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;
+    }
+}

ch_05/alloc_demo/alloc.h

@@ -0,0 +1,10 @@
+
+#define ALLOCSIZE 9500
+
+static char allocbuf[ALLOCSIZE];
+static char *allocp = allocbuf;
+
+
+char *alloc(int n);
+
+void afree(char *p);

ch_05/alloc_demo/main.c

@@ -0,0 +1,69 @@
+/*
+ *
+ * Rudimentary memory allocation demo
+ *
+ * Author: Kun Deng
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+#include "alloc.h"
+
+
+void populate_ptr(char *ptr, const int size);
+void print_ptr(char *ptr, const int size);
+
+
+void populate_ptr(char *ptr, const int size)
+{
+    for (int i = 0; i < size; ++i) {
+        switch (i) {
+            case 0:
+                *(ptr+i) = '1';
+                break;
+            case 1:
+                *(ptr+i) = '9';
+                break;
+            case 2:
+                *(ptr+i) = '7';
+                break;
+            case 3:
+                *(ptr+i) = '2';
+                break;
+            default:
+                *(ptr+i) = ' ';
+                break;
+        }
+    }
+}
+
+void print_ptr(char *ptr, const int size)
+{
+    for (int i = 0; i < size; ++i) {
+        printf("index: %d ptr value: %c\n", i, *(ptr+i));
+    }
+
+    if (strcmp(ptr, "1972") == 0) {
+        printf("Thick as a Brick and Close to the Edge... Yes, %s was a good year for music\n", ptr);
+    } else {
+        printf("Full value: %s\n", ptr);
+    }
+}
+
+
+int main()
+{
+    printf("alloc demo\n");
+
+    const int ptr_size = 4;
+    char *sequence = alloc(ptr_size);
+
+    populate_ptr(sequence, ptr_size);
+    print_ptr(sequence, ptr_size);
+
+    afree(sequence);
+
+
+    return 0;
+}