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       XPAIntro -  Introduction to the XPA Messaging System


       A brief introduction to the XPA messaging system, which provides seamless communication
       between all kinds of Unix event-driven programs, including X programs, Tcl/Tk programs,
       and Perl programs.


       The XPA messaging system provides seamless communication between all kinds of Unix
       programs, including X programs, Tcl/Tk programs, and Perl programs.  It also provides an
       easy way for users to communicate with these XPA-enabled programs by executing XPA client
       commands in the shell or by utilizing such commands in scripts.  Because XPA works both at
       the programming level and the shell level, it is a powerful tool for unifying any analysis
       environment: users and programmers have great flexibility in choosing the best level or
       levels at which to access XPA services, and client access can be extended or modified
       easily at any time.

       A program becomes an XPA-enabled server by defining named points of public access through
       which data and commands can be exchanged with other client programs (and users).  Using
       standard TCP sockets as a transport mechanism, XPA supports both single-point and
       broadcast messaging to and from these servers.  It supports direct communication between
       clients and servers, or indirect communication via an intermediate message bus emulation
       program. Host-based access control is implemented, as is as the ability to communicate
       with XPA servers across a network.

       XPA implements a layered interface that is designed to be useful both to software
       developers and to users.  The interface consists of a library of XPA client and server
       routines for use in programs and a suite of high-level user programs built on top of these
       libraries.  Using the XPA library, access points can be added to Tcl/Tk programs, Xt
       programs, or to Unix programs that use the XPA event loop or any event loop based on
       select().  Client access subroutines can be added to any Tcl/Tk or Unix program. Client
       access also is supported at the command line via a suite of high-level programs.

       The major components of the XPA layered interface are:

       ·   A set of XPA server routines, centered on XPANew(), which are used by XPA server
           programs to tag public access points with string identifiers and to register send and
           receive callbacks for these access points.

       ·   A set of XPA client routines, centered on the XPASet() and XPAGet(), which are used by
           external client applications to exchange data and commands with an XPA server.

       ·   High-level programs, centered on xpaset and xpaget, which allow data and information
           to be exchanged with XPA server programs from the command line and from scripts.
           These programs have the command syntax:

             [data] | xpaset  [qualifiers ...]
                      xpaget  [qualifiers ...]

       ·   An XPA name server program, xpans, through which XPA access point names are registered
           by servers and distributed to clients.

       Defining an XPA access point is easy: a server application calls XPANew(), XPACmdNew(), or
       the experimental XPAInfoNew() routine to create a named public access point.  An XPA
       service can specify "send" and "receive" callback procedures (or an "info" procedure in
       the case of XPAInfoNew()) to be executed by the program when an external process either
       sends data or commands to this access point or requests data or information from this
       access point.  Either of the callbacks can be omitted, so that a particular access point
       can be specified as read-only, read-write, or write-only.  Application-specific client
       data can be associated with these callbacks.  Having defined one or more public access
       points in this way, an XPA server program enters its usual event loop (or uses the
       standard XPA event loop).

       Clients communicate with these XPA public access points using programs such as xpaget,
       xpaset, and xpainfo (at the command line), or routines such as XPAGet(), XPASet(), and
       XPAInfo() within a program.  Both methods require specification of the name of the access
       point.  The xpaget program returns data or other information from an XPA server to its
       standard output, while the xpaset program sends data or commands from its standard input
       to an XPA application. The corresponding API routines set/get data to/from memory,
       returning error messages and other info as needed.  If a template is used to specify the
       access point name (e.g., "ds9*"), then communication will take place with all servers
       matching that template.

       Please note that XPA currently is not thread-safe. All XPA calls must be in the same


       See xpa(7) for a list of XPA help pages