Stores the size of allocated memory before the void * given by memory allocation functions. You can get it easily with msize();

sizeof-heap-var.c

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

void *__malloc__(size_t size) {
	size_t *memory;
	
	memory = (size_t*)malloc(sizeof(size_t) + size);
	if (memory == NULL) return NULL;
	
	*memory = size;
	memory += 1;
	
	return (void*)memory;
}

void *__calloc__(size_t num, size_t size) {
	size_t *memory;
	
	memory = (size_t*)calloc(num, size + sizeof(size_t));
	if (memory == NULL) return NULL;
	
	*memory = num * size;
	memory += 1;
	
	return (void*)memory;
}

void __free__(void *memory) {
	size_t *ptr;
	
	ptr = (size_t*)memory;
	ptr -= 1;
	
	free(ptr);
}

void *__realloc__(void *memory, size_t newSize) {
	size_t *ptr;
	
	ptr = (size_t*)memory;
	ptr -= 1;
	
	ptr = (size_t*)realloc(ptr, newSize + sizeof(size_t));
	if (ptr == NULL) return NULL;
	
	*ptr = newSize;
	ptr += 1;
	
	return (void*)ptr;
}

size_t msize(void *memory) {
	size_t *ptr;
	
	ptr = (size_t*)memory;
	ptr -= 1;
	
	return *ptr;
}

#define malloc(size) 			 __malloc__(size)
#define calloc(num, size) 		 __calloc__(num, size)
#define free(memory) 			 __free__(memory)
#define realloc(memory, newSize) 	 __realloc__(memory, newSize)

/*///////////////////////////////////
	
		Testing:
		
///////////////////////////////////*/

#include <string.h>

int main(void) {
	
	char *str = malloc(1024);
	
	strcpy(str, "Hello, world!");
	
	printf("%s\n", str);
	printf("%i\n", msize(str));
	
	return 0;
}