diff --git a/server.c b/server.c
index f0dddb9..1f9e165 100644
--- a/server.c
+++ b/server.c
@@ -17,8 +17,11 @@
 /*========
  *Operation
  *========*/
-/*send user error message
-80 character limit
+
+/**
+ * send user error message
+ * 80 character limit
+ * thread safe
 */
 int send_fail(int sock,const char * msg){
     struct TransferResult res;
@@ -37,7 +40,10 @@ int send_fail(int sock,const char * msg){
     }
     return 0;
 }
-
+/**
+ * send errno to client
+ * thread safe
+ */
 int send_errno(int sock){
     struct TransferResult r;
     r.res = RES_ERR;
@@ -50,8 +56,9 @@ int send_errno(int sock){
     }
     return 0;
 }
-/* return fd, if success.
-otherwise, return -1.
+/**
+ * return fd, if success. otherwise, return -1.
+ * thread safe
 */
 int read_request(int sock,uint8_t * buf,size_t bufsize){
     struct ReadOp p;
@@ -62,7 +69,7 @@ int read_request(int sock,uint8_t * buf,size_t bufsize){
         else perror("receive fail"); 
         return -1;
     }
-    if(bufsize <= p.file_url_size + sizeof(p) + 1){
+    if(bufsize <= ((size_t)p.file_url_size) + sizeof(p) + 1){
         send_fail(sock,"buffer overflow");
         return -1;
     }
@@ -75,8 +82,8 @@ int read_request(int sock,uint8_t * buf,size_t bufsize){
         return -1;
     }
     n = recv_until_byte(sock,buf,p.file_url_size,TIMEOUT);
-    buf[p.file_url_size] = '\0';
-    printf("str size: %d, request %s\n",p.file_url_size,buf);
+    buf[p.file_url_size] = '\0';    //truncate
+    fprintf(stdout,"str size: %d, request %s\n",p.file_url_size,buf);
     if(strchr((char *)buf,'/') != NULL){
         send_fail(sock,"Illegal character /");
         return -1;
@@ -89,7 +96,10 @@ int read_request(int sock,uint8_t * buf,size_t bufsize){
     }
     return fd;
 }
-
+/**
+ * send response to client
+ * thread safe
+*/
 int send_response(int sock,int fd, uint8_t * buf, size_t bufsize){
     struct TransferResult r;
     struct stat st;
diff --git a/socket_wrapper.c b/socket_wrapper.c
index fc7d19c..dab2d4f 100644
--- a/socket_wrapper.c
+++ b/socket_wrapper.c
@@ -11,6 +11,7 @@
 int getBufferSizeFrom(int sock){
     int buffer_sz;
     socklen_t len = sizeof(buffer_sz);
+    //getsockopt is thread-safe
     if(getsockopt(sock,SOL_SOCKET,SO_SNDBUF,&buffer_sz,&len) < 0){
         perror("failed to get sock buffer size: getsockopt");
         buffer_sz = DEFAULT_BUF_SIZE;/*set to default*/
@@ -37,11 +38,13 @@ ssize_t timeout_recv(int fd,void * buf,size_t n,int timeout)
     case POLLNVAL:
         return -1;
     case POLL_IN:
-        return recv(fd,buf,n,0);
+        ret = recv(fd,buf,n,0);
+        assert(ret != 0);
+        return ret;
     default:
         assert(0 && "unreachable");
     }
-    return ret;
+    assert(0 && "unreachable");
 }
 
 ssize_t recv_until_byte(int fd,void * buf, size_t n,int timeout){
diff --git a/socket_wrapper.h b/socket_wrapper.h
index 9a2fdac..7b41c2b 100644
--- a/socket_wrapper.h
+++ b/socket_wrapper.h
@@ -41,16 +41,22 @@ struct TransferResult{
 
 #endif
 
+/**
+ * find buffer size from sock
+ * thread safe
+*/
 int getBufferSizeFrom(int sock);
 
-/*
-return -2 if timeout occur.
-otherwise, implement equal to 'recv'.
-`timeout` unit is second.
+/**
+ * return -2 if timeout occur.
+ * otherwise, implement equal to 'recv'.
+ * `timeout` unit is second.
+ * thread safe
 */
 ssize_t timeout_recv(int fd,void * buf,size_t n,int timeout);
 /*
-recieve data to buf until all bytes.
+ * recieve data to buf until all bytes.
+ * thread safe
 */
 ssize_t recv_until_byte(int fd,void * buf, size_t n,int timeout);