C function pointer and typedef

Question

I have been given a code. I need to use the following function but I am having problems calling it.

int myFunction(const struct LQuery *pQuery, 
                   f_Callback Callback,
                   const void *pPriv);

And I have the following definition

typedef void (f_Callback) (int NumOfRecordsFound,
                          void *pPriv,
                          const tL_QueryResult *pData);

I understand that Callback should be the the pointer to my_callbackFunction but I can't seem to understand how to set the parameters to it. As far as I understand pQuery is passed to the callback function but f_Callback requires 3 parameters.

ie

int main (int Argc, char *pArgv[])
{
    myFunction(what should go here and WHY)
}

Answer 1

The callback function will be called from the myFunction . You have to pass just your callback function as a parameter. Here's an example:

#include <stdio.h>

typedef void (f_Callback)(int a,
                          int b,
                          int c);

int myFunction(int a, f_Callback* callback)
{
    callback(a, a+1, a+2);
}

void myCallback(int a, int b, int c)
{
    printf("a: %d, b: %d, c: %d\n", a, b, c);
}

int main()
{
    myFunction(5, &myCallback);
    return 0;
}

Expected output:

$./a.out
a: 5, b: 6, c: 7

Answer 2

The typedef is incorrect. It should be

typedef void (*f_Callback) (
              int NumOfRecordsFound,
              f_Callback Callback,
              const void *pPriv);

which is a proper pointer-to-function - note the star; otherwise it will declare a typedef for a function prototype, which you cannot use as a variable as such, though it will work in a function declaration.

If you have a function like

int myFunction(const struct LQuery *pQuery, 
               f_Callback Callback,
               const void *pPriv);

And a callback

 void my_callback_function(int records_found, 
                           f_Callback callback, 
                           const void *priv)
 {
      ...
 }

You can call myfunction simply as

myFunction(the_query, my_callback_function, the_private_pointer);

The lvalue my_callback_function will decay into a pointer-to-function, so no & character is needed.