add exercise of lab5 spoc discussion

related_info/lab5/lab5-spoc-discuss/user/libs/ulib.c

@@ -0,0 +1,48 @@
+#include <defs.h>
+#include <syscall.h>
+#include <stdio.h>
+#include <ulib.h>
+
+void
+exit(int error_code) {
+    sys_exit(error_code);
+    cprintf("BUG: exit failed.\n");
+    while (1);
+}
+
+int
+fork(void) {
+    return sys_fork();
+}
+
+int
+wait(void) {
+    return sys_wait(0, NULL);
+}
+
+int
+waitpid(int pid, int *store) {
+    return sys_wait(pid, store);
+}
+
+void
+yield(void) {
+    sys_yield();
+}
+
+int
+kill(int pid) {
+    return sys_kill(pid);
+}
+
+int
+getpid(void) {
+    return sys_getpid();
+}
+
+//print_pgdir - print the PDT&PT
+void
+print_pgdir(void) {
+    sys_pgdir();
+}
+