Pointers

After this lesson, you will be able to: Use pointers: pointer arithmetic, pointers to arrays, pointers to functions, pass-by-reference idiom.

Pointers are the source of C's power and most of its bugs. Master them.

Prerequisites:Arrays and Strings

What a pointer is

An address in memory.

tsx

int x = 42;
int *p = &x;       // p holds the address of x
printf("%d\n", *p);   // 42, dereference; read what p points at
*p = 100;          // write through p; x is now 100
printf("%d\n", x);    // 100

// Null pointer
int *np = NULL;
if (np == NULL) printf("empty\n");
// *np would crash (segfault on most systems)

Pointers + arrays

Arrays decay to pointers when passed.

tsx

int nums[] = {10, 20, 30};
int *p = nums;          // p points to nums[0]
printf("%d\n", *p);     // 10
printf("%d\n", *(p+1));  // 20, pointer arithmetic
printf("%d\n", p[1]);    // 20, same thing; nums[i] === *(nums + i)

// Pass to function, length must come separately
int sum(int *arr, size_t n) {
    int total = 0;
    for (size_t i = 0; i < n; i++) total += arr[i];
    return total;
}
sum(nums, 3);

Pass-by-reference idiom

C is pass-by-value. To modify a caller's variable, pass its address.

tsx

// WRONG, sets the local copy
void set_zero_bad(int x) { x = 0; }

// RIGHT, modifies the caller's variable
void set_zero(int *x) { *x = 0; }

int main(void) {
    int n = 5;
    set_zero_bad(n);     // n still 5
    set_zero(&n);        // n now 0
    return 0;
}

// Same pattern for swap, returning multiple values, etc.
void swap(int *a, int *b) {
    int tmp = *a;
    *a = *b;
    *b = tmp;
}

Function pointers

Pass behaviour like data, used by qsort.

tsx

#include <stdlib.h>

int compare_ints(const void *a, const void *b) {
    int ai = *(const int *)a;
    int bi = *(const int *)b;
    return (ai > bi) - (ai < bi);   // -1, 0, or 1
}

int main(void) {
    int nums[] = {3, 1, 4, 1, 5, 9, 2, 6};
    size_t n = sizeof(nums) / sizeof(nums[0]);
    qsort(nums, n, sizeof(int), compare_ints);
    return 0;
}

Common mistakes only experienced C devs avoid

Dereferencing NULL or uninitialized pointers. Pointer arithmetic on void*. Not allowed (size unknown). Returning a pointer to a stack local variable. Undefined behavior, the local is gone after return. Wild pointers (uninitialized). Always initialise to NULL.

Quick Check

Why is `int *get_local(void) { int x = 5; return &x; }` undefined behavior?

Pick the C-spec answer.

←Arrays and Strings

Back to C

Memory Management→