private interface with parameters in va

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I want to add a private interface in standard libva but vaGetLibFunc only return a pointer that has no prarameters ,so how can I get a private function with parameters

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may be

typedef int (*VAPrivFunc)(void) VAPrivFunc vaGetLibFunc ( VADisplay dpy, const char *func );

To

typedef int (VAPrivFunc pointer)(void pointer)

VAPrivFunc vaGetLibFunc ( VADisplay dpy, const char *func );

is better

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why pointer symbol can not be showed