What's the correct way to handle dllexport/dllimport in Meson?

[MarkLagodych]

When building a cross-platform library that can be either static or shared, libraries usually prefix all function declarations with a special macro (e.g. "MY_EXPORT") that in case of Windows is defined like so:

#ifdef MY_SHARED
#  ifdef MY_BUILDING_ITSELF
#    define MY_EXPORT __declspec(dllexport)
#  else
#    define MY_EXPORT __declspec(dllimport)
#  endif
#else
#  define MY_EXPORT
#endif

The MY_BUILDING_SELF macro is private, it is only visible to the library code.
The MY_SHARED macro is public (defined for both the library and the code that uses it), but only if the library is built as shared.

The only missing part here is how to define the MY_SHARED in Meson.

Even though on the library side this can be done with c_shared_args/c_static_args in library() of project(..default_options..), I could not figure out how to propagate those args through declare_dependency() in a correct way (i.e. without using add_global_arguments()).

What is the correct way to approach this? Am I missing something about how Meson handles static/shared libraries?

[eli-schwartz]

usually you'd pass the arguments as separate arrays, one passed to the library() function (to indicate BUILDING_ITSELF / dllexport) and a different one passed to the declare_dependency() function (indicating dllimport).

The "building as shared" macro should be passed to c_shared_args, and also to declare_dependency since a declared dependency always links shared if it can (unless you use get_static_lib on a both_libraries() object to explicitly request the static version).

[MarkLagodych]

I was confused about how DLL compilation works.

Turns out, dllimport does not actually do anything because on Windows you must link to a static library (e.g. "mylib.dll.a") that you ship alongside the shared library (e.g. "mylib.dll") anyway.

So, the optimal header code is:

#ifdef MY_SHARED
#  define MY_EXPORT __declspec(dllexport)
#else
#  define MY_EXPORT
#endif

and doing library(... c_shared_args: ['-DMY_SHARED']) in Meson is sufficient.

[eli-schwartz]

Actually the answer is "it is complicated".

There is a very good writeup at libass/libass@c5ee66d

tl;dr omitting dllimport is sometimes allowed, but when it is, the resulting code is slower.

[MarkLagodych]

Thank you! That link explained a lot, but now I wonder: if I really want to use dllimport for my library() and then use the library through declare_dependency(), will it break if the user specifies meson configure --default-library static?

[eli-schwartz]

That's why you will want to check like:

if get_option('default_library') != 'static'`
    # define SHARED, because it is shared, and not
    # BUILDING_ITSELF, because this is the interface dep
    public_flags = ['MYLIBNAME_SHARED']
else
    # static library 
    public_flags = []
endif

mylibname_dep = declare_dependency(
    ...,
    compile_args: public_flags
)

[MarkLagodych]

Got it, thanks!