function pointers in openGL

This is a discussion on function pointers in openGL within the Game Programming forums, part of the General Programming Boards category; Hello. I'm trying to use openGL extension functions in a way that will compile on multiple platforms, hence I am ...

  1. #1
    tall guy
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    function pointers in openGL

    Hello.
    I'm trying to use openGL extension functions in a way that will compile on multiple platforms, hence I am using freeglut's glutGetProcAddress function. However, when I have code like the following:
    Code:
    void (*glConvolutionFilter2D)(GLenum target,
      GLenum internalformat,  GLsizei width,
      GLsizei height,  GLenum format,
      GLenum type,  const GLvoid *image) __attribute__((cdecl));
    glConvolutionFilter2D = glutGetProcAddress("glConvolutionFilter2D");
    The last line generates the compile error:
    invalid conversion from ‘void*’ to ‘void (*)(GLenum, GLenum, GLsizei, GLsizei, GLenum, GLenum, const GLvoid*)'
    I have had a look at function-pointers.org and this is what it suggests.
    What am I doing wrong?

  2. #2
    The Right Honourable psychopath's Avatar
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    glutGetProcAddress() doesn't work that way.

    In your glext.h header, there are definitions and API entrys for all of the GL extensions. the APIENTRY def for glConvolutionFilter2d() is PFNGLCONVOLUTIONFILTER2DPROC. You then use this data type to return the proc address with glutGetProcAddress().

    Code:
    PFNGLCONVOLUTIONFILTER2DPROC glConvolutionFilter2D;
    
    void setupGLExtensions()
    {
      glConvolutionFilter2D=  (PFNGLCONVOLUTIONFILTER2DPROC) glutGetProcAddress("glConvolutionFilter2D");
    }
    If you want it explained better, just let me know.
    Memorial University of Newfoundland
    Computer Science

    Mac and OpenGL evangelist.

  3. #3
    tall guy
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    Thumbs up

    Thanks for that, it compiles now.

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