How can I tell when a socket is closed on the other end?
How can I tell when a socket is closed on the other end?
From Andrew Gierth (andrew@erlenstar.demon.co.uk):
AFAIK:
If the peer calls close() or exits, without having messed with
SO_LINGER, then our calls to read() should return 0. It is less clear
what happens to write() calls in this case; I would expect EPIPE, not
on the next call, but the one after.
If the peer reboots, or sets l_onoff = 1, l_linger = 0 and then
closes, then we should get ECONNRESET (eventually) from read(), or
EPIPE from write().
I should also point out that when write() returns EPIPE, it also
raises the SIGPIPE signal - you never see the EPIPE error unless you
handle or ignore the signal.
If the peer remains unreachable, we should get some other error.
I don't think that write() can legitimately return 0. read() should
return 0 on receipt of a FIN from the peer, and on all following
calls.
So yes, you must expect read() to return 0.
As an example, suppose you are receiving a file down a TCP link; you
might handle the return from read() like this:
rc = read(sock,buf,sizeof(buf));
if (rc > 0)
{
write(file,buf,rc);
/* error checking on file omitted */
}
else if (rc == 0)
{
close(file);
close(sock);
/* file received successfully */
}
else /* rc < 0 */
{
/* close file and delete it, since data is not complete
report error, or whatever */
}
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