C function pointers

Question

static void increment(long long *n){
  (*n)++;
}

struct test{
  void (*work_fn)(long long *);
};

struct test t1;

t1.work_fn = increment;

How do I ac

Answer 1

Yes, that's how to call it. Function names and variables containing function pointers are essentially the same thing.

Answer 2

How do I actually call the function now? t1.work_fn(&n) ?

That'll work just fine.

Function pointers don't need to be explicitly dereferenced. This is because even when calling a function normally (using the actual name of the function), you're really calling it through the pointer to the function. C99 6.5.22 "Function calls" says (emphasis mine):

The expression that denotes the called function (footnote 77) shall have type pointer to function returning void or returning an object type other than an array type

Footnote 77:

Most often, this is the result of converting an identifier that is a function designator.

Note that you still can dereference the function pointer (or a normal function name - though I think you'd cause much confusion doing so) to call a function because C99 6.5.3.2/4 "Address and indirection operators" says:

The unary * operator denotes indirection. If the operand points to a function, the result is a function designator

So all of these will end up doing the same thing (though the compiler might not be able to optimize the calls-through t1.work_fn as well):

t1.work_fn(&n);
(*t1.work_fn)(&n);

increment(&n);
(*increment)(&n);

Answer 3

You can call it as t1.work_fn(&n) or as (*t1.work_fn)(&n), whichever you prefer.

Symmetrically, when assigning the pointer you can do either t1.work_fn = increment or t1.work_fn = &increment. Again, it is a matter of personal coding style.

One can probably argue that for the sake of consistency one should stick to either "minimalistic" style

t1.work_fn = increment;
t1.work_fn(&n);

or to a "maximalistic" style

t1.work_fn = &increment;
(*t1.work_fn)(&n);

but not a mix of the two, so that we can have well-defined holy wars between two distinctive camps instead of four.

P.S. Of course, the "minimalistic" style is the only proper one. And one must crack eggs on the pointy end.