c++ how to use select to see if a socket has closed - c++

Question

Can someone provide me an example of how to use select() to see if a client has closed the connection on a socket?
FYI. I'm using linux.
Thanks!

Answer 1

The below snippet first checks if the socket is marked readable (which it is when closed) and then if there's actually anything to be read.
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/ioctl.h>
bool isclosed(int sock) {
fd_set rfd;
FD_ZERO(&rfd);
FD_SET(sock, &rfd);
timeval tv = { 0 };
select(sock+1, &rfd, 0, 0, &tv);
if (!FD_ISSET(sock, &rfd))
return false;
int n = 0;
ioctl(sock, FIONREAD, &n);
return n == 0;
}

Answer 2

You don't need to do a select() followed by ioctl(). You can instead do a non-blocking peek on the socket to see if it returns 0.
bool isclosed (int sock) {
char x;
interrupted:
ssize_t r = ::recv(sock, &x, 1, MSG_DONTWAIT|MSG_PEEK);
if (r < 0) {
switch (errno) {
case EINTR: goto interrupted;
case EAGAIN: break; /* empty rx queue */
case ETIMEDOUT: break; /* recv timeout */
case ENOTCONN: break; /* not connected yet */
default: throw(errno);
}
}
return r == 0;
}

Answer 3

If you get a read notification and the read returns 0 bytes the client has exited cleanly. If not you will get an exception notification.

Answer 4

Here is an example to checks if the socket is closed by peer WITHOUT reading form it in case of non-empty recv buffer.
#ifndef _GNU_SOURCE
#define _GNU_SOURCE /* FOR: POLLRDHUP define */
#endif
#include <poll.h>
bool connection_peer_closed(int fd)
{
struct pollfd pfd;
int ret;
if (fd < 0) {
return true;
}
pfd.fd = fd;
pfd.events = POLLRDHUP;
pfd.revents = 0;
ret = poll(&pfd, 1, 0);
if (ret > 0 && (pfd.revents & (POLLHUP | POLLERR | POLLRDHUP))) {
return true;
} else {
return false;
}
}