Callback without function pointer

It is usually said callbacks are implemented with function pointers. When I check PortAudio's source code, I see that callback function is declared as an ordinary function (not a f. pointer). Is it n开发者_如何学Goormal/legal/advisable?

typedef int PaStreamCallback(
const void *input, void *output,
unsigned long frameCount,
const PaStreamCallbackTimeInfo* timeInfo,
PaStreamCallbackFlags statusFlags,
void *userData );

It is fine as long as the parameter is used as PaStreamCallback* (which is a pointer to a function), like

PaError Pa_OpenStream   (
        PaStream **      stream,
        const PaStreamParameters *      inputParameters,
        const PaStreamParameters *      outputParameters,
        double      sampleRate,
        unsigned long   framesPerBuffer,
        PaStreamFlags   streamFlags,
        PaStreamCallback *      streamCallback,   // <---
        void *      userData     
    )