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: http://hea-www.harvard.edu/RD/xpa/intro.html
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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:
[data] | xpaset[qualifiers ...] xpaget [qualifiers ...]
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 thread.