CODEWITHSUNDEEP
c++csocketsunixnetwork-programming
Kiril Kirov
Kiril Kirov

Reputation: 38173

send succeeds even when the connection is no longer available?

What am I missing?

I have very simple client and server. The server uses recv with timeout (using select) for 3 seconds. Then it shutdowns and closes the sockets and exits.

The client connects to the server, sleeps 30 seconds and sends very short message. The sending is about 27seconds after the server has closed the sockets and exited.
And send does not fail..? Why? Why it does not return -1 for error?

Please note: I cut all checks for return codes and removed all logs, to make this as short as I can. Also, I removed all includes, to make this shorter. But it IS a real code.

client code:

int main( int argc, char* argv[] )
{
    addrinfo hints;
    memset(&hints, 0, sizeof hints); // make sure the struct is empty

    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM; // TCP stream sockets

    addrinfo *res;
    getaddrinfo( "127.0.0.1", "1313", &hints, &res );
    int nSocketFD = socket( res->ai_family, res->ai_socktype, res->ai_protocol );
    assert( -1 != connect( nSocketFD, res->ai_addr, res->ai_addrlen) );
    freeaddrinfo( res ); // free the linked-list, we don't need it anymore

    sleep( 30 );
    if( send( nSocketFD, "bla", 4, 0 ) > 0 )
    {
        printf( "Message successfully sent!\n" );
    }   

    close( nSocketFD );

    return 0;
}

and server:

int main()
{
    addrinfo hints;
    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;

    addrinfo *res;  // will point to the results
    getaddrinfo( NULL, "1313", &hints, &res );
    int nSocketFD = socket( res->ai_family, res->ai_socktype, res->ai_protocol );
    bind( nSocketFD, res->ai_addr, res->ai_addrlen );
    freeaddrinfo( res ); // free the linked-list
    listen( nSocketFD, 1 );

    sockaddr_storage their_addr;
    socklen_t addr_size = sizeof( sockaddr_storage );
    int nAcceptedSocket = accept( nSocketFD, (sockaddr*)&their_addr, &addr_size );
    assert( -1 != nAcceptedSocket );

    fd_set fds;
    FD_ZERO( &fds );
    FD_SET( nAcceptedSocket, &fds );
    timeval tv; 
    tv.tv_sec = 3;
    tv.tv_usec = 0;

    if( 0 == select( nAcceptedSocket + 1, &fds, NULL, NULL, &tv) )
    {   
        printf( "recv timeout! Exiting..\n" );
        shutdown( nSocketFD, SHUT_RDWR );
        close( nSocketFD );
        shutdown( nAcceptedSocket, SHUT_RDWR );
        close( nAcceptedSocket );
        return 1;
    }   
    assert( false );
    return 0;
}

When I execute it, I see the messages for recv timeout and for successful sent message.

Sorry for the long and probably stupid question.

Upvotes: 3

Views: 1073

Answers (2)

Adrian Cornish
Adrian Cornish

Reputation: 23886

You didnt not close the socket in your client - so this a valid socket. From send()'s man page

No indication of failure to deliver is implicit in a send().  Locally detected errors are indicated by a return value of -1.

We have this issue with some of our software - if network hardware fails somewhere along the line and it is brought back up again - then either end still thinks the socket is valid - and without keepalives probes - it will stay this way.

ADDING: Have a look at socket options (man setsockopt and man tcp) SO_KEEPALIVE

From TCP man page tcp_keepalive_intvl (integer; default: 75; since Linux 2.4) The number of seconds between TCP keep-alive probes.

   tcp_keepalive_probes (integer; default: 9; since Linux 2.2)
          The  maximum  number  of  TCP  keep-alive  probes  to  send before giving up and killing the connection if no response is obtained from the other end.

   tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
          The number of seconds a connection needs to be idle before TCP begins sending out  keep-alive  probes.   Keep-alives  are only  sent when the SO_KEEPALIVE socket option is enabled.  The default value is 7200 seconds (2 hours).  An idle connection is terminated after approximately an additional 11 minutes (9 probes an interval of 75  seconds  apart)  when  keep-alive is enabled.

          Note that underlying connection tracking mechanisms and application timeouts may be much shorter.

Upvotes: 1

Some programmer dude
Some programmer dude

Reputation: 409404

In general you need to read from a socket to get it to notice that the remote end has closed the connection.

From the manual page of send (which is just write but with flags):

No indication of failure to deliver is implicit in a send().  Locally
detected errors are indicated by a return value of -1.

Upvotes: 3

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