add exercise of lab5 spoc discussion

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

@@ -0,0 +1,22 @@
+#include <stdio.h>
+#include <ulib.h>
+
+int
+main(void) {
+    int pid, exit_code;
+    if ((pid = fork()) == 0) {
+        cprintf("fork ok.\n");
+        int i;
+        for (i = 0; i < 10; i ++) {
+            yield();
+        }
+        exit(0xbeaf);
+    }
+    assert(pid > 0);
+    assert(waitpid(-1, NULL) != 0);
+    assert(waitpid(pid, (void *)0xC0000000) != 0);
+    assert(waitpid(pid, &exit_code) == 0 && exit_code == 0xbeaf);
+    cprintf("badarg pass.\n");
+    return 0;
+}
+