in Code, Geeklife

Function callbacks in C

Ever since I started programming in Javascript, and doing asynchronous function calls, I’ve found myself to be addicted to passing functions as parameters.

I do it a lot in python and php, it’s very easy to do this on all these dynamic typed languages.

I never had this concept of passing functions as parameters, or pointers to functions as parameters when I was a kid in school and we were doing stuff in C or Pascal, I’d deal with it with ifs and switches.

So, this afternoon I decided to read a little bit and give it a try in C.

Here’s some code for future reference If I ever need it, it’s pretty easy.

#include 
void this() { printf("Thisn"); }
void that() { printf("Thatn"); }

int sum(int x, int y) {	return x+y; }

int mul(int x, int y) { return x*y; }

//Function that takes a callback that uses no parameters
void callanother(void (*callback)()) {
  (*callback)();
}

//Function that takes a callback that
//takes 2 int parameters and returns int
int callComplexCallback(int (*callback)(),int a, int b) {
  return (*callback)(a,b);
}

int main (int argc, char** argv) {
  callanother(this);
  callanother(that);

  printf("n");

  int w = 20;
  int h = 30;

  printf("%dn",callComplexCallback(sum,w,h));
  printf("%dn",callComplexCallback(mul,w,h));

  //this also works
  printf("%dn",callComplexCallback((*sum),w,h));
  printf("%dn",callComplexCallback((*mul),w,h));

  return 0;
}

The output is this:

~$ ./a.out 
This
That

50
600
50
600

The whole trick is how you define the function that will take the other function as a parameter.

If you have a function:

void whatever();

The function that’s supposed to use “whatever()” like-functions should look:

void useWhateverLikeFunctions(void (*f)()) {
  ...
  (*f)();
}

If you have a callback function that needs parameters, then you define the caller as:

void callerFunction(void (*f),int paramA, int* paramB, char paramC) {
  ...
  (*f)(paramA,paramB,paramC);
}

Then you’d use the function

void someCallback(int a, int* b, char c);

...
callerFunction(someCallback,a,b,c);
...

I know this is the oldest thing in the world to C programmers, but it never crossed my mind before, so here it is for my own personal reference, I hope it serves others.

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