title: C socket client
body: <pre><code>#include &lt;stdio.h&gt;
  #include &lt;stdlib.h&gt;
  #include &lt;unistd.h&gt;
  #include &lt;string.h&gt;
  #include &lt;sys/types.h&gt;
  #include &lt;sys/socket.h&gt;
  #include &lt;netinet/in.h&gt;
  #include &lt;netdb.h&gt; 
  
  void error(const char *msg) {
      perror(msg);
      exit(0);
  }
  
  int main(int argc, char *argv[]) {
      int sockfd, portno, n;
      struct sockaddr_in serv_addr;
      struct hostent *server;
  
      char buffer[256];
      if (argc &lt; 3) {
          fprintf(stderr, "usage %s hostname port\n", argv[0]);
          exit(0);
      }
      portno = atoi(argv[2]);
      sockfd = socket(AF_INET, SOCK_STREAM, 0);
      if (sockfd &lt; 0)
          error("ERROR opening socket");
      server = gethostbyname(argv[1]);
      if (server == NULL) {
          fprintf(stderr, "ERROR, no such host\n");
          exit(0);
      }
      bzero((char *) &amp;serv_addr, sizeof (serv_addr));
      serv_addr.sin_family = AF_INET;
      bcopy((char *) server-&gt;h_addr,
              (char *) &amp;serv_addr.sin_addr.s_addr,
              server-&gt;h_length);
      serv_addr.sin_port = htons(portno);
  
      if (connect(sockfd, (struct sockaddr *) &amp;serv_addr, sizeof (serv_addr)) &lt; 0) error("ERROR connecting");
  
      while (1) {
          printf("Please enter the message: ");
          bzero(buffer, 256);
          fgets(buffer, 255, stdin);
          // n = write(sockfd, buffer, strlen(buffer));
          n = send(sockfd, buffer, strlen(buffer), 0);
          if (n &lt; 0) error("ERROR writing to socket");
  
          bzero(buffer, 256);
          // n = read(sockfd, buffer, 255);
          n = recv(sockfd, buffer, 255, 0);
          if (n &lt; 0) error("ERROR reading from socket");
  
          printf("%s\n", buffer);
      }
      close(sockfd);
  
      return 0;
  }
  </code></pre>
  
  <p>The code shown above works only at the first iteration. However, At second iteration it blocks at <code>recv()</code> or <code>read()</code>.</p>
  
  <p>Could someone please explain the reason for this behaviour?  </p>
  
  <p><strong>Update:</strong> here is the server code (not the one I'm using, but the same principles) and it works like the one I mentioned above:</p>
  
  <pre><code>/* A simple server in the internet domain using TCP
     The port number is passed as an argument */
  #include &lt;stdio.h&gt;
  #include &lt;stdlib.h&gt;
  #include &lt;string.h&gt;
  #include &lt;unistd.h&gt;
  #include &lt;sys/types.h&gt; 
  #include &lt;sys/socket.h&gt;
  #include &lt;netinet/in.h&gt;
  
  void error(const char *msg) {
      perror(msg);
      exit(1);
  }
  
  int main(int argc, char *argv[]) {
      int sockfd, newsockfd, portno;
      socklen_t clilen;
      char buffer[256];
      struct sockaddr_in serv_addr, cli_addr;
      int n;
      if (argc &lt; 2) {
          fprintf(stderr, "ERROR, no port provided\n");
          exit(1);
      }
      sockfd = socket(AF_INET, SOCK_STREAM, 0);
      if (sockfd &lt; 0)
          error("ERROR opening socket");
      memset((char*) &amp;serv_addr, 0, sizeof(serv_addr));
      portno = atoi(argv[1]);
      serv_addr.sin_family = AF_INET;
      serv_addr.sin_addr.s_addr = INADDR_ANY;
      serv_addr.sin_port = htons(portno);
      if (bind(sockfd, (struct sockaddr *) &amp;serv_addr,
              sizeof (serv_addr)) &lt; 0)
          error("ERROR on binding");
      listen(sockfd, 5);
      clilen = sizeof (cli_addr);
      while (1) {
          newsockfd = accept(sockfd,
                  (struct sockaddr *) &amp;cli_addr,
                  &amp;clilen);
          if (newsockfd &lt; 0)
              error("ERROR on accept");
          memset(buffer, 0, sizeof(buffer));
          n = read(newsockfd, buffer, 255);
          if (n &lt; 0) error("ERROR reading from socket");
          printf("Here is the message: %s\n", buffer);
          n = write(newsockfd, "I got your message", 18);
          if (n &lt; 0) error("ERROR writing to socket");
      }
      close(newsockfd);
      close(sockfd);
      return 0;
  }
  </code></pre>