How thread-safe are AF_INET sockets?

Hi all,

This is my first post to this forum.

I'm writing an application in which I need to have "thread A" to receive UDP packets and "thread B" to send UDP packets, using the same socket.

Kernel: 2.6.20.x .

The question is, is it safe to have B calling sendto() while A is blocked in recvfrom() on the same socket?

Thank you in advance,
best regards.

--
Filippo

yes it should be possible.

create the socket in thread A.

while creating the thread B, pass this as parameter or save it in a global variable so that both the threads will be able to access it.

care should be take for proper synchronization.

You might actually have to make the socket non-blocking. This would require a slight change to thread A to check for data on the socket since it would no longer just block on the recvfrom.

I don't see why non-blocking sockets are needed here...

I do what OP is trying to do all the time. I almost always use non-blocking sockets with select for reading, though it does complicate writing if the other end is too slow to clear the buffer. I generally prefer non-blocking because it forces you to consider timing, which is important to threaded socket programs. Also, blocking isn't always reliable by itself since signals in one thread can cause system calls in another thread to interrupt, causing an EINTR, especially when using sockets.
ta0kira

Thank you all for your replies.

I've tested a lot this application using blocking sockets and didn't get any single error. So, I guess it's safe to use blocking sockets.

Btw, can you give me an example usage of non-blocking sockets and select() for major robustness?
Thanks.

Best regards.

--
Filippo

Originally Posted by fzangheri Thank you all for your replies. I've tested a lot this application using blocking sockets and didn't get any single error. So, I guess it's safe to use blocking sockets. Btw, can you give me an example usage of non-blocking sockets and select() for major robustness? Thanks. Best regards. -- Filippo

hi

if you don't mine.
can i have you have compiled socket programming code..
that you are using threads..

may be it will helpful for me.

Originally Posted by swift2008 hi if you don't mine. can i have you have compiled socket programming code.. that you are using threads.. may be it will helpful for me.

Hi swift2008,

May I ask you to reformulate your question, please? I'm not an English native speaker and I'm not sure I understand correctly your post.

Thanks.

--
Filippo

Originally Posted by fzangheri Hi swift2008, May I ask you to reformulate your question, please? I'm not an English native speaker and I'm not sure I understand correctly your post. Thanks. -- Filippo

can i see your socket program code.

Originally Posted by swift2008 can i see your socket program code.

Yes.

After removing all synchronization and communication mechanisms, the code looks kind of this:

/*
 * Thread A
 */
void *A()
{
	// Initialize UDP socket:
	sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
		
	// Bind socket
	
	while (1)
	{
		// Wait for UDP packet:
		recvfrom(rdfsock, rdfbuf, MAX_PACKET_SIZE, 0, (struct sockaddr *)&peerAddr, &peerAddr_len);
		// Send message to thread B
	}
	return NULL;
}

/*
 * Thread B
 */
void *B()
{
	while (1)
	{
		// Receive request from "A" via internal mechanism
		// Elaborate request and create response
		// Send response:
		sendto(sock, response, strlen(response), 0, (struct sockaddr *)&peerAddr, peerAddr_len);
	}
	return NULL;
}

int main()
{	
	pthread_t thread_A, thread_B;
	
	pthread_create(&thread_A, NULL, A, NULL);	
	pthread_create(&thread_B, NULL, B, NULL);
	pthread_join(thread_A, NULL);
	pthread_join(thread_B, NULL);
	
	pthread_exit(NULL);
	exit(0);
}