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Dynamic Memory

Allocate memory at run time with malloc and release it with free.

Ordinary arrays need their size fixed when you write the program. When the size is only known while the program runs, for example after asking the user how many items there are, you can request memory from the heap with malloc from stdlib.h.

C
Open in playground
#include <stdio.h>
#include <stdlib.h>

int main(void) {
    int n = 5;
    int *squares = malloc(n * sizeof(int));
    if (squares == NULL) {
        printf("Out of memory\n");
        return 1;
    }

    for (int i = 0; i < n; i++) {
        squares[i] = (i + 1) * (i + 1);
    }
    for (int i = 0; i < n; i++) {
        printf("%d ", squares[i]);
    }
    printf("\n");

    free(squares);   /* give the memory back */
    return 0;
}
Output
1 4 9 16 25
  • malloc(bytes) returns a pointer to a new block, or NULL if it fails.
  • The block can be used just like an array.
  • free(pointer) returns the memory when you no longer need it.

calloc and realloc

calloc allocates memory and sets it all to zero. realloc resizes a block you already have, keeping its contents.

C
Open in playground
#include <stdio.h>
#include <stdlib.h>

int main(void) {
    int size = 2;
    int *list = calloc(size, sizeof(int));
    if (list == NULL) return 1;

    printf("after calloc: %d %d\n", list[0], list[1]);

    int *bigger = realloc(list, 4 * sizeof(int));
    if (bigger == NULL) {
        free(list);
        return 1;
    }
    list = bigger;
    list[2] = 30;
    list[3] = 40;
    for (int i = 0; i < 4; i++) {
        printf("%d ", list[i]);
    }
    printf("\n");

    free(list);
    return 0;
}
Output
after calloc: 0 0
0 0 30 40

Rules to remember

  • Every malloc, calloc or realloc must be matched by exactly one free.
  • Forgetting to free causes a memory leak: the program holds memory it can never use again.
  • Never use a pointer after freeing it, and never free the same block twice.
  • Setting a pointer to NULL right after free makes accidental reuse easier to spot.

GATE-style practice questions

GATE-styleNAT · numerical answer

What does the following program print?

#include <stdio.h>
#include <stdlib.h>
int main(void) {
    int *a = malloc(4 * sizeof(int));
    for (int i = 0; i < 4; i++) a[i] = i * i;
    int *b = realloc(a, 6 * sizeof(int));
    b[4] = 16;
    b[5] = 25;
    int s = 0;
    for (int i = 0; i < 6; i++) s += b[i];
    printf("%d\n", s);
    free(b);
    return 0;
}
Show answer and explanationAnswer: 55

malloc gives room for 4 ints, filled with 0, 1, 4, 9. realloc grows the block to 6 ints and keeps the old values; b[4] and b[5] become 16 and 25. The sum is 0 + 1 + 4 + 9 + 16 + 25 = 55.

GATE-styleMSQ · one or more correct answers

Which statements about dynamic memory in C are true? (Select all that apply.)

Show answer and explanationAnswer: A, B, D

free releases the memory but leaves p holding the old address (a dangling pointer), which is why setting p = NULL afterwards is good practice. The other statements are true.