Reading Function Pointer Types

A function has an address too

Code lives in memory. A function pointer stores the address of the code — which is all a callback is: "call whatever code this address names."

The syntax reads inside-out:

int (*fp)(int, int);        /* fp is a pointer to a function taking two ints
                               and returning int */
int *fp(int, int);          /* WITHOUT parens: a function returning int* —
                               entirely different! */

The parentheses around (*fp) are not decoration; they bind fp to "pointer" before int can bind to "returning". C precedence has opinions.

typedef makes it humane

typedef int (*Compare)(const void *, const void *);
Compare cmp = my_compare;   /* now it reads like any other type */

Every C codebase with callbacks defines a typedef. Raw function-pointer syntax in an API is a code smell.

The name of a function decays to a pointer

int add(int a, int b) { return a + b; }
int (*fp)(int, int) = add;     /* &add is the same address */
int v = fp(3, 4);              /* call through the pointer: 7 */
int w = (*fp)(3, 4);           /* identical: explicit deref is optional */

Both fp(...) and (*fp)(...) work — the standard defines the call operator to handle either. Pick one style; mixing reads badly.

Where they live

Function pointers can be parameters, struct members, array elements, return values — full citizens:

typedef struct {
    const char *name;
    int (*op)(int, int);
} BinOp;

Check your understanding

• What is char *(*maker)(size_t)? (Pointer to function taking size_t, returning char*.)
• Is fp == &fp legal? (Trick: no — &fp is the address of the pointer variable; fp is the code address it holds. But fp == add and *fp == add both hold.)