function pointers in openGL

**leggy** · 04-07-2006

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?

**psychopath** · 04-07-2006

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.

**leggy** · 04-08-2006

Thanks for that, it compiles now.

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