How to prevent program being blocked by socket listen()

Question

I'm writing a server program, at the initialization, I want to bind() and listen() at certain ip/port. But then the whole program is blocked by listen(). Is there any way can make the program check for new incoming connection every, say, 5ms?

My code is currently like this:

int main(){
    initialize();
    do_something();

    return 0;
}

In initialize(), socket is set up:

void initialize(){
    sockfd = sock_create_bind(ip_addr[nodeid], port_addr[nodeid]);
    listen(sockfd, 5);
    peer_sockfd = accept(sockfd, (struct sockaddr *)&peer_addr, &peer_addr_len);
}

But initialize() never returns, and do_something() never get called. So I'm wondering is there anyway to prevent this blocking? Thanks.

Use `select()` or `poll()` to see which file descriptors can be read without blocking, or use `O_NONBLOCK` (`fcntl()` et al) to make the file descriptor non-blocking (but be careful with non-blocking I/O). — Jonathan Leffler, Sep 06 '19 at 17:57
Could you spawn a separate thread and have it block listening for connections? — ggorlen, Sep 06 '19 at 18:00
Specifically you can use `select` with a timeout, or even better, have **everything** in your program handled by the `select` — Antti Haapala -- Слава Україні, Sep 06 '19 at 18:01

score 2 · Accepted Answer · answered Sep 06 '19 at 18:42

2

You're blocking here

peer_sockfd = accept(sockfd, (struct sockaddr *)&peer_addr, &peer_addr_len);

To avoid blocking you can use select

fd_set fds;
FD_ZERO(fds);
FD_SET(sockfd, fds);
struct timeval tv = { 0, 0 };
select(sockfd + 1, &fds, NULL, NULL, &tf);
if (FD_ISSET(socktf, fds)) {
    peer_sockfd = accept(sockfd, (struct sockaddr *)&peer_addr, &peer_addr_len);
}

To decide the interval to check for, write your code so this is called at the appropriate interval.

You'll need to do a similar thing for read() and write() on peer_sockfd. If you assume that sockfd isn't from a hostile host OS, you can get away with only checking on read().

answered Sep 06 '19 at 18:42

Joshua

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`write()` will also block if the remote peer isn't processing the incoming data fast enough -- that might happen if the remote program has (for some reason) stopped calling `recv()` on a timely basis, or (more commonly) if the network between your program and the remote peer is flaky and dropping packets, causing the TCP transmission rate to become slower than the rate at which your program is `send()`-ing data. – Jeremy Friesner Sep 06 '19 at 19:26
@JeremyFriesner: Correct. In my experience this rarely is important to most cases. It's the `read()` or `accept()` blocked for an hour that's bad. – Joshua Sep 06 '19 at 19:28
An easy way to observe `write()` getting blocked is to pull the ethernet cable out of its socket and leave it disconnected for 15-30 seconds, while your program is trying to send a non-trivial amount of TCP data to some destination elsewhere on the network. – Jeremy Friesner Sep 08 '19 at 01:44

How to prevent program being blocked by socket listen()

1 Answers1