Function Pointers

In a nutshell

A function pointer holds the address of a function. Declaring one requires a syntax you have not seen: int (*cmp)(const void *, const void *); reads “cmp is a pointer to a function that takes two const void * and returns int.” You assign it by writing the function name without the call parens (cmp = compare_ints;) and you call through it exactly like a regular function (int r = cmp(a, b);). The Java analogue is a lambda or functional-interface reference; the mechanical picture is “an entry point address you can pass around.”

Why it matters

Function pointers are how C does callbacks. qsort takes one. Event handlers take one. State machines use tables of them. Every C library that wants to let the caller customize behavior without rewriting the library uses function pointers at its API. They are also how C programs implement polymorphism (the Linux kernel’s VFS is a table of function pointers; so is every device driver interface).

Key takeaways

• Declaration syntax. return_type (*name)(param_types);. The parens around *name are required; without them it parses as a function returning a pointer.
• typedef makes declarations readable. typedef int (*Comparator)(const void *, const void *); then Comparator cmp = my_compare;.
• Call through is just like a regular call. cmp(a, b) or the older-syntax (*cmp)(a, b) — both work.
• Function names decay to function pointers in most expressions, similar to arrays. cmp = strcmp; works because strcmp is implicitly &strcmp in that context.
• Signatures must match exactly. A function pointer typed for int(int, int) cannot hold long foo(int, int) without an explicit cast and undefined behavior.

Lessons in this topic

Practice and deep dives

Practice this topic: Function Pointers drill, or browse the practice gallery.

What comes next

Generic Containers & qsort — putting void * and function pointers together into libraries that work on any type.