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non-blocking connect() behaviour

Posted on 1997-12-11
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Last Modified: 2013-12-26
I've got a quick question regarding the connect() behavior in the
following situation. In a client program I open a socket, set some
options on it (KEEPALIVE, LINGER, REUSEADDR), set it to the
non-blocking mode with fcntl() and then try to connect(). While
everything is fine if a server is running, the situation is not that
great if there is no server.

If there is not server listening on a specified port, then I get the
following results from connect():

1. DG/UX, Solaris:      ECONNREFUSED (Connection refused) - quite
obvious and expected.

2. Linux 2.0.29            EINPROGRESS (Operation in progress)

And the latter seems *really weird* to me. EINPROGRESS is not supposed
to be an error per se, so I don't terminate. Later, I am trying to
check whether the socket is ready to send (using select()) and on
Linux it says that it *is* ready (this is probably because of the
non-blocking mode. Am I right?). Due to some circumstances I am not
allowed to catch SIGPIPE. This basically means, that on Linux the
client program, instead of exiting nicely with a message: "server not
running" and doing some clean-up, it dies with a very "clear"
diagnostics "Broken pipe".

My question is: is there any "standard" regarding the connect()
behavior in such situation (non-blocking mode, no server listening)
and the ERROR to be returned: ECONNREFUSED or EINPROGRESS?

Thanks in advance,

PS. I'm attaching a tiny program, which is doing exactly what I
described. On Solaris and DG/UX it terminates with a 'Connection
refused' message, whereas on Linux, it procceeds further and dies of
SIGPIPE. The only requirement is that nobody is listening on the port
6779.

========================================================================

#include <stdio.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>
#include <netinet/in.h>


void
pexit(char* s)
{
    perror(s);
    exit(1);
}


int
main(void)
{
    int      sockfd, res;
    struct sockaddr_in      servaddr;
    struct linger Linger;
    fd_set fds;
   
    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
      pexit("socket error");
   
    res = 0;
   
    if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &res, sizeof(int)) < 0)
      pexit("setsockopt REUSEADDR error");
      
    Linger.l_onoff    = 1;
    Linger.l_linger   = 0;

    if (setsockopt(sockfd, SOL_SOCKET, SO_LINGER, &Linger, sizeof(Linger)) < 0)
      pexit("setsockopt LINGER error");
      
    res = 1;
    if (setsockopt(sockfd, SOL_SOCKET, SO_KEEPALIVE, &res, sizeof(int)) < 0)
      pexit("setsockopt KEEPALIVE error");
      
    res = fcntl(sockfd, F_GETFL, 0);

    if (fcntl(sockfd, F_SETFL, O_NDELAY | res) < 0)
      pexit("fcntl error");
      
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
    servaddr.sin_port = htons(6779);
      
    if (connect(sockfd, (struct sockaddr*)&servaddr, sizeof(servaddr)) < 0 &&
      errno != EINPROGRESS)
      pexit("connect error");
      
    FD_ZERO(&fds);
    FD_SET(sockfd, &fds);
   
    if (select(sockfd + 1, NULL, &fds, NULL, NULL) < 0)
      pexit("select error");

    if (FD_ISSET(sockfd, &fds))
      printf("Socket %d is ready to send\n", sockfd);
   
    if (write(sockfd, &res, sizeof(res)) < 0)
      pexit("write error");
   
    exit(0);
}



========================================================================

--
Alexander L. Belikoff
Berger Financial Research Ltd.
abel@bfr.co.il
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Question by:belikoff
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1 Comment
 
LVL 1

Accepted Solution

by:
hno earned 130 total points
ID: 1295800
A connect() call on a non blocking socket should always be a non-blocking call, and as such it can't return a correct response other than "connection in progress" since it takes time to complete a connect... Don't ever count on it returning ECONNREFUSED. This is only possible in a VERY fast LAN environment where the network is faster than you own CPU, or on blocking connects.

In the programs I have been involved in we have always used write to detect failing connetcs after a select(), but it should be possible to use getsockopt(s,SOL_SOCKET,SO_ERROR,..) as well (and this does not generate SIGPIPE).

On Linux getsockopt is even documented on the connect() man page as the way to find the result of a non-blocking connect.

On some platforms (esp Solaris) you can use a second connect() to check the final status of the connect() call.

A note regarding SIGPIPE:
Unless the contacted server is extremly robust, or that it doesn't matter that the application is abruptly terminated when the server aborts the connection, you should take care of SIGPIPE.
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