Adding swap

swap.c

@@ -0,0 +1,21 @@
+#include <stdio.h>
+
+void swap(int a, int b) {
+  int t = a;
+
+  a = b;
+  b = t;
+
+  printf("swap: a = %d, b = %d\n",a,b);
+}
+
+int main(void) {
+  int a = 21;
+  int b = 17;
+
+  printf("main: a = %d, b = %d\n",a,b);
+  
+  swap(a, b);
+
+  return 0;
+}