在侦听套接字上使用select（）

Question

I want to open a port and wait for incoming connections, however I can't get select() to work. I had it working with poll() but I need select() for portability. What am I doing wrong?

Code for waiting for the connection looks like this (I need to check for interruptions every 200ms):

/* Wait for a descriptor */
int wait_for_fd(int fd){
  int waitms = 200;
  struct timeval tv;
  tv.tv_sec = 0;
  tv.tv_usec = waitms * 1000;
  fd_set rfds;
  FD_ZERO(&rfds);
  FD_SET(fd, &rfds);
  int active = 0;
  while(active == 0){
    active = select(fd+1, &rfds, NULL, NULL, &tv);
    bail_for(active < 0, "select()");
    if(pending_interrupt())
      break;
  }
  return active;
}

And then my code to actually open a port and wait for a connection:

int open_port(int port){

  // define server socket
  struct sockaddr_in serv_addr;
  memset(&serv_addr, '0', sizeof(serv_addr));
  serv_addr.sin_family = AF_INET;
  serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
  serv_addr.sin_port = htons(port);

  //creates the listening socket
  int listenfd = socket(AF_INET, SOCK_STREAM, 0);
  bail_for(listenfd < 0, "socket()");
  bail_for(bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr)) < 0, "bind()");
  bail_for(listen(listenfd, 10) < 0, "listen()");

  //each accept() is a new incoming connection
  printf("Waiting for connetion on port %d...\n", port);
  wait_for_fd(listenfd);
  int connfd = accept(listenfd, NULL, NULL);
  bail_for(connfd < 0, "accept()");
  printf("Incoming connection!\n");

  //do not allow additional client connetions
  close(listenfd);
  return connfd;
} 

However wait_for_fd() never returns (due to select always returning 0) even when a client is connecting.

Answer 1

This must be on every iteration:

FD_ZERO(&rfds);
FD_SET(fd, &rfds);

Because rfds is an in/out parameter for select() . It actually tells with it which fds were affected.

Answer 2

According to manpage of select

On exit, the sets are modified in place to indicate which file descriptors actually changed status. Each of the three file descriptor sets may be specified as NULL if no file descriptors are to be watched for the corresponding class of events.

This means, that, when you have called select and no file-descriptors have changed, no file-descriptors are set in rfds . Therefor you'll have to set them on each iteration

while(active == 0){
    FD_ZERO(&rfds);
    FD_SET(fd, &rfds);
    active = select(fd+1, &rfds, NULL, NULL, &tv);
    bail_for(active < 0, "select()");
    if(pending_interrupt())
      break;
  }