Thread: EPOLL echo server not concurrent

  1. #1
    Registered User lantzvillian's Avatar
    Join Date
    Sep 2010

    EPOLL echo server not concurrent

    Hi all,

    I am working on an epoll version of an echoserver that I am porting from a multithreaded version I wrote.

    What it should do:
    The server should get a connection from a client > say x client connected > print x message from said client.

    What it is doing:
    The server looks like it is only accepting one connection at a time, and any other clients are queued. I thought maybe at first that it was because it was running on the same host, but I tried from another box and it looks like it is still blocking.

    Any suggestions?

    #include <assert.h>
    #include <errno.h>
    #include <fcntl.h>
    #include <netinet/in.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/epoll.h>
    #include <sys/socket.h>
    #include <sys/types.h>
    #include <unistd.h>
    #include <netdb.h>
    #include <strings.h>
    #include <string.h>
    #include <arpa/inet.h>
    #include <unistd.h>
    #include <signal.h>
    #define TRUE 		1
    #define FALSE 		0
    #define EPOLL_QUEUE_LEN 10
    #define BUFLEN		1024
    #define SERVER_PORT	7000
    int fd_server;
    struct clientStuff
      int sockfd;
      struct sockaddr_in client;
    // Function prototypes
    static void SystemFatal (const char* message);
    static void* ClearSocket (void* arg);
    void close_fd (int);
    int main (int argc, char* argv[]) 
      int i, arg, visits; 
      int num_fds, fd_new, epoll_fd;
      static struct epoll_event events[EPOLL_QUEUE_LEN], event;
      int port = SERVER_PORT;
      struct sockaddr_in addr, remote_addr;
      socklen_t addr_size = sizeof(struct sockaddr_in);
      struct sigaction act;
      FILE *fp;
      visits =  0;
      // set up the signal handler to close the server socket when CTRL-c is received
      act.sa_handler = close_fd;
      act.sa_flags = 0;
      if ((sigemptyset (&act.sa_mask) == -1 || sigaction (SIGINT, &act, NULL) == -1))
        perror ("Failed to set SIGINT handler");
        exit (EXIT_FAILURE);
      // Create the listening socket
      fd_server = socket (AF_INET, SOCK_STREAM, 0);
      if (fd_server == -1) 
      // set SO_REUSEADDR so port can be resused imemediately after exit, i.e., after CTRL-c
      arg = 1;
      if (setsockopt (fd_server, SOL_SOCKET, SO_REUSEADDR, &arg, sizeof(arg)) == -1) 
      // Make the server listening socket non-blocking
      if (fcntl (fd_server, F_SETFL, O_NONBLOCK | fcntl (fd_server, F_GETFL, 0)) == -1) 
      // Bind to the specified listening port
      memset (&addr, 0, sizeof (struct sockaddr_in));
      addr.sin_family = AF_INET;
      addr.sin_addr.s_addr = htonl(INADDR_ANY);
      addr.sin_port = htons(port);
      if (bind (fd_server, (struct sockaddr*) &addr, sizeof(addr)) == -1) 
      // Listen for fd_news; SOMAXCONN is 128 by default
      if (listen (fd_server, SOMAXCONN) == -1) 
      // Create the epoll file descriptor
      epoll_fd = epoll_create(EPOLL_QUEUE_LEN);
      if (epoll_fd == -1) 
      // Add the server socket to the epoll event loop = EPOLLIN | EPOLLERR | EPOLLHUP | EPOLLET; = fd_server;
      if (epoll_ctl (epoll_fd, EPOLL_CTL_ADD, fd_server, &event) == -1) 
      // Execute the epoll event loop
      while (TRUE) 
         // signal( SIGABRT, function);
        //struct epoll_event events[256];
        num_fds = epoll_wait (epoll_fd, events, EPOLL_QUEUE_LEN, -1);
        if (num_fds < 0) 
          SystemFatal ("Error in epoll_wait!");
        for (i = 0; i < num_fds; i++) 
          // Case 1: Error condition
          if (events[i].events & (EPOLLHUP | EPOLLERR)) 
    	fputs("epoll: EPOLLERR", stderr);
          assert (events[i].events & EPOLLIN);
          // Case 2: Server is receiving a connection request
          if (events[i].data.fd == fd_server) 
    	struct clientStuff c1;
    	fd_new = accept (fd_server, (struct sockaddr*) &remote_addr, &addr_size);
    	  printf(">> Initializing remote address: %s\n", inet_ntoa(remote_addr.sin_addr));
    	  fp=fopen("server-output.txt", "a"); /* Open file */
    	  fprintf(fp, "Connections-to-date,%u\n",visits);
    	  fprintf(fp, "%s,connect\n",inet_ntoa(remote_addr.sin_addr));
    	  printf("Connections to date: %u \n",visits);	  
    	c1.sockfd = fd_new;
    	printf("sockfd: %u\n",c1.sockfd);
    	c1.client = remote_addr;
    	if (fd_new == -1) 
    	  if (errno != EAGAIN && errno != EWOULDBLOCK) 
    	// Make the fd_new non-blocking
    	if (fcntl (fd_new, F_SETFL, O_NONBLOCK | fcntl(fd_new, F_GETFL, 0)) == -1) 
    	// Add the new socket descriptor to the epoll loop = fd_new;
    	  if (epoll_ctl (epoll_fd, EPOLL_CTL_ADD, fd_new, &event) == -1) 
    	    SystemFatal ("epoll_ctl");
          // Case 3: One of the sockets has read data
          if (!ClearSocket(events[i].data.fd)) 
    	// epoll will remove the fd from its set
    	// automatically when the fd is closed
    	close (events[i].data.fd);
      exit (EXIT_SUCCESS);
    static void* ClearSocket (void* arg) 
     struct clientStuff *ptr;
      struct sockaddr_in client_addr;
      int bytes_to_read, sockfd;
      char *bp, buf[BUFLEN];
      ssize_t n;
      FILE *fp;
      //ptr = (struct clientStuff *)arg;
      sockfd = (int) arg;
        //printf("sockfd: %u",ptr->sockfd);
     // client_addr = ptr->client;
      bp = buf;
      bytes_to_read = BUFLEN -1;
      while(1) {
        n = read(sockfd, bp, bytes_to_read);
          bp += n;
          bytes_to_read -= n;
          if (bytes_to_read <= 0)
    	/* File I/O Preparations */
    	fp=fopen("server-output.txt", "a");
    	fprintf(fp,"%s,Recv,%s\n",inet_ntoa(client_addr.sin_addr), buf);	
    	printf("%s,Recv,%s\n", inet_ntoa(client_addr.sin_addr), buf);
          if((write(sockfd, "DATA-ACK", strlen("DATA-ACK"))) == -1) {
    	  fprintf(fp, "%s,Disconnect\n",inet_ntoa(client_addr.sin_addr));
    	  printf("Error on read\n");
    	  return 1;
          fprintf(fp, "%s,Sent,DATA-ACK\n",inet_ntoa(client_addr.sin_addr));
      return 1;
    // Prints the error stored in errno and aborts the program.
    static void SystemFatal(const char* message) 
      perror (message);
      exit (EXIT_FAILURE);
    // close fd
    void close_fd (int signo)
      exit (EXIT_SUCCESS);

  2. #2
    Registered User
    Join Date
    Mar 2011
    I know this is a old thread and probably may answer will not help because is to late but I considered a interesting challenge to find the issue.

    At first everything seem to be done by the book... but the issue is actually simple.

    The problem is the while(1) in the ClearSocket function which will keep looping until the client close the connection (write fails). And while doing this no other connections are handled.

    The correct approach is to use separate buffers and states for each connection and as soon as we manage to read any data from the socket break the loop and allow other connection to be handled.

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. RPC concurrent server in C...???
    By mr_m4x in forum C Programming
    Replies: 1
    Last Post: 03-28-2009, 11:10 PM
  2. server client application - (i really need your help)
    By sarahnetworking in forum C Programming
    Replies: 3
    Last Post: 03-01-2008, 09:54 PM
  3. TCP/IP client & echo server
    By Jordban in forum C++ Programming
    Replies: 2
    Last Post: 06-06-2005, 06:39 PM
  4. A breakthrough
    By loopy in forum Linux Programming
    Replies: 4
    Last Post: 11-26-2003, 05:46 PM
  5. socket question
    By Unregistered in forum C Programming
    Replies: 3
    Last Post: 07-19-2002, 01:54 PM