Address arithmetic

Added example for address arithmetic demo using alloc

ch_05/alloc_demo/alloc.c

@@ -0,0 +1,21 @@
+#include "alloc.h"
+
+#include <stdio.h>
+
+
+char *alloc(int n)
+{
+    if (allocbuf + ALLOCSIZE - allocp >= n) {
+        allocp += n;
+        return allocp - n; /* old pointer */
+    } else {
+        return 0;
+    }
+}
+
+void afree(char *p)
+{
+    if (p >= allocbuf && p < allocbuf + ALLOCSIZE) {
+        allocp = p;
+    }
+}