LinuxQuestions.org - nonblocking select() + socket()

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nonblocking select() + socket()

I would like to use the following example to timeout a connection attempt, made by a TCP socket:

Code:

/*

** select.c -- a select() demo

*/



#include <stdio.h>

#include <sys/time.h>

#include <sys/types.h>

#include <unistd.h>



#define STDIN 0  // file descriptor for standard input



int main(void){

    struct timeval tv;

    fd_set readfds;



    tv.tv_sec = 2;

    tv.tv_usec = 500000;



    FD_ZERO(&readfds);

    FD_SET(STDIN, &readfds);



    // don't care about writefds and exceptfds:

    select(STDIN+1, &readfds, NULL, NULL, &tv);



    if (FD_ISSET(STDIN, &readfds))

        printf("A key was pressed!\n");

    else

        printf("Timed out.\n");



    return 0;

}

my socket function looks like this:

Code:

int checkprt(int port, char *ip){

  int test = 0;

  int sockfd;

  struct sockaddr_in servaddr;

  sockfd = socket(AF_INET, SOCK_STREAM, 0);

    if(sockfd < 0) {

      printf("error. cannot creat socket\n");

      return ERROR;

    }

  servaddr.sin_family = AF_INET;

  servaddr.sin_addr.s_addr = inet_addr(ip);

  servaddr.sin_port = htons(port);

  test = connect(sockfd, (struct sockaddr_in *)&servaddr, sizeof(servaddr));

  if(test == -1){

    close(sockfd);

    return CLOSED;

  }

  close(sockfd);

  return OPEN;

}

As you probably have guessed I am in the middle of learning socket programming. Any help here on how to use the select() example to timeout a connect attempt would be appreciated.

You can't do it the way you're thinking.

There's a non-portable (read LINUX) way of doing something similar:

The socket option TCP_SYNCNT. It's defaulted to 5, which corresponds to 180 seconds. However, you could easily make this number.. say.. 2. And your connect will time out much more quickly.

This may be worth a read http://www.beej.us/guide/bgnet/

Quote:

Originally Posted by orgcandman (Post 3696370)

I've found out that on my system (gentoo updated), its defaulted to 7.
What would the best method of altering this value be without doing it permanently?

man setsockopt

Just set the TCP_SYNCNT socket option. This has the advantage of working the way you expect on any linux machine (but other systems probably will fail).

Quote:

Originally Posted by orgcandman (Post 3696772)

man setsockopt

Just set the TCP_SYNCNT socket option. This has the advantage of working the way you expect on any linux machine (but other systems probably will fail).

This can probably be averted with #ifdef TCP_SYNCNT /*...*/ #endif around the corresponding code. Those without Linux will just be out of luck!
Kevin Barry

Much more easy and standard way for this. Put the socket in non-blocking mode and call the connect after that. Connect will return immediately with EINPROGRESS. Now you call the select again for this socket, with the desired timeout. Now the select will wake up once the socket gets connected or it will timeout.

This is the standard async socket programming way and I hope this is the way you wanted to follow.

-jork