Hi

I'm having trouble with fd_set's, select and the FS_ISSET macro. I have two separate fd_set's one for the listen socket and one for connected sockets. Once I have accepted a connection that socket is added to the fd_set of connected sockets. A message is then sent by the remote client, FD_ISSET(...) recognizes this and I read the data. The problem is that once the remote socket has been closed FD_ISSET(...) keeps signaling that I have new data waiting on this socket, but when I try to read it I get a return value of 0 which is correct for a disconnected socket. I noticed this because I first had forgotten to remove the socket from the fd_set, which I now have corrected, but the question remains, why does FD_ISSET(...) keep signaling new data on a closed socket?

/Kristofer

I don't know the internals of the TCP stack but my guess is that you don't close the socket once you've found that the other side has disconnected (read returns 0).

This behavior is by design. Here's a short explanation:
http://www.developerweb.net/forum/ar...hp/t-3316.html

Thx for the good replies, helped me find the explanation I was looking for:
http://lkml.org/lkml/2002/6/10/68.

Basically it's a implementation dispute, where one implementation exists on Linux and another on "Other UNIX".

Here are some citations from the discussions in the above mentioned thread:
"The problem stems from the fact that Linux does not make distinction between 'not yet connected' and 'already closed' states.

What you say is true and correct for 'already closed' state. In this case
select _must_ return 'ready to read' at least once so that application can
properly sense EOF condition. What happens on subsequent select's/read's is
strictly speaking implementation defined (access to a file after EOF has
been seen is undefined) but natural implementation is "sticky EOF" that
always returns EOF after socket has been closed."

Example output from a test application:
"Linux runs:

bor@cooker% ./a.out
testing with a normal, unconnected socket:
socket is readable
socket is writeable
created socket, select() returned 2
read() returned 0, errno = 0 (Success)
write() returned -1, errno = 32 (Broken pipe)
testing with a non-blocking, unconnected socket:
socket is readable
socket is writeable
socket nonblocking, select() returned 2
read() returned 0, errno = 0 (Success)
write() returned -1, errno = 32 (Broken pipe)

"Other UNIX" runs:

bor@itsrm2% /tmp/a.out
testing with a normal, unconnected socket:
created socket, select() returned 0
read() returned -1, errno = 134 (Transport endpoint is not connected)
write() returned -1, errno = 134 (Transport endpoint is not connected)
testing with a non-blocking, unconnected socket:
socket nonblocking, select() returned 0
read() returned -1, errno = 134 (Transport endpoint is not connected)
write() returned -1, errno = 134 (Transport endpoint is not connected)"