the build process to the task of building Python wheels.

The default options specified by `meson-python` are overridden by package specific options specified in `pyproject.toml` and by options provided by the user at build time via the Python build front-end. Refer to the Passing arguments to Meson guide for details.

The options used to build the project are summarized in the User defined options section of the output of the `meson setup` stage of the build, for example when running `python -m build -w`. This will look something like:

    User defined options
      Native files: $builddir/meson-python-native-file.ini
      buildtype   : release
      b_ndebug    : if-release
      b_vscrt     : md

where the path to the build directory has been replaced with `$builddir` for clarity.

The options that `meson-python` specifies by default are:

native-file=$builddir/meson-python-native-file.ini

  `meson-python` uses a native file to point Meson at the `python` interpreter that the build must target. This is the Python interpreter that is used to run the Python build front-end. Meson would otherwise look for the first Python interpreter on the `$PATH`, which may not be the same.

  Additional `--native-file` options can be passed to `meson setup` if further adjustments to the native environment need to be made. Meson will merge the contents of all machine files. To ensure everything works as expected, the `meson-python` native file is last in the command line, overriding the `python` binary path that may have been specified in user supplied native files.

buildtype=release

  The Meson default is to produce a debug build with binaries compiled with debug symbols and, when compiling with MSVC, linking to the Visual Studio debug runtime, see below. The main purpose of `meson-python` is to build release artifacts, therefore a more appropriate build type is selected. A release build is compiled without debug symbols and with compiler optimizations. Refer to the Meson documentation for more details.

b_ndebug=if-release

  For reasons related to backward compatibility, Meson does not disable assertions for release builds. For most users this is a surprising and undesired behavior. This option instructs Meson to pass the `-DNDEBUG` option to the compilers, unless the build type is set to something else than release.

b_vscrt=md

  With the default options, when compiling a debug build, Meson instructs the MSVC compiler to use the debug version of the Visual Studio runtime library. This causes the MSVC linker to look for the debug build of all the linked DLLs. The Python distribution for Windows does not contain a debug version of the Python DLL and linking fails. These linking failures are surprising and hard to diagnose. To avoid this issue when users explicitly asks for a debug build, `meson-python` sets this options to instruct Meson to compile with the release version of the Visual Studio runtime. For more details, refer to the Meson documentation and to the Visual Studio documentation . This option is ignored when other compilers are used.