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64bit osx (part II) linking



  • I am trying to link the libx264 library to one of my apps but I am getting linking errors saying it can not find various symbols for the x86_64 architecture:

    Undefined symbols for architecture x86_64:
    "x264_param_default_preset(x264_param_t*, char const*, char const*)", referenced from:

    I have verified that I compiled the libx264 library using x86_64:

    lipo -info libx264.a
    input file libx264.a is not a fat file
    Non-fat file: libx264.a is architecture: x86_64

    I also checked to see if the missing symbols exist in the library:

    nm libx264.a | grep x264_param_default_preset
    0000000000001710 T _x264_param_default_preset
    0000000000005e88 S _x264_param_default_preset.eh

    I am trying to link as a static library.

    Are there some additional parameters I need to set when compiling libraries to get them to be compatible with Qt or can someone point me in the right direction?

    Thanks again.

    -=ben



  • Could you post the part of your .pro file, where you are linking your libs?
    Also, there was a similar thread before (maybe you have the same problem):
    http://developer.qt.nokia.com/forums/viewthread/5665



  • Sure thing:

    @
    TARGET = myLib
    TEMPLATE = lib

    ... headers/source removed for readability ...

    macx {

    LIBS += /Developer/SDKs/MacOSX10.6.sdk/System/Library/Frameworks/AppKit.framework/Versions/Current/AppKit
    /Developer/SDKs/MacOSX10.6.sdk/System/Library/Frameworks/OpenGL.framework/Versions/Current/OpenGL
    ../../../common/x264/libx264.a
    ../../../common/ffmpeg/libswscale/libswscale.a

        QMAKE_LFLAGS_SONAME = -Wl,-install_name,@executable_path/../Frameworks/
    

    }@

    Thanks for the other posting link, unfortunately it has not helped me.

    Thanks.

    -=ben



  • I can reproduce the problem. It's nothing Qt specific, though.

    The x264 library does not support C++ out of the box. It is a pure C library, so it uses the C style "name mangling":http://en.wikipedia.org/wiki/Name_mangling, if you just include the headers in a C++ source, the compiler wants to apply C++ style name mangling - so it tries to link different names which are not available in the lib.

    So, the solution simple: Wrap the #include with extern "C" this way:

    @
    #include <stdint.h>

    // enforce C style name mangling
    extern "C" {
    #include "x264.h"
    }
    @

    This switches on C name mangling for the functions in this header file.



  • That was it! You made my day and I learned something to boot. :)

    Are there any ways to tell what type of name mangling is being used to help troubleshoot this in the future?

    Thanks again!

    -=ben



  • If you do not setup anything, then the default name mangling is used. If you have a C library that is usesful from C++ as well, it is good practice to put that extern stuff into the header of your libraray. The usual pattern is:

    @
    #ifdef __cplusplus
    extern "C" {
    #endif

    // your actual header files goes here

    #ifdef __cplusplus
    }
    #endif
    @

    See the "C++ FAQ on 'How to mix C and C++'":http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html for some more details.

    Unfortunately there is no easy way to tell if the header has the "extern" other than actually looking into it.


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