calloc and realloc

Learn memory management with calloc and realloc.

calloc: allocating zeroed memory

calloc also allocates memory on the heap like malloc; but it zeroes the allocated area. It comes with <stdlib.h>.

int *arr = calloc(3, sizeof(int));
// arr[0], arr[1], arr[2] are all 0

It takes two arguments: the element count and the size of each element. malloc takes the total byte count in one argument, while calloc splits it into two. Zero initialization is useful for arrays and counters; it avoids surprises from garbage data.

Resizing with realloc

realloc grows or shrinks previously allocated memory. It preserves the content; if needed, it moves the data to a new location.

int *arr = malloc(2 * sizeof(int));
arr[0] = 1;
arr[1] = 2;
arr = realloc(arr, 4 * sizeof(int));
arr[2] = 3;
arr[3] = 4;

Here the array starts with 2 elements; realloc makes it 4 elements. The first two are preserved, the last two are explicitly assigned. realloc may invalidate the old address; so always assign the return value back to the same pointer.

Exercise Print a zeroed array with calloc

Allocate an array of 3 ints with calloc (initial values become zero). Print the three elements with printf. Finally release the memory with free.

Show example solution

int *arr = calloc(3, sizeof(int));
printf("%d %d %d\n", arr[0], arr[1], arr[2]);
free(arr);

Test yourself

What is the main difference between calloc and malloc?
- calloc zeroes memory, malloc does not
- calloc is slower
- calloc is faster
- calloc needs no free
Show answer

calloc zeroes memory, malloc does not — calloc zeroes memory; malloc leaves old data.
Match function to property

malloc

Does not zero
calloc

Zeroes
realloc

Resizes
free

Releases

Fill in the blank: calloc call

int *arr = ____(3, sizeof(int));

Show answer

int *arr = calloc(3, sizeof(int));

Next lesson: Summary - Memory and stack

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