C Allocate Memory

To allocate memory means to ask the system for a block of free space that your program can use. The most common function for this is malloc(), which stands for memory allocation. You tell it how many bytes you want, and it hands you a pointer to the start of that block.

Using malloc() and sizeof()

Because different data types take up different amounts of space, we use the sizeof operator to ask C how many bytes a type needs. Multiplying sizeof by the number of items gives the exact amount to request. This way your code works correctly no matter what computer it runs on.

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

int main() {
    int count = 5;

    // Ask for room to hold 5 ints
    int *numbers = malloc(count * sizeof(int));
    if (numbers == NULL) {
        printf("Not enough memory!\n");
        return 1;
    }

    // Use it just like a normal array
    for (int i = 0; i < count; i++) {
        numbers[i] = i * 10;
        printf("%d\n", numbers[i]);
    }

    free(numbers);
    return 0;
}

One important detail: malloc() does not clean the memory it gives you, so it may contain leftover garbage values. If you want the memory to start out filled with zeros, use calloc() instead. calloc() takes two arguments: the number of items and the size of each item.

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

int main() {
    int count = 4;

    // Ask for 4 ints, all set to 0 automatically
    int *scores = calloc(count, sizeof(int));
    if (scores == NULL) {
        return 1;
    }

    // Every value is already 0
    for (int i = 0; i < count; i++) {
        printf("scores[%d] = %d\n", i, scores[i]);
    }

    free(scores);
    return 0;
}

Growing Memory with realloc()

• realloc() lets you change the size of a block you already allocated, keeping the data that was there.
• It is handy when you discover you need more room than you first asked for.
• Always store the result of realloc() back into your pointer, because the block may move to a new location.