Program:      gdsserver

Purpose:      Handles GDS requests from GIPSY tasks.

Category:     SYSTEM

File:         gdsserver.src

Author:       J.P. Terlouw
 
Use:          $gip_exe/gdsserver

Description:  The server is normally started by the first task which needs it.
              This is done transparantly when any GDS routine is called.
              After startup it can also process requests from other tasks.
              For every user who runs one or more GIPSY sessions there is
              a server for every directory in which sets are being accessed.

              Tasks can communicate with the server in one of two ways:
              via a Unix socket or via an Internet socket. Using the Unix
              socket is the most efficient. This method is chosen when the
              server is started and will be used by all subsequent clients
              which run on the same host. Clients on other hosts will use
              the Internet socket.

              When the server does not have any clients anymore, it will
              exit after 5 minutes, except if the server has only had remote 
              clients during the last 5 minutes. In this case it will exit
              immediately so that clients have the opportunity to start
              a local server with which communication is more efficient.

              To connect to the server, a task uses the following files in the
              user's home directory, which are created by the server:

                 .gds_sockets<directory_id>
                 .gds_read
                 .gds_write
                
              .gds_sockets* contain information about the Internet address of
              the host on which the server runs, the Internet port number and
              the path of the Unix socket. This file is only present when the
              server is active.
               
              .gds_read contains a 'password' which the client software needs
              to read from a GDS set using the server. The protection of this
              file determines who can access the server to read from any sets
              it serves. The default protection of this file is 644, but this
              can be changed by the user with the chmod command.
               
              .gds_write contains a 'password' which the client software needs
              to write to a GDS set using the server. The default protection 
              of this file is 600, but this can also be changed.

              In every directory which is being or has been served, there is
              a file with the name .gds_ident .

              This version only serves descriptor files. Image I/O is done
              by a routine in the client part of GDS.
              
Updates:      Apr  9, 1994: JPT original document created.
              Mar 17, 1995: JPT fixed byte-swap bug in DescrFind (descr.c);
                                added server id command line argument.
              Mar 28, 1995: JPT do not change existing .gds_read and .gds_write
              May  3, 1995: JPT improved consistency checks (descr.c).
              Aug 24, 1995: JPT inter-architecture bug fixed (clientmgr.c).
              Mar 26, 1996: JPT null pointer bug fixed (linkedlist.c).
              Sep  2, 1996: JPT Small code change by KGB.
              Jun 26, 1998: JPT Fixed bug in low-high-endian inter-operability. 
              Sep 19, 2000: JPT Exit on sockets file write error.
              Feb  8, 2002: JPT Fixed byte-swap bug in DescrDelall (descr.c).
              May  4, 2007: JPT Included conditional code for Apple Mac.
              Feb 19, 2009: JPT Implemented temporary (FITS pseudo-) sets.