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Troubles loading dll

  • Here I've got a dll written in C downloaded from the internet. The library is successfully loaded in Qt, functions resolved. But calling these methods causes the app to crash with "Segmentation fault". Besides, I've built a C# project in VS08 and loaded the library. Everything works again.

    Here's the code:
    1.dll library:

    void fdlib_detectfaces(unsigned char *imagedata, int imagewidth, int imageheight, int threshold);

    2.Qt app:

    ushort graydata[10*10];
    QLibrary mylib("\fdlib.dll");
    bool okLoad = mylib.load();
    bool loaded = mylib.isLoaded();
    typedef void (FdetectFace)(ushort,int,int,int);
    FdetectFace detectFace = (FdetectFace)mylib.resolve("fdlib_detectfaces");

    //app crashes here

    3.C# implementation:

    unsafe public static extern void fdlib_detectfaces(ushort*data,int w,int h,int threshold);

    UInt16[] image = new ushort[10 * 10];
    fixed (ushort* ptr = &image[0])
    //app works fine here
    fdlib_detectfaces(ptr, 10, 10, 0);

    [EDIT: fixed code formatting, pleas use @-tags, Volker]
    [EDIT: fixed title, Andre]

  • Why don't you link against the library, if you have the headers available? This way you can use the functions directly.

    Also, the signature of the function is

    void fdlib_detectfaces(unsigned char *imagedata, int imagewidth, int imageheight, int threshold);

    but you search for


    in resolve. I suspect resolve() to return a null pointer. You did not check the function pointer!

  • Thanx 4 the reply!
    I've checked the returned function pointer and it's not null.

    Could you talk more specifically about linking against the library?
    Does it means include the header in the project?:D

  • Then you should run a debugger and look in the backtrace where the error occurs.

    And, yes, linking means including the header and link against the library.

    I suggest you make yourself comfortable with C/C++ basics first (includes, linking libs, etc.), before you dive into the real advanced topic of dynamically resolving library symbols.

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