Address Arithmetic of K&R

alloc_afree.c

#include <stdio.h>
#include <string.h>
#define ALLOCSIZE 10000

static char allocbuf[ALLOCSIZE];
static char *allocp = allocbuf;

char *alloc(int n)
{
	if (allocbuf + ALLOCSIZE - allocp < n) {
		return 0;
	}

	char *oldp = allocp;
	allocp += n;
	return oldp;
}

void afree(char *p)
{
	if (allocbuf <= p && p < allocbuf + ALLOCSIZE) {
		allocp = p;
	}
}