meson/docs/markdown/Compiler-properties.md

Compiler properties

Not all compilers and platforms are alike. Therefore Meson provides the tools to detect properties of the system during configure time. To get most of this information, you first need to extract the compiler object from the main meson variable.

compiler = meson.get_compiler('c')

Here we extract the C compiler. We could also have given the argument cpp to get the C++ compiler, objc to get the objective C compiler and so on. The call is valid for all languages specified in the project declaration. Trying to obtain some other compiler will lead to an unrecoverable error.

System information

This is a bit complex and more thoroughly explained on the page on cross compilation. But if you just want to know the operating system your code will run on, issue this command:

host_machine.system()

Compiler id

The compiler object method get_id returns a lower case string describing the "family" of the compiler. Since 0.53.0 get_linker_id returns a lower case string with the linker name. Since compilers can often choose from multiple linkers depending on operating system, get_linker_id can be useful for handling or mitigating effects of particular linkers.

The compiler object also has a method get_argument_syntax which returns a lower case string of gcc, msvc, or another undefined string value; identifying whether the compiler arguments use the same syntax as either gcc or msvc, or that its arguments are not like either. This should only be used to select the syntax of the arguments, such as those to test with has_argument.

See reference tables for a list of supported compiler ids and their argument type.

Does code compile?

Sometimes the only way to test the system is to try to compile some sample code and see if it works. For example, this can test that a "C++17" compiler actually supports a particular C++17 feature, without resorting to maintaining a feature list vs. compiler vendor, compiler version and operating system. Testing that a code snippet runs is a two-phase operation. First we define some code using the multiline string operator:

code = '''#include<stdio.h>
void func() { printf("Compile me.\n"); }
'''

Then we can run the test.

result = compiler.compiles(code, name : 'basic check')

The variable result will now contain either true or false depending on whether the compilation succeeded or not. The keyword argument name is optional. If it is specified, Meson will write the result of the check to its log.

Does code compile and link?

Sometimes it is necessary to check whether a certain code fragment not only compiles, but also links successfully, e.g. to check if a symbol is actually present in a library. This can be done using the '''.links()''' method on a compiler object like this:

code = '''#include<stdio.h>
void func() { printf("Compile me.\n"); }
'''

Then we can run the test.

result = compiler.links(code, args : '-lfoo', name : 'link check')

The variable result will now contain either true or false depending on whether the compilation and linking succeeded or not. The keyword argument name is optional. If it is specified, Meson will write the result of the check to its log.

Compile and run test application

Here is how you would compile and run a small test application. Testing if a code snippets runs versus merely that it links is particularly important for some dependencies such as MPI.

code = '''#include<stdio.h>
int main(int argc, char **argv) {
  printf("%s\n", "stdout");
  fprintf(stderr, "%s\n", "stderr");
  return 0;
}
'''
result = compiler.run(code, name : 'basic check')

The result variable encapsulates the state of the test, which can be extracted with the following methods. The name keyword argument works the same as with compiles.

Method	Return value
compiled	True if compilation succeeded. If false then all other methods return undefined values.
returncode	The return code of the application as an integer
stdout	Program's standard out as text.
stderr	Program's standard error as text.

Here is an example usage:

if result.stdout().strip() == 'some_value'
  # do something
endif

Does a header exist?

Header files provided by different platforms vary quite a lot. Meson has functionality to detect whether a given header file is available on the system. The test is done by trying to compile a simple test program that includes the specified header. The following snippet describes how this feature can be used.

if compiler.has_header('sys/fstat.h')
  # header exists, do something
endif

Expression size

Often you need to determine the size of a particular element (such as int, wchar_t or char*). Using the compiler variable mentioned above, the check can be done like this.

wcharsize = compiler.sizeof('wchar_t', prefix : '#include<wchar.h>')

This will put the size of wchar_t as reported by sizeof into variable wcharsize. The keyword argument prefix is optional. If specified its contents is put at the top of the source file. This argument is typically used for setting #include directives in configuration files.

In older versions (<= 0.30) Meson would error out if the size could not be determined. Since version 0.31 it returns -1 if the size could not be determined.

Does a function exist?

Just having a header doesn't say anything about its contents. Sometimes you need to explicitly check if some function exists. This is how we would check whether the function open_memstream exists in header stdio.h

if compiler.has_function('open_memstream', prefix : '#include <stdio.h>')
  # function exists, do whatever is required.
endif

Note that, on macOS programs can be compiled targeting older macOS versions than the one that the program is compiled on. It can't be assumed that the OS version that is compiled on matches the OS version that the binary will run on.

Therefore when detecting function availability with has_function, it is important to specify the correct header in the prefix argument.

In the example above, the function open_memstream is detected, which was introduced in macOS 10.13. When the user builds on macOS 10.13, but targeting macOS 10.11 (-mmacosx-version-min=10.11), this will correctly report the function as missing. Without the header however, it would lack the necessary availability information and incorrectly report the function as available.

Does a structure contain a member?

Some platforms have different standard structures. Here's how one would check if a struct called mystruct from header myheader.h contains a member called some_member.

if compiler.has_member('struct mystruct', 'some_member', prefix : '#include<myheader.h>')
  # member exists, do whatever is required
endif

Type alignment

Most platforms can't access some data types at any address. For example it is common that a char can be at any address but a 32 bit integer only at locations which are divisible by four. Determining the alignment of data types is simple.

int_alignment = compiler.alignment('int') # Will most likely contain the value 4.

Has argument

This method tests if the compiler supports a given command line argument. This is implemented by compiling a small file with the given argument.

has_special_flags = compiler.has_argument('-Wspecialthing')

Note: some compilers silently swallow command line arguments they do not understand. Thus this test can not be made 100% reliable.