Cross compilation

Meson has full support for cross compilation through the use of a cross build definition file. An minimal example of one such file x86_64-w64-mingw32.txt for a GCC/MinGW cross compiler targeting 64-bit Windows could be:

[binaries]
c = 'x86_64-w64-mingw32-gcc'
cpp = 'x86_64-w64-mingw32-g++'
ar = 'x86_64-w64-mingw32-ar'
strip = 'x86_64-w64-mingw32-strip'
exe_wrapper = 'wine64'

[host_machine]
system = 'windows'
cpu_family = 'x86_64'
cpu = 'x86_64'
endian = 'little'

Which is then used during the setup phase.

meson setup --cross-file x86_64-w64-mingw32.txt build-mingw
meson compile -C build-mingw

Since cross compiling is more complicated than native building, let's first go over some nomenclature. The three most important definitions are traditionally called build, host and target. This is confusing because those terms are used for quite many different things. To simplify the issue, we are going to call these the build machine, host machine and target machine. Their definitions are the following:

• build machine is the computer that is doing the actual compiling.
• host machine is the machine on which the compiled binary will run.
• target machine is the machine on which the compiled binary's output will run, only meaningful if the program produces machine-specific output.

The tl/dr summary is the following: if you are doing regular cross compilation, you only care about build_machine and host_machine. Just ignore target_machine altogether and you will be correct 99% of the time. Only compilers and similar tools care about the target machine. In fact, for so-called "multi-target" tools the target machine need not be fixed at build-time like the others but chosen at runtime, so target_machine still doesn't matter. If your needs are more complex or you are interested in the actual details, do read on.

This might be easier to understand through examples. Let's start with the regular, not cross-compiling case. In these cases all of these three machines are the same. Simple so far.

Let's next look at the most common cross-compilation setup. Let's suppose you are on a 64 bit OSX machine and you are cross compiling a binary that will run on a 32 bit ARM Linux board. In this case your build machine is 64 bit OSX, your host machine is 32 bit ARM Linux and your target machine is irrelevant (but defaults to the same value as the host machine). This should be quite understandable as well.

The usual mistake in this case is to call the OSX system the host and the ARM Linux board the target. That's because these were their actual names when the cross-compiler itself was compiled! Let's assume the cross-compiler was created on OSX too. When that happened the build and host machines were the same OSX and different from the ARM Linux target machine.

In a nutshell, the typical mistake assumes that the terms build, host and target refer to some fixed positions whereas they're actually relative to where the current compiler is running. Think of host as a child of the current compiler and target as an optional grand-child. Compilers don't change their terminology when they're creating another compiler, that would at the very least make their user interface much more complex.

The most complicated case is when you cross-compile a cross compiler. As an example you can, on a Linux machine, generate a cross compiler that runs on Windows but produces binaries on MIPS Linux. In this case build machine is x86 Linux, host machine is x86 Windows and target machine is MIPS Linux. This setup is known as the Canadian Cross. As a side note, be careful when reading cross compilation articles on Wikipedia or the net in general. It is very common for them to get build, host and target mixed up, even in consecutive sentences, which can leave you puzzled until you figure it out.

Again note that when you cross-compile something, the 3 systems (build, host, and target) used when building the cross compiler don't align with the ones used when building something with that newly-built cross compiler. To take our Canadian Cross scenario from above (for full generality), since its host machine is x86 Windows, the build machine of anything we build with it is x86 Windows. And since its target machine is MIPS Linux, the host machine of anything we build with it is MIPS Linux. Only the target machine of whatever we build with it can be freely chosen by us, say if we want to build another cross compiler that runs on MIPS Linux and targets Aarch64 iOS. As this example hopefully makes clear to you, the machine names are relative and shifted over to the left by one position.

If you did not understand all of the details, don't worry. For most people it takes a while to wrap their head around these concepts. Don't panic, it might take a while to click, but you will get the hang of it eventually.

Defining the environment

Meson requires you to write a cross build definition file. It defines various properties of the cross build environment. The cross file consists of different sections.

There are a number of options shared by cross and native files, here. It is assumed that you have read that section already, as this documentation will only call out options specific to cross files.

Binaries

[binaries]
exe_wrapper = 'wine' # A command used to run generated executables.

The exe_wrapper option defines a wrapper command that can be used to run executables for this host. In this case we can use Wine, which runs Windows applications on Linux. Other choices include running the application with qemu or a hardware simulator. If you have this kind of a wrapper, these lines are all you need to write. Meson will automatically use the given wrapper when it needs to run host binaries. This happens e.g. when running the project's test suite.

Properties

In addition to the properties allowed in all machine files, the cross file may contain specific information about the cross compiler or the host machine. It looks like this:

[properties]
sizeof_int = 4
sizeof_wchar_t = 4
sizeof_void* = 4

alignment_char = 1
alignment_void* = 4
alignment_double = 4

has_function_printf = true

sys_root = '/some/path'
pkg_config_libdir = '/some/path/lib/pkgconfig'

In most cases you don't need the size and alignment settings, Meson will detect all these by compiling and running some sample programs. If your build requires some piece of data that is not listed here, Meson will stop and write an error message describing how to fix the issue. If you need extra compiler arguments to be used during cross compilation you can set them with [langname]_args = [args]. Just remember to specify the args as an array and not as a single string (i.e. not as '-DCROSS=1 -DSOMETHING=3').

Since 0.52.0 The sys_root property may point to the root of the host system path (the system that will run the compiled binaries). This is used internally by Meson to set the PKG_CONFIG_SYSROOT_DIR environment variable for pkg-config. If this is unset the host system is assumed to share a root with the build system.

Since 0.54.0 The pkg_config_libdir property may point to a list of path used internally by Meson to set the PKG_CONFIG_LIBDIR environment variable for pkg-config. This prevents pkg-config from searching cross dependencies in system directories.

One important thing to note, if you did not define an exe_wrapper in the previous section, is that Meson will make a best-effort guess at whether it can run the generated binaries on the build machine. It determines whether this is possible by looking at the system and cpu_family of build vs host. There will however be cases where they do match up, but the build machine is actually not compatible with the host machine. Typically this will happen if the libc used by the build and host machines are incompatible, or the code relies on kernel features not available on the build machine. One concrete example is a macOS build machine producing binaries for an iOS Simulator x86-64 host. They're both darwin and the same architecture, but their binaries are not actually compatible. In such cases you may use the needs_exe_wrapper property to override the auto-detection:

[properties]
needs_exe_wrapper = true

Machine Entries

The next bit is the definition of host and target machines. Every cross build definition must have one or both of them. If it had neither, the build would not be a cross build but a native build. You do not need to define the build machine, as all necessary information about it is extracted automatically. The definitions for host and target machines look the same. Here is a sample for host machine.

[host_machine]
system = 'windows'
subsystem = 'windows'
kernel = 'nt'
cpu_family = 'x86'
cpu = 'i686'
endian = 'little'

These values define the machines sufficiently for cross compilation purposes. The corresponding target definition would look the same but have target_machine in the header. These values are available in your Meson scripts. There are three predefined variables called, surprisingly, build_machine, host_machine and target_machine. Determining the operating system of your host machine is simply a matter of calling host_machine.system(). Starting from version 1.2.0 you can get more fine grained information using the .subsystem() and .kernel() methods. The return values of these functions are documented in the reference table page.

There are two different values for the CPU. The first one is cpu_family. It is a general type of the CPU. This should have a value from the CPU Family table. Note that Meson does not add el to end cpu_family value for little endian systems. Big endian and little endian mips are both just mips, with the endian field set appropriately.

The second value is cpu which is a more specific subtype for the CPU. Typical values for a x86 CPU family might include i386 or i586 and for arm family armv5 or armv7hl. Note that CPU type strings are very system dependent. You might get a different value if you check its value on the same machine but with different operating systems.

If you do not define your host machine, it is assumed to be the build machine. Similarly if you do not specify target machine, it is assumed to be the host machine.

Starting a cross build

Once you have the cross file, starting a build is simple

$ meson setup builddir --cross-file cross_file.txt

Once configuration is done, compilation is started by invoking meson compile in the usual way.

Introspection and system checks

The main meson object provides two functions to determine cross compilation status.

meson.is_cross_build()        # returns true when cross compiling
meson.can_run_host_binaries() # returns true if the host binaries can be run, either with a wrapper or natively

You can run system checks on both the system compiler or the cross compiler. You just have to specify which one to use.

build_compiler = meson.get_compiler('c', native : true)
host_compiler = meson.get_compiler('c', native : false)

build_int_size = build_compiler.sizeof('int')
host_int_size  = host_compiler.sizeof('int')

Mixing host and build targets

Sometimes you need to build a tool which is used to generate source files. These are then compiled for the actual target. For this you would want to build some targets with the system's native compiler. This requires only one extra keyword argument.

native_exe = executable('mygen', 'mygen.c', native : true)

You can then take native_exe and use it as part of a generator rule or anything else you might want.

Using a custom standard library

Sometimes in cross compilation you need to build your own standard library instead of using the one provided by the compiler. Meson has built-in support for switching standard libraries transparently. The invocation to use in your cross file is the following:

[properties]
c_stdlib = ['mylibc', 'mylibc_dep'] # Subproject name, variable name

This specifies that C standard library is provided in the Meson subproject mylibc in internal dependency variable mylibc_dep. It is used on every cross built C target in the entire source tree (including subprojects) and the standard library is disabled. The build definitions of these targets do not need any modification.

Note that it is supported for any language, not only c, using <lang>_stdlib property.

Since 0.56.0 the variable name parameter is no longer required as long as the subproject calls meson.override_dependency('c_stdlib', mylibc_dep). The above example becomes:

[properties]
c_stdlib = 'mylibc'

Changing cross file settings

Cross file settings are only read when the build directory is set up the first time. Any changes to them after the fact will be ignored. This is the same as regular compiles where you can't change the compiler once a build tree has been set up. If you need to edit your cross file, then you need to wipe your build tree and recreate it from scratch.

Custom data

You can store arbitrary data in properties and access them from your Meson files. As an example if your cross file has this:

[properties]
somekey = 'somevalue'

then you can access that using the meson object like this:

myvar = meson.get_external_property('somekey')
# myvar now has the value 'somevalue'

Cross file locations

As of version 0.44.0 Meson supports loading cross files from system locations (except on Windows). This will be $XDG_DATA_DIRS/meson/cross, or if XDG_DATA_DIRS is undefined, then /usr/local/share/meson/cross and /usr/share/meson/cross will be tried in that order, for system wide cross files. User local files can be put in $XDG_DATA_HOME/meson/cross, or ~/.local/share/meson/cross if that is undefined.

The order of locations tried is as follows:

• A file relative to the local dir
• The user local location
• The system wide locations in order

Distributions are encouraged to ship cross files either with their cross compiler toolchain packages or as a standalone package, and put them in one of the system paths referenced above.

These files can be loaded automatically without adding a path to the cross file. For example, if a ~/.local/share/meson/cross contains a file called x86-linux, then the following command would start a cross build using that cross files:

meson setup builddir/ --cross-file x86-linux