diff --git a/LICENSE.txt b/LICENSE.txt
index ffe4849..d5a757c 100644
--- a/LICENSE.txt
+++ b/LICENSE.txt
@@ -3,9 +3,12 @@ Copyright License
 ==========================================================================
 The PKE software is:
 
-Copyright (c) 2021, Zhiyuan Shao (zyshao@hust.edu.cn), 
-                    Yi Gui (gy163email@163.com), 
-                    Yan Jiao (773709579@qq.com),
+Copyright (c) 2021, Zhiyuan Shao (zyshao@hust.edu.cn),
+                    Yi Gui (gy163email@163.com),
+                    Ziming Yuan (1223962053@qq.com),
+                    Yixin Song (yixinsong@hust.edu.cn),
+                    Boyang Li (liboyang_hust@163.com),
+                    Zichen Xu (xuzichen@hust.edu.cn),
                    Huazhong University of Science and Technology
 
 Permission is hereby granted, free of charge, to any person obtaining
diff --git a/Makefile b/Makefile
index 5ed973d..4edd2a4 100644
--- a/Makefile
+++ b/Makefile
@@ -20,7 +20,7 @@ ifneq (,)
   mabi := -mabi=$(if $(is_32bit),ilp32,lp64)
 endif
 
-CFLAGS        := -Wall -Werror  -fno-builtin -nostdlib -D__NO_INLINE__ -mcmodel=medany -g -Og -std=gnu99 -Wno-unused -Wno-attributes -fno-delete-null-pointer-checks -fno-PIE $(march)
+CFLAGS        := -Wall -Werror  -fno-builtin -nostdlib -D__NO_INLINE__ -mcmodel=medany -g -Og -std=gnu99 -Wno-unused -Wno-attributes -fno-delete-null-pointer-checks -fno-PIE $(march) -fno-omit-frame-pointer
 COMPILE       	:= $(CC) -MMD -MP $(CFLAGS) $(SPROJS_INCLUDE)
 
 #---------------------	utils -----------------------
@@ -71,7 +71,7 @@ USER_OBJS  		:= $(addprefix $(OBJ_DIR)/, $(patsubst %.c,%.o,$(USER_CPPS)))
 
 
 
-USER_TARGET 	:= $(OBJ_DIR)/app_long_loop
+USER_TARGET 	:= $(OBJ_DIR)/app_print_backtrace
 #------------------------targets------------------------
 $(OBJ_DIR):
 	@-mkdir -p $(OBJ_DIR)	
diff --git a/user/app_long_loop.c b/user/app_long_loop.c
deleted file mode 100644
index 8b4d873..0000000
--- a/user/app_long_loop.c
+++ /dev/null
@@ -1,20 +0,0 @@
-/*
- * Below is the given application for lab1_3.
- * This app performs a long loop, during which, timers are
- * generated and pop messages to our screen.
- */
-
-#include "user_lib.h"
-#include "util/types.h"
-
-int main(void) {
-  printu("Hello world!\n");
-  int i;
-  for (i = 0; i < 100000000; ++i) {
-    if (i % 5000000 == 0) printu("wait %d\n", i);
-  }
-
-  exit(0);
-
-  return 0;
-}
diff --git a/user/app_print_backtrace.c b/user/app_print_backtrace.c
new file mode 100644
index 0000000..588cfb5
--- /dev/null
+++ b/user/app_print_backtrace.c
@@ -0,0 +1,23 @@
+/*
+ * Below is the given application for lab1_challenge1_backtrace.
+ * This app prints all functions before calling print_backtrace().
+ */
+
+#include "user_lib.h"
+#include "util/types.h"
+
+void f8() { print_backtrace(7); }
+void f7() { f8(); }
+void f6() { f7(); }
+void f5() { f6(); }
+void f4() { f5(); }
+void f3() { f4(); }
+void f2() { f3(); }
+void f1() { f2(); }
+
+int main(void) {
+  printu("back trace the user app in the following:\n");
+  f1();
+  exit(0);
+  return 0;
+}