add exercise of lab5 spoc discussion

related_info/lab5/lab5-spoc-discuss/user/exit.c

@@ -0,0 +1,34 @@
+#include <stdio.h>
+#include <ulib.h>
+
+int magic = -0x10384;
+
+int
+main(void) {
+    int pid, code;
+    cprintf("I am the parent. Forking the child...\n");
+    if ((pid = fork()) == 0) {
+        cprintf("I am the child.\n");
+        yield();
+        yield();
+        yield();
+        yield();
+        yield();
+        yield();
+        yield();
+        exit(magic);
+    }
+    else {
+        cprintf("I am parent, fork a child pid %d\n",pid);
+    }
+    assert(pid > 0);
+    cprintf("I am the parent, waiting now..\n");
+
+    assert(waitpid(pid, &code) == 0 && code == magic);
+    assert(waitpid(pid, &code) != 0 && wait() != 0);
+    cprintf("waitpid %d ok.\n", pid);
+
+    cprintf("exit pass.\n");
+    return 0;
+}
+