How do C library writers support callbacks?

This is common pattern of design that one sees with C libraries to solve this problem:

The library writer needs to make it possible for the user of the library to consume data coming back from say:
- Reading off a socket or file.
- Parsing a document - say XML or JSON
- etc.

So generally the user of the library needs to:

Receive callbacks from the library
Have a means of putting the information from those callbacks in an appropriate part of the users application.

So the common way we do it in C is this, here is the API:

/* C API */

typedef void (CLIBcallback*)(void* pUserPointer, const char* pSomeDataInCallback);
void CLIBstartProcess(void* pUserPointer, CLIBcallback* pCallback);

And this is how a user application might use this API.

struct FOOconsumer{
   int Count;
};

void FOOcallback(void* pUserPointer, const char* pSomeDataInCallback);

int main(){
    FOOconsumer Foo;
    Foo.Count = 0;
    CLIBstartProcess((void*)&Foo, &FOOcallback);
}

void FOOcallback(void* pUserPointer, const char* pSomeDataInCallback){
   FOOconsumer* pConsumer = (FOOconsumer*)pUserPointer; 
   pConsumer.Count = pConsumer.Count + 1;
}

See how:

We use the pUserPointer to give the address of the Foo instance of the FOOconsumer struct
The library writer passes this through to the callback function as void* pointer
The application writer can then cast back to a FOOconsumer pointer
And thus consume the information.