send( ) problem

Question

When I test my program using a web browser I can write to the socket/FD just fine so i decided to loop it and cut the connection mid-connection and I noticed a problem. send() is capable of closing down the entire program when the socket is unavailable. I thought the problem was that the program caught itself in a catch-22 and closed itself. So I set the socket to not block. No change. Any ideas of why this is happening?

else if ( b->temp_socket_list[read].revents & POLLOUT ) {  
    printf ( "#Write#\n" );  
    char *done = "Done!";  
    int sent = send ( sock, done, 5, 0 );  
    printf ( "end\n", sent );  
}

And, additionally, run `strace` with the program, so that we can see what really happens. — Roland Illig, Apr 12 '11 at 20:37
@Roland +1. This was the only way I could figure out a process was getting a SIGPIPE some months ago. It simply seemed to die shortly after system initialization. — Jeff, Apr 12 '11 at 20:49
As an alternative to ignoring the SIGPIPE, [this post](https://stackoverflow.com/q/108183/631065) has some good answers on how to avoid them. — Jeff, Apr 12 '11 at 20:35

score 5 · Accepted Answer · answered Apr 12 '11 at 20:35

5

This is likely due to the default action of the SIGPIPE signal. To ignore this signal, use something like:

signal(SIGPIPE, SIG_IGN);

Socket errors will then be reported as return values from socket functions, rather than a signal.

answered Apr 12 '11 at 20:35

Greg Hewgill

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Note that you cannot do this from library code, which should not be modifying the caller's signal dispositions, at least not without documenting it and including a big warning. However if you use `sendto` rather than send, you can specify a flag indicating that you don't want `SIGPIPE` to be generated. I believe there's also a solution involving `setsockopt` and, and using `pthread_sigmask` and `sigtimedwait` to block and clear the pending `SIGPIPE`. – R.. GitHub STOP HELPING ICE Apr 12 '11 at 21:53

score 3 · Answer 2 · answered Apr 12 '11 at 20:35

3

Which platform is this?

On UNIX in some cases you can get a signal when the connection goes down (SIGPIPE) and this terminates the program by default... the solution is to install a signal handler for SIGPIPE that does nothing.

answered Apr 12 '11 at 20:35

Antti Huima

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score 0 · Answer 3 · answered Apr 12 '11 at 22:10

Try this:

sigset_t set, oldset;
sigemptyset(&set);
sigaddset(&set, SIGPIPE);
pthread_sigmask(SIG_BLOCK, &set, &oldset);
/* use send all you like here */
sigtimedwait(&set, 0, (struct timespec [1]){0});
pthread_sigmask(SIG_SETMASK, &oldset, 0);

I'm not 100% sure it works, but I believe it should, and if it's correct then it's a solution that can be used from library code without messing up the state of the caller or other potentially-signal-using threads.

Also note that if the program (or even just the current thread) does not want to make use of SIGPIPE, you can simplify this a lot by just leaving SIGPIPE permanently blocked:

sigset_t set;
sigemptyset(&set);
sigaddset(&set, SIGPIPE);
pthread_sigmask(SIG_BLOCK, &set, &oldset);

send( ) problem

3 Answers3