diff --git a/Makefile b/Makefile
index 6afcc54..183c8c9 100644
--- a/Makefile
+++ b/Makefile
@@ -3,7 +3,7 @@ CFLAGS=-g -Wall -Wextra -std=gnu99
 BIN_DIR=bin
 SRC_DIR=src
 TARGET=$(BIN_DIR)/netsl
-OBJECTS=$(BIN_DIR)/main.o $(BIN_DIR)/msg.o $(BIN_DIR)/net.o
+OBJECTS=$(BIN_DIR)/main.o $(BIN_DIR)/msg.o $(BIN_DIR)/net.o $(BIN_DIR)/display.o
 
 all: $(BIN_DIR) $(TARGET) 
 
diff --git a/src/display.c b/src/display.c
new file mode 100644
index 0000000..593c2a1
--- /dev/null
+++ b/src/display.c
@@ -0,0 +1,6 @@
+
+#include "display.h"
+
+void callback(long tst) {
+	printf("in callback, tst=%ld\n", tst);
+}
diff --git a/src/display.h b/src/display.h
new file mode 100644
index 0000000..c671667
--- /dev/null
+++ b/src/display.h
@@ -0,0 +1,8 @@
+#ifndef __DISPLAY
+#define __DISPLAY
+
+#include <stdio.h>
+
+void callback(long tst);
+
+#endif
diff --git a/src/main.c b/src/main.c
index 9e20df9..28365da 100644
--- a/src/main.c
+++ b/src/main.c
@@ -5,6 +5,7 @@
 
 #include "misc.h"
 #include "net.h"
+#include "display.h"
 
 #define MODE_SERVER 0
 #define MODE_CLIENT 1
@@ -57,9 +58,6 @@ int parseArgs(struct prog_info *pinfo, int argc, char **argv) {
 	return 0;
 }
 
-void callback(long tst) {
-	printf("in callback, tst=%ld\n", tst);
-}
 
 int main(int argc, char **argv) {
 
diff --git a/src/msg.h b/src/msg.h
index eb7b8ee..24f14a9 100644
--- a/src/msg.h
+++ b/src/msg.h
@@ -1,3 +1,5 @@
+#ifndef __MSG
+#  define __MSG
 
 //typedef struct Buffer {
 //	int size;
@@ -23,3 +25,5 @@ void serialize_int(int x,Buffer *b);
 void serialize_string(char *str,Buffer *b);
 void serialize_message(struct message *msg,Buffer *b);
 */
+
+#endif
diff --git a/src/net.h b/src/net.h
index 76353eb..f396641 100644
--- a/src/net.h
+++ b/src/net.h
@@ -1,6 +1,10 @@
+#ifndef __NET
+#  define __NET
+
 #include <sys/socket.h> // struct sockaddr
 
 void *get_in_addr(struct sockaddr *sa);
 int run_server(const struct prog_info *pinfo);
 int run_client(const struct prog_info *pinfo,void(*framecallback)(long));
 
+#endif