The Ivy C library guide <author>Stéphane Chatty, <tt/chatty@cena.dgac.fr/ <date>1 April 1999 <abstract> This document is a programmer's guide that describes how to use the Ivy C library to connect applications to an Ivy bus. This guide describes version 3.0 of the library. </abstract> <toc> <sect>General information <sect1>What is Ivy? Ivy is a software bus designed at CENA (France). A software bus is a system that allows software applications to exchange information with the illusion of broadcasting that information, selection being performed by the receiving applications. Using a software bus is very similar to dealing with events in a graphical toolkit: on one side, messages are emitted without caring about who will handle them, and on the other side, one decide to handle the messages that have a certain type or follow a certain pattern. Software buses are mainly aimed at facilitating the rapid development of new agents, and at managing a dynamic collection of agents on the bus: agents show up, emit messages and receive some, then leave the bus without blocking the others. <sect2>Architecture and principles As opposed to other software buses, Ivy does not depend on a centralised server. Actually, Ivy is mostly a communication convention between processes, implemented through a collection of libraries in several languages. From the programmer's point of view, Ivy is an information broadcasting channel. The main functions are: <itemize> <item> connecting to a bus. Example: IvyInit (b, 2011) <item> sending a message. Example: IvySend (b, "HELLO %s", world) <item> bind a message pattern to a callback function. Example: IvyBind (b, "HELLO (.*)", cb) <item> the main loop. Example : IvyLoop () </itemize> The messages are exchanged in text format, and bindings are based on regular expressions with captures. If an application subscribes to <tt/HELLO (.*)/ an if another application emits the message <tt/HELLO WORLD/, a callback will be called in the first application with <tt/WORLD/ as an argument. <sect2>Using Ivy Libraries that implement Ivy are available in the following environments: <itemize> <item> in C on Unix and Windows platforms, with it own communication library <item> in C++ on Windows platforms <item> in C++ on Unix platforms, integrated with the Uch communication library <item> in C++ on Unix platforms, integrated with OpenInventor <item> in C++ on Macintosh <item> in Perl and in Perl/Tk <item> integrated with Object Caml on Unix platforms <item> in Scheme on Unix platforms <item> in Java </itemize> <sect1>The Ivy C library The Ivy C library (aka Ivy-C or ivy-c) is a C library that allows you to connect applications to an Ivy bus. You can use it to write applications in C or any other language that supports C extensions. <sect>Getting and installing the Ivy C library <sect>Basic functions <sect1>Initialization <sect1>Emitting messages <sect1>Subscribing to messages <sect>Managing timers and other channels In your applications, you may need to manage other input/output channels than an Ivy bus: a serial driver, the channels defined by a graphical toolkit, or simply stdin and stdout. The same applies for timers. You can either manage those channels or timers from the Ivy main loop, or instead use the main loop provided by another library. <sect1>Adding channels and timers to the Ivy main loop <sect2>Channels You can get a channel to be managed from the Ivy main loop by using functions <tt/IvyChannelSetUp/ and <tt/IvyChannelClose/. <tscreen><verb> Channel IvyChannelSetUp (HANDLE fd, void* data, ChannelHandleDelete handle_delete, ChannelHandleRead handle_read); </verb></tscreen> ensures that function <tt/handle_read/ is called whenever data is read on file descriptor <tt/fd/, and function <tt/handle_delete/ whenever <tt/fd/ is closed, and <tscreen><verb> void IvyChannelClose (Channel ch); </verb></tscreen> terminates the management of channel <tt/ch/. In what precedes, <tt/data/ is a pointer that will be passed to functions <tt/handle_read/ and <tt/handle_delete/. It can be defined at will by users. The types HANDLE, ChannelHandleDelete and ChannelHandleRead are as follows: <tscreen>Unix: <verb>typedef int HANDLE;</verb> Windows: <verb>typedef SOCKET HANDLE;</verb> <verb> typedef void (*ChannelHandleDelete)(void *data); typedef void (*ChannelHandleRead)(Channel ch, HANDLE fd, void* data); </verb></tscreen> <sect2>Timers You can get a function to be repeatedly called by using function <tt/TimerRepeatAfter/: <tscreen><verb> TimerId TimerRepeatAfter (int nbticks, long delay, TimerCb handle_timer, void* data); </verb></tscreen> ensures that function <tt/handle_timer/ is called <tt/nbticks/ times at intervals of <tt/delay/ seconds, thus creating a timer. <tscreen><verb> void TimerModify (TimerId id, long delay); </verb></tscreen> changes the delay used for timer <tt/id/. <tscreen><verb> void TimerRemove (TimerId id); </verb></tscreen> deletes timer <tt/id/, thus stopping it. In what precedes, <tt/data/ is passed to <tt/handle_timer/ every time it is called. <tt/delay/ is expressed in milliseconds. If <tt/nbticks/ is set to <tt/TIMER_LOOP/, then <tt/handle_timer/ will be called forever. <tt/TimerCb/ is as follows: <tscreen><verb> typedef void (*TimerCb)(TimerId id, void *data, unsigned long delta); </verb></tscreen> <sect1>Adding Ivy to another main loop <sect2>Functions to be provided You can decide to use the main loop from another toolkit such as the X Toolkit or the Tk toolkit. If you do that, you'll have to define three functions that Ivy will use to get its own channels managed by the other toolkit. The three following global variables should be defined: <tscreen><verb> ChannelInit channel_init; ChannelSetUp channel_setup; ChannelClose channel_close; </verb></tscreen> They should point to functions that respectively: <itemize> <item> make the necessary global initializations before entering the main loop <item> initialize a channel and ensure that it is managed by the main loop <item> close a channel </itemize> The types <tt/ChannelInit/, <tt/ChannelSetUp/ and <tt/ChannelClose/ are defined as follows: <tscreen><verb> typedef void (*ChannelInit)(void); typedef Channel (*ChannelSetUp)( HANDLE fd, void *data, ChannelHandleDelete handle_delete, ChannelHandleRead handle_read); typedef void (*ChannelClose)( Channel channel ); </verb></tscreen> <sect2>Type to be defined In order to implement the three previous functions, you will need to define the hidden type <tt/struct _channel/ (the type <tt/Channel/ is defined as <tt/struct _channel*/). Use it to store the data provided by the other toolkit. <sect2>Overriding the Ivy main loop In order to override the default definition of the three previous variables, you will need: <itemize> <item> either to create a new library by replacing file <tt/ivyloop.o/ with the file that contains your definitions <item> or ... </itemize> <sect>Conventions for writing applications ... the environment variable <tt/IVYBUS/ ..., ... the option <tt/-b/ ... </article>