write to socket problem

[Andreas Höschler]

Hi all,

I have an Objective-C tool that offers its service (route-calculation) 
via a TCP-socket. A client connects, orders the calculation of a route 
and when the method has calculated the result it is written back with 
the following code:

      FD_ZERO(&fdset);
      FD_SET(_sock, &fdset);
      switch (select(_sock+1, NULL, &fdset, NULL, (_timeout.tv_sec == 0 
? NULL : &_timeout)))
        {
         case -1:
            [_writeLock unlock];
            [NSException raise:SRFileHandleWriteException 
format:@"poll() call in %@ returned error: %i", 
NSStringFromSelector(_cmd), errno];

         case 0:
            [_writeLock unlock];
            [NSException raise:SRFileHandleTimeoutException 
format:@"poll() call in %@ timed out after %u seconds.", 
NSStringFromSelector(_cmd), _timeout.tv_sec];
        }
     ret = send(_sock, dataStart + pos, length - pos, 0);

This usually works. However the client disconnects after 75s (this 
seems to be a built-in timeout on Solaris). If this happens and 
calculation takes more than 75s and finally after may be 2 minutes the 
service tries to write the result to the (already closed on the reading 
end) socket, the

     ret = send(_sock, dataStart + pos, length - pos, 0);

call blocks and never returns and thus renders the service inoperable. 
I have already spent a couple of hours with the select, write, send,... 
documentation but haven't found a solution for this problem yet. Any 
idea how I can find out whether the reading end has closed? I expected 
select to do this, but it does not. select returns and FD_ISSET(_sock, 
&fdset)) gives me TRUE. But then the send blocks!

What can I do?

Thanks a lot,

 Andreas