• Martin Storsjö's avatar
    Add support for building shared libraries with MSVC · d66c52c2
    Martin Storsjö authored
    This requires the makedef perl script by Derek, from the
    c89-to-c99 repo. That scripts produces a .def file, listing
    the symbols to be exported, based on the gcc version scripts
    and the built object files.
    
    To properly load non-function symbols from DLL files, the
    data symbol declarations need to have the attribute
    __declspec(dllimport) when building the calling code. (On mingw,
    the linker can fix this up automatically, which is why it has not
    been an issue so far. If this attribute is omitted, linking
    actually succeeds, but reads from the table will not produce the
    desired results at runtime.)
    
    MSVC seems to manage to link DLLs (and run properly) even if
    this attribute is present while building the library itself
    (which normally isn't recommended) - other object files in the
    same library manage to link to the symbol (with a small warning
    at link time, like "warning LNK4049: locally defined symbol
    _avpriv_mpa_bitrate_tab imported" - it doesn't seem to be possible
    to squelch this warning), and the definition of the tables
    themselves produce a warning that can be squelched ("warning C4273:
    'avpriv_mpa_bitrate_tab' : inconsistent dll linkage, see previous
    definition of 'avpriv_mpa_bitrate_tab').
    
    In this setup, mingw isn't able to link object files that refer to
    data symbols with __declspec(dllimport) without those symbols
    actually being linked via a DLL (linking avcodec.dll ends up with
    errors like "undefined reference to `__imp__avpriv_mpa_freq_tab'").
    The dllimport declspec isn't needed at all in mingw, so we simply
    choose not to declare it for other compilers than MSVC that requires
    it. (If ICL support later requires it, the condition can be extended
    later to include both of them.)
    
    This also implies that code that is built to link to a certain
    library as a DLL can't link to the same library as a static library.
    Therefore, we only allow building either static or shared but not
    both at the same time. (That is, static libraries as such can be,
    and actually are, built - this is used for linking the test tools to
    internal symbols in the libraries - but e.g. libavformat built to
    link to libavcodec as a DLL cannot link statically to libavcodec.)
    
    Also, linking to DLLs is slightly different from linking to shared
    libraries on other platforms. DLLs use a thing called import
    libraries, which is basically a stub library allowing the linker
    to know which symbols exist in the DLL and what name the DLL will
    have at runtime.
    
    In mingw/gcc, the import library is usually named libfoo.dll.a,
    which goes next to a static library named libfoo.a. This allows
    gcc to pick the dynamic one, if available, from the normal -lfoo
    switches, just as it does for libfoo.a vs libfoo.so on Unix. On
    MSVC however, you need to literally specify the name of the import
    library instead of the static library.
    Signed-off-by: 's avatarMartin Storsjö <martin@martin.st>
    d66c52c2
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