# How do I do X in Meson? This page lists code snippets for common tasks. These are written mostly using the C compiler, but the same approach should work on almost all other compilers. ## Set compiler When first running Meson, set it in an environment variable. ```console $ CC=mycc meson ``` Note that environment variables like `CC` only works in native builds. The `CC` refers to the compiler for the host platform, that is the compiler used to compile programs that run on the machine we will eventually install the project on. The compiler used to build things that run on the machine we do the building can be specified with `CC_FOR_BUILD`. You can use it in cross builds. Note that environment variables are never the idiomatic way to do anything with Meson, however. It is better to use the native and cross files. And the tools for the host platform in cross builds can only be specified with a cross file. There is a table of all environment variables supported [Here](Reference-tables.md#compiler-and-linker-selection-variables) ## Set linker *New in 0.53.0* Like the compiler, the linker is selected via the `_LD` environment variable, or through the `_ld` entry in a native or cross file. You must be aware of whether you're using a compiler that invokes the linker itself (most compilers including GCC and Clang) or a linker that is invoked directly (when using MSVC or compilers that act like it, including Clang-Cl). With the former `c_ld` or `CC_LD` should be the value to pass to the compiler's special argument (such as `-fuse-ld` with clang and gcc), with the latter it should be an executable, such as `lld-link.exe`. *NOTE* In Meson 0.53.0 the `ld` entry in the cross/native file and the `LD` environment variable were used, this resulted in a large number of regressions and was changed in 0.53.1 to `_ld` and `_LD`. ```console $ CC=clang CC_LD=lld meson ``` or ```console $ CC=clang-cl CC_LD=link meson ``` or in a cross or native file: ```ini [binaries] c = 'clang' c_ld = 'lld' ``` There is a table of all environment variables supported [Here](Reference-tables.md#compiler-and-linker-selection-variables) ## Set default C/C++ language version ```meson project('myproj', 'c', 'cpp', default_options : ['c_std=c11', 'cpp_std=c++11']) ``` The language version can also be set on a per-target basis. ```meson executable(..., override_options : ['c_std=c11']) ``` ## Enable threads Lots of people seem to do this manually with `find_library('pthread')` or something similar. Do not do that. It is not portable. Instead do this. ```meson thread_dep = dependency('threads') executable(..., dependencies : thread_dep) ``` ## Set extra compiler and linker flags from the outside (when e.g. building distro packages) The behavior is the same as with other build systems, with environment variables during first invocation. Do not use these when you need to rebuild the source ```console $ CFLAGS=-fsomething LDFLAGS=-Wl,--linker-flag meson ``` ## Use an argument only with a specific compiler First check which arguments to use. ```meson if meson.get_compiler('c').get_id() == 'clang' extra_args = ['-fclang-flag'] else extra_args = [] endif ``` Then use it in a target. ```meson executable(..., c_args : extra_args) ``` If you want to use the arguments on all targets, then do this. ```meson if meson.get_compiler('c').get_id() == 'clang' add_global_arguments('-fclang-flag', language : 'c') endif ``` ## Set a command's output to configuration ```meson txt = run_command('script', 'argument').stdout().strip() cdata = configuration_data() cdata.set('SOMETHING', txt) configure_file(...) ``` ## Generate configuration data from files `The [fs module](#Fs-modules) offers the `read` function` which enables adding the contents of arbitrary files to configuration data (among other uses): ```meson fs = import('fs') cdata = configuration_data() copyright = fs.read('LICENSE') cdata.set('COPYRIGHT', copyright) if build_machine.system() == 'linux' os_release = fs.read('/etc/os-release') cdata.set('LINUX_BUILDER', os_release) endif configure_file(...) ``` ## Generate a runnable script with `configure_file` `configure_file` preserves metadata so if your template file has execute permissions, the generated file will have them too. ## Producing a coverage report First initialize the build directory with this command. ```console $ meson -Db_coverage=true ``` Then issue the following commands. ```console $ meson compile $ meson test $ meson compile coverage-html (or coverage-xml) ``` The coverage report can be found in the meson-logs subdirectory. *New in 0.55.0* llvm-cov support for use with clang ## Add some optimization to debug builds By default the debug build does not use any optimizations. This is the desired approach most of the time. However some projects benefit from having some minor optimizations enabled. GCC even has a specific compiler flag `-Og` for this. To enable its use, just issue the following command. ```console $ meson configure -Dc_args=-Og ``` This causes all subsequent builds to use this command line argument. ## Use address sanitizer Clang comes with a selection of analysis tools such as the [address sanitizer](https://clang.llvm.org/docs/AddressSanitizer.html). Meson has native support for these with the `b_sanitize` option. ```console $ meson -Db_sanitize=address ``` After this you just compile your code and run the test suite. Address sanitizer will abort executables which have bugs so they show up as test failures. ## Use Clang static analyzer Install scan-build and configure your project. Then do this: ```console $ meson compile scan-build ``` You can use the `SCANBUILD` environment variable to choose the scan-build executable. ```console $ SCANBUILD= meson compile scan-build ``` ## Use profile guided optimization Using profile guided optimization with GCC is a two phase operation. First we set up the project with profile measurements enabled and compile it. ```console $ meson setup -Db_pgo=generate $ meson compile -C builddir ``` Then we need to run the program with some representative input. This step depends on your project. Once that is done we change the compiler flags to use the generated information and rebuild. ```console $ meson configure -Db_pgo=use $ meson compile ``` After these steps the resulting binary is fully optimized. ## Add math library (`-lm`) portably Some platforms (e.g. Linux) have a standalone math library. Other platforms (pretty much everyone else) do not. How to specify that `m` is used only when needed? ```meson cc = meson.get_compiler('c') m_dep = cc.find_library('m', required : false) executable(..., dependencies : m_dep) ``` ## Install an executable to `libexecdir` ```meson executable(..., install : true, install_dir : get_option('libexecdir')) ``` ## Use existing `Find.cmake` files Meson can use the CMake `find_package()` ecosystem if CMake is installed. To find a dependency with custom `Find.cmake`, set the `cmake_module_path` property to the path in your project where the CMake scripts are stored. Example for a `FindCmakeOnlyDep.cmake` in a `cmake` subdirectory: ```meson cm_dep = dependency('CmakeOnlyDep', cmake_module_path : 'cmake') ``` The `cmake_module_path` property is only needed for custom CMake scripts. System wide CMake scripts are found automatically. More information can be found [here](Dependencies.md#cmake) ## Get a default not-found dependency? ```meson null_dep = dependency('', required : false) ``` This can be used in cases where you want a default value, but might override it later. ```meson # Not needed on Windows! my_dep = dependency('', required : false) if host_machine.system() in ['freebsd', 'netbsd', 'openbsd', 'dragonfly'] my_dep = dependency('some dep', required : false) elif host_machine.system() == 'linux' my_dep = dependency('some other dep', required : false) endif executable( 'myexe', my_sources, deps : [my_dep] ) ```