Here is some code I’ve used on many Unix-like systems (e.g SunOS 4, SGI IRIX, HPUX 10.20, CentOS 5, Cygwin) to close a socket:
int getSO_ERROR(int fd) {
int err = 1;
socklen_t len = sizeof err;
if (-1 == getsockopt(fd, SOL_SOCKET, SO_ERROR, (char *)&err, &len))
FatalError("getSO_ERROR");
if (err)
errno = err; // set errno to the socket SO_ERROR
return err;
}
void closeSocket(int fd) { // *not* the Windows closesocket()
if (fd >= 0) {
getSO_ERROR(fd); // first clear any errors, which can cause close to fail
if (shutdown(fd, SHUT_RDWR) < 0) // secondly, terminate the 'reliable' delivery
if (errno != ENOTCONN && errno != EINVAL) // SGI causes EINVAL
Perror("shutdown");
if (close(fd) < 0) // finally call close()
Perror("close");
}
}
But the above does not guarantee that any buffered writes are sent.
Graceful close: It took me about 10 years to figure out how to close a socket. But for another 10 years I just lazily called usleep(20000)
for a slight delay to ‘ensure’ that the write buffer was flushed before the close. This obviously is not very clever, because:
- The delay was too long most of the time.
- The delay was too short some of the time–maybe!
- A signal such SIGCHLD could occur to end
usleep()
(but I usually calledusleep()
twice to handle this case–a hack). - There was no indication whether this works. But this is perhaps not important if a) hard resets are perfectly ok, and/or b) you have control over both sides of the link.
But doing a proper flush is surprisingly hard. Using SO_LINGER
is apparently not the way to go; see for example:
- http://msdn.microsoft.com/en-us/library/ms740481%28v=vs.85%29.aspx
- https://www.google.ca/#q=the-ultimate-so_linger-page
And SIOCOUTQ
appears to be Linux-specific.
Note shutdown(fd, SHUT_WR)
doesn’t stop writing, contrary to its name, and maybe contrary to man 2 shutdown
.
This code flushSocketBeforeClose()
waits until a read of zero bytes, or until the timer expires. The function haveInput()
is a simple wrapper for select(2), and is set to block for up to 1/100th of a second.
bool haveInput(int fd, double timeout) {
int status;
fd_set fds;
struct timeval tv;
FD_ZERO(&fds);
FD_SET(fd, &fds);
tv.tv_sec = (long)timeout; // cast needed for C++
tv.tv_usec = (long)((timeout - tv.tv_sec) * 1000000); // 'suseconds_t'
while (1) {
if (!(status = select(fd + 1, &fds, 0, 0, &tv)))
return FALSE;
else if (status > 0 && FD_ISSET(fd, &fds))
return TRUE;
else if (status > 0)
FatalError("I am confused");
else if (errno != EINTR)
FatalError("select"); // tbd EBADF: man page "an error has occurred"
}
}
bool flushSocketBeforeClose(int fd, double timeout) {
const double start = getWallTimeEpoch();
char discard[99];
ASSERT(SHUT_WR == 1);
if (shutdown(fd, 1) != -1)
while (getWallTimeEpoch() < start + timeout)
while (haveInput(fd, 0.01)) // can block for 0.01 secs
if (!read(fd, discard, sizeof discard))
return TRUE; // success!
return FALSE;
}
Example of use:
if (!flushSocketBeforeClose(fd, 2.0)) // can block for 2s
printf("Warning: Cannot gracefully close socket\n");
closeSocket(fd);
In the above, my getWallTimeEpoch()
is similar to time(),
and Perror()
is a wrapper for perror().
Edit: Some comments:
-
My first admission is a bit embarrassing. The OP and Nemo challenged the need to clear the internal
so_error
before close, but I cannot now find any reference for this. The system in question was HPUX 10.20. After a failedconnect()
, just callingclose()
did not release the file descriptor, because the system wished to deliver an outstanding error to me. But I, like most people, never bothered to check the return value ofclose.
So I eventually ran out of file descriptors(ulimit -n),
which finally got my attention. -
(very minor point) One commentator objected to the hard-coded numerical arguments to
shutdown()
, rather than e.g. SHUT_WR for 1. The simplest answer is that Windows uses different #defines/enums e.g.SD_SEND
. And many other writers (e.g. Beej) use constants, as do many legacy systems. -
Also, I always, always, set FD_CLOEXEC on all my sockets, since in my applications I never want them passed to a child and, more importantly, I don’t want a hung child to impact me.
Sample code to set CLOEXEC:
static void setFD_CLOEXEC(int fd) {
int status = fcntl(fd, F_GETFD, 0);
if (status >= 0)
status = fcntl(fd, F_SETFD, status | FD_CLOEXEC);
if (status < 0)
Perror("Error getting/setting socket FD_CLOEXEC flags");
}