cannot connect as existing user

src/chat.h

@@ -8,6 +8,8 @@
 
 #define LIST "list\n"
 
+#define ERROR "error\n"
+
 #define USERNAMExMESSAGE ": "
 
 // General

src/client.c

@@ -147,7 +147,15 @@ void *receiver() {
         }
 
         receiveBuffer[nbytes] = '\0';
-        printf("%s", receiveBuffer);
-        pthread_mutex_unlock(&mutexsum);
+        if(strcmp(ERROR, receiveBuffer) == 0) {
+            printf("Error: The username %s is already taken.\n", sendBuffer);
+            done = TRUE;
+            pthread_mutex_destroy(&mutexsum);
+            pthread_exit(NULL);
+        }
+        else {
+            printf("%s", receiveBuffer);
+            pthread_mutex_unlock(&mutexsum);
+        }
     }
 }

src/server.c

@@ -100,6 +100,15 @@ int connectClient(struct sockaddr_in newClient, char *username) {
     while(element != NULL) {
         if(strcmp(element->username, username) == 0) {
             printf("Cannot connect client user already exists\n");
+            strcpy(responseBuffer, "");
+            strcat(responseBuffer, ERROR);
+            if((sendto
+                (sockfd, responseBuffer, strlen(responseBuffer), 0, (struct sockaddr *) &newClient,
+                 sizeof(struct sockaddr))) == SYSERR) {
+                perror("sendto");
+                close(sockfd);
+                exit(EXIT_FAILURE);
+            }
             return SYSERR;
         }
         element = element->next;