calloc.c

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#include <stdbool.h>
#include <stdlib.h>
#include <string.h>

/*
 * This is sqrt(SIZE_MAX+1), as s1*s2 <= SIZE_MAX
 * if both s1 < MUL_NO_OVERFLOW and s2 < MUL_NO_OVERFLOW
 */
#define MUL_NO_OVERFLOW (1UL << (sizeof(size_t) * 4))

void* calloc(size_t num, size_t size)
{
    /* num * size unsigned integer wrapping check */
    if((num >= MUL_NO_OVERFLOW || size >= MUL_NO_OVERFLOW) && num > 0 && SIZE_MAX / num < size)
    {
        return NULL;
    }

    size_t total_size = num * size;
    void* ptr = malloc(total_size);

    if(ptr)
    {
        memset(ptr, 0, total_size);
    }

    return ptr;
}