Function Pointers in C

Function pointers are like having a pet dragon; dangerous, unpredictable, and often unnecessary. So, if you find yourself dealing with function pointers, embrace the chaos and remember to keep a fire extinguisher handy! For your difficult journey ahead, here is a brief reference guide.

A very basic example of how you can assign a pointer to a function.

void (*FunctionPtr)()= &Function;

Here FunctionPtr points to the adress of Function of type void with no arguments. It can be invoked by simply calling (*FunctionPtr)().

void (*FunctionPtr)()=Function;

This is also a valid statement, behaves exactly the same as previous one. In fact, all of the statements below behave the same.

void (*FunctionPtr1)() = Function;
void (*FunctionPtr2)() = &Function;
void (*FunctionPtr1)() = *Function;
void (*FunctionPtr2)() = *******Function;

or alternatively and interestingly with typedef

typedef void (FunctionType) ();
typefef void (*FunctionPtrType)();

FunctionType FunctionPtr = Function; // <-- This is an error
FunctionType* FubctionPtr = Function;
FunctionPtrType FunctionPtr = *Function;

Hence, can be invoked as follows

(*FunctionPtr)();
FunctionPtr();
(**FunctionPtr)();

In short, dereferencing a function or address of a function evaluates to the pointer to that function.

On an unrelated note, typedef void (FunctionType)() can be used to declare a function of the same type, so FunctionType Function evaluates to void Function().

You can also do a bit of hack to force a pointer to take a value of different type than it’s own. It’s useful invoking symbols from dynamic libraries.

int (*funcPtr)();
int value;
void (*funcPtr2)() = function;

*(void**)(&funcPtr) = funcPtr2;

value = functionPtr2();

As you can see funcPtr2 is of type void but can be assigned to a function pointer of type int. It is useful to silence the compiler warnings for mismatch of pointer types i-e void * to int(*functionPtr).

You can also return a function pointer of any type, preferred way to do it is with typedef, consider this:

typedef int(*functPtrType)(int,int)

int add(int a, int b)
{
    return a+b;
}

funcPtrType returnfunction(functPtrType funcPtr)
{
    return funcPtr;
}

/* In another function you can call
funcPtr2=returnfunction(add);
*/

or another way to go about this is without using typedef

int add(int a, int b)
{
    return a+b;
}

int(*returnfunction(int(*funcPtr)(int,int))(int,int)
{
    return funcPtr;
}

This code is equivalent to what is written above.

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