• Tim-Philipp Müller's avatar
    libs: fix API export/import and 'inconsistent linkage' on MSVC · b6411ae7
    Tim-Philipp Müller authored
    For each lib we build export its own API in headers when we're
    building it, otherwise import the API from the headers.
    
    This fixes linker warnings on Windows when building with MSVC.
    
    The problem was that we had defined all GST_*_API decorators
    unconditionally to GST_EXPORT. This was intentional and only
    supposed to be temporary, but caused linker warnings because
    we tell the linker that we want to export all symbols even
    those from externall DLLs, and when the linker notices that
    they were in external DLLS and not present locally it warns.
    
    What we need to do when building each library is: export
    the library's own symbols and import all other symbols. To
    this end we define e.g. BUILDING_GST_FOO and then we define
    the GST_FOO_API decorator either to export or to import
    symbols depending on whether BUILDING_GST_FOO is set or not.
    That way external users of each library API automatically
    get the import.
    
    While we're at it, add new GST_API_EXPORT in config.h and use
    that for GST_*_API decorators instead of GST_EXPORT.
    
    The right export define depends on the toolchain and whether
    we're using -fvisibility=hidden or not, so it's better to set it
    to the right thing directly than hard-coding a compiler whitelist
    in the public header.
    
    We put the export define into config.h instead of passing it via the
    command line to the compiler because it might contain spaces and brackets
    and in the autotools scenario we'd have to pass that through multiple
    layers of plumbing and Makefile/shell escaping and we're just not going
    to be *that* lucky.
    
    The export define is only used if we're compiling our lib, not by external
    users of the lib headers, so it's not a problem to put it into config.h
    
    Also, this means all .c files of libs need to include config.h
    to get the export marker defined, so fix up a few that didn't
    include config.h.
    
    This commit depends on a common submodule commit that makes gst-glib-gen.mak
    add an #include "config.h" to generated enum/marshal .c files for the
    autotools build.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=797185
    b6411ae7