C Pointers

A pointer is a variable that stores the address of another variable. & gives an address; * dereferences it (accesses the value at that address).

p stores the address of x, so *p gives the value of x

#include <stdio.h>
int main() {
    int x = 42;
    int *p = &x;
    printf("x = %d, *p = %d\n", x, *p);
    *p = 100;                 // changes x through the pointer
    printf("x is now %d\n", x);

    int a[] = {10, 20, 30};
    int *q = a;
    printf("%d %d %d\n", *q, *(q + 1), q[2]);  // pointer arithmetic
    return 0;
}

x = 42, *p = 42 x is now 100 10 20 30

Dynamic Memory Allocation

Function	Purpose
`malloc(n)`	Allocate n bytes (uninitialised)
`calloc(k, size)`	Allocate k blocks, zero-initialised
`realloc(p, n)`	Resize a block
`free(p)`	Release memory

#include <stdio.h>
#include <stdlib.h>
int main() {
    int n = 5;
    int *arr = (int *) malloc(n * sizeof(int));
    if (arr == NULL) return 1;
    for (int i = 0; i < n; i++) arr[i] = (i + 1) * (i + 1);
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    free(arr);
    return 0;
}

1 4 9 16 25

Pointer pitfallsDangling pointers (used after free), memory leaks (never freed) and wild pointers (uninitialised) are common bugs.

Track your progress through the C course

← Prev Next →