I developed a server with C code.
I used the accept() function to keep my server listening on a giving socket.
My server is launched in a thread.
Now in other thread and for some condition I want to stop the accept() blocking and then close the related socket.
How I can do that? Could shutdown() do that?
[This does not work on Windows]
Use sigaction() to install a signal handler for let's say SIGUSR1 doing nothing, but having the SA_RESTART option unset (also see section "Interruption of system calls and library functions by signal handlers" on this man-page).
Then send the blocking process a SIGUSR1 signal.
accept() will then return -1 and set errno to EINTR.
Classically, closing the socket from another thread causes the accept() call to return with an error. I have been told that this does not work on some releases of Linux, but have seen no evidence of that myself - every time, on Windows/Linux, the accept() returns with an error/exception.
The other common solution is to check some 'shutdown' atomic boolean after every accept() return. When you want to stop it, set the boolean and perform a connect() on the localhost stack, so causing the accept() to return in the 'normal' way.
I was having trouble with accept not terminating after I only called close on the socket under Linux Mint 18.3. I solved it by also calling shutdown(socket_fd, SHUT_RD); before the close
mah already mentioned it, but I wanted to highlight this, because this is the first SO result in a google search for "socket stop accept".
Related
I have a server program which looks like this
{
socket();
bind();
listen();
while(1)
{
accept();
recv();
send();
close();
}
close();
}
Let's say the server is running, listening at the specified port. How can I close it by pressing a keypad? I mean a proper closure, not by Ctrl+C.
When you close() a socket that is blocking in accept(), then the accept() call will return immediately with -1.
If your program is single threaded like you show, then you can't do the above. You would need to introduce at least one additional thread to actually do the close().
Have the program install a signal handler (for SIGUSR1 for example) doing nothing.
Use setsockopt() to unset the option SA_RESTART for the sockets in use.
Make the code issuing socket related system calls aware that they might return with -1 and errno set to EINTR.
Run the program.
Send it a signal for which the program has a handler installed (in 1.) from the outside (by for example using the kill <pid> -USR1 command).
Detect the reception of a signal (see 3.) and react, for example by close()ing the socket in question.
This is a question similar to Proper way to close a blocking UDP socket. I have a thread in C which is reading from a UDP socket. The read is blocking. I would like to know if it is possible to be able to exit the thread, without relying on the recv() returning? For example can I close the socket from another thread and safely expect the socket read thread to exit? Didn't see any high voted answer on that thread, thats why I am asking it again.
This really depends on what system you're running under. For example, if you're running under a POSIX-compliant system and your thread is cancelable, the recv() call will be interrupted when you cancel the thread since it's a cancel point.
If you're using an older socket implementation, you could set a signal handler for your thread for something like SIGUSR1 and hope nobody else wanted it and signal, since recv() will interrupt on a signal. Your best option is not to block, if at all possible.
I don't think closing a socket involved in a blocking operation is a safe guaranteed way of terminating the operation. For instance, kernel.org warns darkly:
It is probably unwise to close file descriptors while they may be in
use by system calls in other threads in the same process. Since a
file descriptor may be reused, there are some obscure race conditions
that may cause unintended side effects.
Instead you could use a signal and make recv fail with EINTR
(make sure SA_RESTART is not enabled). You can send a signal to a
specific thread with pthread_kill
You could enable SO_RCVTIMEO on the socket before starting the recv
call
Personally I usually try to stay clear of all the signal nastiness but it's a viable option.
You've got a couple of options for that. A signal will interrupt the read operation, so all you need to do is make sure a signal goes off. The recv operation should fail with error number EINTR.
The simplest option is to set up a timer to interrupt your own process after some timeout e.g. 30 seconds:
itimerval timer
timeval time;
time.tv_sec = 30;
time.tv_usec = 0;
timer.it_value = time;
if( setitimer( ITIMER_REAL, &timer, NULL ) != 0 )
printf( "failed to start timer\n" );
You'll get a SIGALRM after the specified time, which will interrupt your blocking operation, and give you the chance to repeat the operation or quit.
You cannot deallocate a shared resource while another thread is or might be using it. In practice, you will find that you cannot even write code to do what you suggest.
Think about it. When you go to call close, how can you possibly know that the other thread is actually blocked in recv? What if it's about to call recv, but then another thread calls socket and gets the descriptor you just closed? Now, not only will that thread not detect any error, but it will be calling recv on the wrong socket!
There is probably a good way to solve your outer problem, the reason you need to exit from a blocking UDP socket read. There are also several ugly hacks available. The basic approach is to make the socket non-blocking and instead of making a blocking UDP socket read, fake a blocking read with select or poll. You can then abort this loop several ways:
One way is to have select time out and check an 'abort' flag when select returns.
Another way is to also select on the read end of a pipe. Send a single byte to the pipe to abort the select.
If posix complient system, you can try to monitor your thread:
pthread_create with a function that makes your recv and pthread_cond_signal just after, then returns.
The calling thread makes a pthread_cond_timedwait with the desired timeout and terminates the called thread if timed_out.
I have a thread that is essentially just for listening on a socket. I have the thread blocking on accept() currently.
How do I tell the thread to finish any current transaction and stop listening, rather than staying blocked on accept?
I don't really want to do non-blocking if I don't have to...
Use the select(2) call to check which fd are ready to read.
The file descriptors from call can be read with out it blocking. eg accept() on the returned fd will immediately create a new connection.
Basically you have two options, the first one is to use interrupts: i.e
http://www.cs.cf.ac.uk/Dave/C/node32.html (see the signal handler section, it also supply a th_kill example).
From accept man page:
accept() shall fail if:
EINTR
The system call was interrupted by a signal that was caught before a valid connection arrived.
Another option is to use Non blocking sockets and select(): i.e.:
http://publib.boulder.ibm.com/infocenter/iseries/v5r3/index.jsp?topic=%2Frzab6%2Frzab6xnonblock.htm
Anyhow, usually in multi-threaded servers there's one thread which accepts new connections and spawns other threads for each connections. Since accept()ing and than recv()ing, can delay new connections requests... (Unless you're working with one client, and then accept()ing and recieving might be OK)
Use pthread_cancel on the thread. You'll need to make sure you've installed appropriate cancellation handlers (pthread_cleanup_push) to avoid resource leaks, and you should disable cancellation except for the duration of the accept call to avoid race conditions where the cancellation request might get acted upon later by a different function than accept.
Note that, due to bugs in glibc's implementation of cancellation, this approach could lead to lost connections and file descriptor leaks. This is because glibc/NPTL provides no guarantee that accept did not already finish execution and allocate a new file descriptor for the new connection before the cancellation request is acted upon. It should be a fairly rare occurrence but it's still an issue to consider...
See: http://sourceware.org/bugzilla/show_bug.cgi?id=12683
and for a discussion of the issue: Implementing cancellable syscalls in userspace
From Wake up thread blocked on accept() call
I just used the shutdown() system call and it seems to work...
Can a socket be closed from another thread when a send / recv on the same socket is going on?
Suppose one thread is in blocking recv call and another thread closes the same socket, will the thread in the recv call know this and come out safely?
I would like to know if the behavior will differ between different OS / Platforms. If yes, how will it behave in Solaris?
In linux closing a socket won't wake up recv(). Also, as #jxh says:
If a thread is blocked on recv() or send() when the socket is closed
by a different thread, the blocked thread will receive an error.
However, it is difficult to detect the correct remedial action after
receiving the error. This is because the file descriptor number
associated with the socket may have been picked up by yet a different
thread, and the blocked thread has now been woken up on an error for a
"valid" socket. In such a case, the woken up thread should not call
close() itself.
The woken up thread will need some way to differentiate whether the
error was generated by the connection (e.g. a network error) that
requires it to call close(), or if the error was generated by a
different thread having called close() on it, in which case it should
just error out without doing anything further to the socket.
So the best way to avoid both problems is to call shutdown() instead of close(). shutdown() will make the file descriptor still available, so won't be allocated by another descriptor, also will wake up recv() with an error and the thread with the recv() call can close the socket the normal way, like a normal error happened.
I don't know Solaris network stack implementation but I'll throw out my theory/explanation of why it should be safe.
Thread A enters some blocking system call, say read(2), for this given socket. There's no data in socket receive buffer, so thread A is taken off the processor an put onto wait queue for this socket. No network stack events are initiated here, connection state (assuming TCP) has not changed.
Thread B issues close(2) on the socket. While kernel socket structure should be locked while thread B is accessing it, no other thread is holding that lock (thread A released the lock when it was put to sleep-wait). Assuming there's no outstanding data in the socket send buffer, a FIN packet is sent and the connection enters the FIN WAIT 1 state (again I assume TCP here, see connection state diagram)
I'm guessing that socket connection state change would generate a wakeup for all threads blocked on given socket. That is thread A would enter a runnable state and discover that connection is closing. The wait might be re-entered if the other side has not sent its own FIN, or the system call would return with eof otherwise.
In any case, internal kernel structures will be protected from inappropriate concurrent access. This does not mean it's a good idea to do socket I/O from multiple threads. I would advise to look into non-blocking sockets, state machines, and frameworks like libevent.
For me, shutdown() socket from another thread do the job in Linux
If a thread is blocked on recv() or send() when the socket is closed by a different thread, the blocked thread will receive an error. However, it is difficult to detect the correct remedial action after receiving the error. This is because the file descriptor number associated with the socket may have been picked up by yet a different thread, and the blocked thread has now been woken up on an error for a "valid" socket. In such a case, the woken up thread should not call close() itself.
The woken up thread will need some way to differentiate whether the error was generated by the connection (e.g. a network error) that requires it to call close(), or if the error was generated by a different thread having called close() on it, in which case it should just error out without doing anything further to the socket.
Yes, it is ok to close the socket from another thread. Any blocked/busy threads that are using that socket will report a suitable error.
I'm just starting to learn how network programming in C works, and I've written a small program that sends messages to and from a UNIX terminal. I'm using pthreads in my program, one of which essentially just waits on recvfrom() to receive a message.
However, I want to be able to close all threads properly if the users chooses to quit the program. The way I have it set up right now, a different thread just cancels the thread waiting on recvfrom, but I'm worried this might not be a good idea since I'm leaving sockets unclosed and I'm not freeing all the memory I allocated.
Is there a way to cancel a recvfrom() call, or some way to run a certain routine upon cancelling a pthread?
Thanks.
If you use pthread_kill to send a signal to the thread, recvfrom() should return -1 with errno set to EINTR - you can then check for the "application is now exiting" condition and finish up gracefully.
Another approach would be to maintain a list of open connection matched with thread id's. Then whenever you cancel a thread go through the list to find it's socket and call close on the socket.