Function Pointers

A function pointer is a pointer that stores the address of a function. You use it to switch which function to call at runtime, or to implement callbacks by passing a function as an argument.

Syntax

// Declare a function pointer type.
// return_type (*pointer_name)(parameter_type, ...);
int (*fp)(int, int);

// Assign a function's address (the function name itself is the address).
fp = function_name;

// Call a function through the function pointer.
fp(arg1, arg2);
(*fp)(arg1, arg2); // Explicit dereference — works the same way.

// Using typedef to give a function pointer type a readable name.
typedef return_type (*TypeName)(parameter_type, ...);
TypeName variable_name = function_name;

Uses of Function Pointers

Use	Description
Callbacks	Inject part of the logic from the caller. Commonly used in sorting, filtering, and event handling.
Dispatch table	Create an array of function pointers and switch which function to call by index. Can serve as an alternative to switch statements.
Strategy pattern	Swap out algorithms via function pointers to give structs different behaviors.
qsort / bsearch	These standard library functions accept a comparison function as a function pointer to sort or search data of any type.

Sample Code

#include <stdio.h>
#include <stdlib.h>

// Define the functions to operate on.
int add(int a, int b) { return a + b; }
int sub(int a, int b) { return a - b; }
int mul(int a, int b) { return a * b; }

// A callback function that takes a function pointer as an argument.
void apply(int x, int y, int (*op)(int, int), const char *name) {
    printf("%s(%d, %d) = %d\n", name, x, y, op(x, y));
}

// Use typedef for readability.
typedef int (*BinaryOp)(int, int);

// Comparison function for qsort (ascending order).
int compare_asc(const void *a, const void *b) {
    return (*(int *)a - *(int *)b);
}

// Comparison function for qsort (descending order).
int compare_desc(const void *a, const void *b) {
    return (*(int *)b - *(int *)a);
}

int main(void) {
    // Assign a function to a function pointer and call it.
    int (*fp)(int, int) = add;
    printf("add(3, 5) = %d\n", fp(3, 5)); // Prints: add(3, 5) = 8

    fp = sub;
    printf("sub(10, 3) = %d\n", fp(10, 3)); // Prints: sub(10, 3) = 7

    // Pass functions as callbacks.
    apply(6, 2, add, "add"); // Prints: add(6, 2) = 8
    apply(6, 2, sub, "sub"); // Prints: sub(6, 2) = 4
    apply(6, 2, mul, "mul"); // Prints: mul(6, 2) = 12

    // Dispatch table (array of function pointers).
    BinaryOp ops[] = {add, sub, mul};
    const char *names[] = {"add", "sub", "mul"};
    for (int i = 0; i < 3; i++) {
        printf("%s(10, 4) = %d\n", names[i], ops[i](10, 4));
    }

    // Use function pointers with qsort.
    int arr[] = {5, 1, 4, 2, 3};
    qsort(arr, 5, sizeof(int), compare_asc);
    printf("Ascending: ");
    for (int i = 0; i < 5; i++) printf("%d ", arr[i]); // Prints: 1 2 3 4 5
    printf("\n");

    qsort(arr, 5, sizeof(int), compare_desc);
    printf("Descending: ");
    for (int i = 0; i < 5; i++) printf("%d ", arr[i]); // Prints: 5 4 3 2 1
    printf("\n");

    return 0;
}

Notes

Function pointers are how C implements higher-order functions — the ability to pass functions as arguments or return values. The standard library functions qsort() and bsearch() accept a comparison function via a function pointer, which allows them to sort or search data of any type generically.

Using typedef to give a function pointer type a meaningful name greatly improves code readability. Parentheses placement in function pointer declarations is critical: int *fp(int) declares a function that returns int *, while int (*fp)(int) declares a pointer to a function — they mean completely different things.

Embedding function pointers in structs enables object-oriented-style design in C. For more on structs, see struct. For type aliases, see typedef.

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C Language Dictionary

Function Pointers

Syntax

Uses of Function Pointers

Sample Code

Notes