Objective-C callback block for BASS FX

Question

I'm trying to use the audio library BASS in my application. I'm trying to use a method which triggers a callback when it detects a beat in the music.

This is my current code:

void* (^myBlock)(DWORD, double, void *) = ^(DWORD handle, double time, void *user) {
    return nil;
};

BASS_FX_BPM_BeatDecodeGet(bpmStream, 0.0, playBackDuration, BASS_FX_BPM_BKGRND, myBlock,NULL);

The callback is defined in the header file as:

typedef void (CALLBACK BPMBEATPROC)(DWORD chan, double beatpos, void *user);

The error message is:

Passing 'void *(^)(DWORD, double, void *)' to parameter of incompatible type 'BPMBEATPROC *' (aka 'void (*)(DWORD, double, void *)')

I'm pretty sure the block only needs a small modification, but I'm not familiar with Objective-C.

your block is returning a `void*` when the typedef is expecting just `void`. — Daniel A. White, Jan 30 '15 at 20:24
@Razib He is asking why he can't pass his block as a callback. — NobodyNada, Jan 30 '15 at 20:28
@Razib Yes, sorry. I would like to know how to change the block so it will work. I changed void* to void. This is the new error message: Passing 'void (^)(DWORD, double, void *)' to parameter of incompatible type 'BPMBEATPROC *' (aka 'void (*)(DWORD, double, void *)') — LucidLime, Jan 30 '15 at 20:32

score 0 · Accepted Answer · edited May 23 '17 at 11:49

You are passing a block that returns a void* to a parameter that expects a function pointer that returns nothing (void).

You need to declare your block as a function returning void and pass it as the callback:

void myFunction(DWORD handle, double time, void *user) {
    //function code here
}

And then pass it to the library just like the block:

BASS_FX_BPM_BeatDecodeGet(bpmStream, 0.0, playBackDuration, BASS_FX_BPM_BKGRND, myFunction,NULL);

For more information about function pointers, see How do function pointers in C work?.

Objective-C callback block for BASS FX

1 Answers1