Thread: recv returns with 0

Hello.

I have a strange problem with TCP sockets. I have a client which connects to the server and sends some 8-byte string message. the send() function returns value 8. Now on the server side when the recv() function unblocks it returns 0, meaning the client has disconnected. But that is not the case since the client loops and no close() or shutdown() was called.

I've also checked the packets with sharkwire and it turns out there was no error at all. There's the 3-way handshake upon connection and then the PSH packet from the client. After that the server responses with ACK and that's that. I made a syscall trace but between connect() and recvfrom() (or sendto() on the client side) there is no single call to close() on the socket.

What could be wrong?

Greets,
Luka

code?

The code is too large to post. With syscall trace I can assume a simple tcp server/client code can be taken. I'm not doing anything special witch the code. Just initialization, connection and sending/recieving. All other functions return without errors(socket(), connect(), bind(), accept(), listen()). The problem is just with the recv() on the server side. It should not return 0 because I'm not calling close() anywhere.

You sure you couldn't trim it down? You could attach it or post it up on a file sharing site.

It's kind of hard to figure it out without the code.

I'm afraid trimming down the source takes too much time. But like I said the could could look as a simple tcp server/client.

I've made some more packet inspection and found out that after the server ACK-s the receiving packet, it then sends a RST,ACK packet back to the client and terminates the connection.

Originally Posted by Salem

Are you passing any values for the 'flags' ?
What is the value of errno when it returns 0?
Is the socket non-blocking?

The sockets are blocking. When receiving, I pass MSG_WAITALL to recv(), the send() flag is 0. The errno value is 2.

> Is the socket non-blocking?

I thought this too, but the man page said it would return -1 and set errno to EAGAIN on nonblock, not return 0.

Well there are several "modules" that use the same network library for network IO. The strange thing is while other modules work, it's only when I began writing a new module this error appeared.

Originally Posted by robwhit

Is the code multithreaded? Do you have well defined boundaries for how to handle networking code across thread and/or "modules"?

> The errno value is 2.

You have to find out what error that corresponds to on your system. When did it get set?

It's "No such file or directory"