In a non blocking socket connect, select() always returns 1

Question

I have this code segment that is designed to connect to a server using a socket connection. However if it can not connect to the server within a certain amount of time I would like it to stop trying. I tried to do this with this nonblocking socket and the select command but select is always returning 1 indicating that the server exists when nothing exists at the address I give it. Any Ideas?

SOCKET tcp_client( char *hname, char *sname )  {    
    fd_set fdset;
    struct sockaddr_in peer;
    SOCKET s;
    FD_ZERO(&fdset);
    // FD_SET(STDIN, &fdset);
    FD_SET(s, &fdset);
    errno=1;
    struct timeval tv;
    tv.tv_sec = 15;
    set_address( hname, sname, &peer, "tcp" );
    s = socket( AF_INET, SOCK_STREAM, 0 );

    int n = 1;
    fcntl(s, F_SETFL, O_NONBLOCK);

    if ( !isvalidsock( s ) )
    {
        printf("Socket Call Failed: %s\n", strerror(errno));
        return(0);
    }

    int x = 0;

    int status = connect( s, ( struct sockaddr * )&peer, sizeof( peer ) );

    if(status < 0) {
        printf("Status: %i\n", status); 
    }

    int retVal = select(s+1, &fdset, NULL, NULL, &tv);
    printf("retVal: %i\n", retVal);

    if (retVal == 1) {
        int so_error;
        socklen_t slen = sizeof so_error;
        getsockopt(s, SOL_SOCKET, SO_ERROR, &so_error, &slen);
        if (so_error == 0) {
            printf("work\n");

            x =1;
        } else {
            printf("fail\n");
            x = 0;
        }
    } else {     
        printf("noSocks\n"); 
    }


    if (x ==0 )
    {
        printf("Connect Failed: %s\n", strerror(errno));
        L("libOnexc: Connect to socket failed");
        close(s);
        return(0);
    } 

    return s;
}

Well I'm not waiting on it more of waiting for 15 seconds using select() to see if it has connected, but select is always returning that its connected when its not. — Daniel, Apr 30 '11 at 18:41

Brian Rothstein · Accepted Answer · 2011-04-30T18:48:09.240

13

One problem I see is that you stick s in the fdset before you have created the socket. You need to do the

FD_SET(s, &fdset);

after you've created the socket because s is just an integer and so will not be the right value until after the call to socket().

EDIT

Like this:

.
.
.
SOCKET s;
errno=1;
struct timeval tv;
tv.tv_sec = 15;
set_address( hname, sname, &peer, "tcp" );
s = socket( AF_INET, SOCK_STREAM, 0 );

int n = 1;
fcntl(s, F_SETFL, O_NONBLOCK);

if ( !isvalidsock( s ) )
{
    printf("Socket Call Failed: %s\n", strerror(errno));
    return(0);
}    

FD_ZERO(&fdset);
FD_SET(s, &fdset); // don't put socket in set until it is actually created

edited Apr 30 '11 at 18:48

answered Apr 30 '11 at 18:41

Brian Rothstein

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I moved it to after the socket is created but now select always returns 0 even when trying to connect to a machine I know exists... – Daniel Apr 30 '11 at 18:45
`fcntl(s, F_SETFL, O_NONBLOCK);` is incorrect. You overwrite other flags that may be set. Also see [How do I change a TCP socket to be non-blocking?](https://stackoverflow.com/q/1543466/608639) – jww Mar 03 '19 at 05:46

score 13 · Answer 2 · answered Apr 30 '11 at 18:45

13

You need to check whether the socket is ready for writing (the second fd_set * argument to select), not reading (the first one).

answered Apr 30 '11 at 18:45

R.. GitHub STOP HELPING ICE

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2

@Daniel Then give the guy a +1 ;-) I'll do it for you. – Richard Brightwell Apr 30 '11 at 18:50

In a non blocking socket connect, select() always returns 1

2 Answers2

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