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I am using a loop to read message out from a c Berkeley socket but I am not able to detect when the socket is disconnected so I would accept a new connection. please help
while(true) {
bzero(buffer,256);
n = read(newsockfd,buffer,255);
printf("%s\n",buffer);
}
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In TCP there is only one way to detect an orderly disconnect, and that is by getting zero as a return value from read()/recv()/recvXXX() when reading. There is also only one reliable way to detect a broken connection: by writing to it.
You could check if the socket is still connected by trying to write to the file descriptor for each socket. Then if the return value of the write is -1 or if errno = EPIPE, you know that socket has been closed.
If you need to determine the current state of the connection, make a nonblocking, zero-byte Send call. If the call returns successfully or throws a WAEWOULDBLOCK error code (10035), then the socket is still connected; otherwise, the socket is no longer connected.
The only way you can detect that a socket is connected is by writing to it.
Getting a error on read()/recv() will indicate that the connection is broken, but not getting an error when reading doesn't mean that the connection is up.
You may be interested in reading this: http://lkml.indiana.edu/hypermail/linux/kernel/0106.1/1154.html
In addition, using TCP Keep Alive may help distinguish between inactive and broken connections (by sending something at regular intervals even if there's no data to be sent by the application).
(EDIT: Removed incorrect sentence as pointed out by @Damon, thanks.)
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