C Allocator that tracks alloc size

gistfile1.txt

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

typedef struct 
{ 
	size_t alloc_size;
} block_header;

void *my_alloc( size_t sz )
{
	block_header *hdr = (block_header*)malloc(sizeof(block_header) + sz );
	hdr->alloc_size = sz;
	printf("hdr is %p (%p)\n", hdr, hdr+1);
	return hdr+1;
}

size_t my_buffsize( void *ptr )
{
	block_header *hdr = (block_header*)ptr;
	return (hdr-1)->alloc_size;
}

void my_free( void *ptr )
{
	block_header *hdr = (block_header*)ptr;
	free(hdr-1);
}

int main()
{
	void *ptr = my_alloc( 8762 );
	printf("buffer size of ptr is %ld\n", my_buffsize(ptr) );
	my_free( ptr );

	return 0;
}