A function’s name decays to its address. Storing that in a function pointer lets you pass behaviour as data — callbacks, dispatch tables, plug-in strategies, qsort comparators.

Syntax

C

#include <stdio.h>

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

int main(void) {
    int (*op)(int, int) = add;        /* pointer to function taking (int,int) returning int */
    printf("%d\n", op(3, 4));
    op = &mul;                        /* & is optional */
    printf("%d\n", (*op)(3, 4));      /* explicit * is optional too */
    return 0;
}

Output

7
12

Callbacks: qsort

C

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

int cmp_int_asc(const void *a, const void *b) {
    int x = *(const int *)a, y = *(const int *)b;
    return (x > y) - (x < y);         /* avoids overflow of x - y */
}

int cmp_str(const void *a, const void *b) {
    return strcmp(*(const char *const *)a, *(const char *const *)b);
}

int main(void) {
    int nums[] = {42, -7, 19, 0, 3};
    qsort(nums, 5, sizeof nums[0], cmp_int_asc);
    for (int i = 0; i < 5; i++) printf("%d ", nums[i]);
    printf("\n");

    const char *words[] = {"pear", "apple", "fig"};
    qsort(words, 3, sizeof words[0], cmp_str);
    printf("%s %s %s\n", words[0], words[1], words[2]);

    int key = 19;
    int *hit = bsearch(&key, nums, 5, sizeof nums[0], cmp_int_asc);
    printf("19 at index %td\n", hit - nums);
    return 0;
}

Output

-7 0 3 19 42
apple fig pear
19 at index 3

Your own higher-order function

C

#include <stdio.h>

typedef int (*transform_fn)(int);

int square(int x) { return x * x; }
int negate(int x) { return -x; }

void map(int *a, int n, transform_fn f) {
    for (int i = 0; i < n; i++) a[i] = f(a[i]);
}

int main(void) {
    int v[] = {1, 2, 3};
    map(v, 3, square);
    map(v, 3, negate);
    printf("%d %d %d\n", v[0], v[1], v[2]);
    return 0;
}

Output

-1 -4 -9

Dispatch tables

C

#include <stdio.h>

double add(double a, double b) { return a + b; }
double sub(double a, double b) { return a - b; }
double mul(double a, double b) { return a * b; }
double dvd(double a, double b) { return a / b; }

int main(void) {
    struct { char sym; double (*fn)(double, double); } ops[] = {
        {'+', add}, {'-', sub}, {'*', mul}, {'/', dvd},
    };
    for (int i = 0; i < 4; i++)
        printf("8 %c 2 = %g\n", ops[i].sym, ops[i].fn(8, 2));
    return 0;
}

Output

Callbacks with context

C

#include <stdio.h>

/* C has no closures: pass state through a void * "user data" pointer */
void for_each(const int *a, int n, void (*visit)(int, void *), void *ctx) {
    for (int i = 0; i < n; i++) visit(a[i], ctx);
}

void accumulate(int x, void *ctx) { *(long *)ctx += x; }

int main(void) {
    int a[] = {5, 10, 20};
    long total = 0;
    for_each(a, 3, accumulate, &total);
    printf("total = %ld\n", total);
    return 0;
}

Output

total = 35

How do you declare a function that returns a function pointer?

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