<P>This command is used to create <EM>file event handlers</EM>. A file event
handler is a binding between a filehandle and a callback, such that the callback
is evaluated whenever the filehandle becomes readable or writable. File event
handlers are most commonly used to allow data to be received from another
process on an event-driven basis, so that the receiver can continue to
interact with the user while waiting for the data to arrive. If an
application invokes <CODE><></CODE>, <A HREF="../../../lib/Pod/perlfunc.html#item_sysread"><CODE>sysread</CODE></A> or <A HREF="../../../lib/Pod/perlfunc.html#item_read"><CODE>read</CODE></A> on a blocking filehandle when
there is no input data available, the process will block; until the input
data arrives, it will not be able to service other events, so it will
appear to the user to ``freeze up''. With <STRONG>fileevent</STRONG>, the process can
tell when data is present and only invoke <STRONG>gets</STRONG> or <STRONG>read</STRONG> when
they won't block.</P>
<P>The <EM>fileHandle</EM> argument to <STRONG>fileevent</STRONG> refers to an open filehandle,
such as the return value from a previous <STRONG>open</STRONG> or <STRONG>socket</STRONG>
command.
If the <EM>callback</EM> argument is specified, then <STRONG>fileevent</STRONG>
creates a new event handler: <EM>callback</EM> will be evaluated
whenever the filehandle becomes readable or writable (depending on the
argument to <STRONG>fileevent</STRONG>).
In this case <STRONG>fileevent</STRONG> returns an empty string.
The <STRONG>readable</STRONG> and <STRONG>writable</STRONG> event handlers for a file
are independent, and may be created and deleted separately.
However, there may be at most one <STRONG>readable</STRONG> and one <STRONG>writable</STRONG>
handler for a file at a given time in a given interpreter.
If <STRONG>fileevent</STRONG> is called when the specified handler already
exists in the invoking interpreter, the new callback replaces the old one.</P>
<P>If the <EM>callback</EM> argument is not specified, <STRONG>fileevent</STRONG>
returns the current callback for <EM>fileHandle</EM>, or an empty string
if there is none.
If the <EM>callback</EM> argument is specified as an empty string
then the event handler is deleted, so that no callback will be invoked.
A file event handler is also deleted automatically whenever
its filehandle is closed or its interpreter is deleted.</P>
<P>A filehandle is considered to be readable if there is unread data
available on the underlying device.
A filehandle is also considered to be readable if an end of file or
error condition is present on the underlying file or device.
It is important for <EM>callback</EM> to check for these conditions
and handle them appropriately; for example, if there is no special
check for end of file, an infinite loop may occur where <EM>callback</EM>
reads no data, returns, and is immediately invoked again.</P>
<P>A filehandle is considered to be writable if at least one byte of data
can be written to the underlying file or device without blocking,
or if an error condition is present on the underlying file or device.</P>
<P>Event-driven I/O works best for filehandles that have been
placed into nonblocking mode.
In blocking mode, a <A HREF="../../../lib/Pod/perlfunc.html#item_print"><CODE>print</CODE></A> command may block if you give it
more data than the underlying file or device can accept, and a
<CODE><></CODE>, <A HREF="../../../lib/Pod/perlfunc.html#item_sysread"><CODE>sysread</CODE></A> or <A HREF="../../../lib/Pod/perlfunc.html#item_read"><CODE>read</CODE></A> command will block if you attempt to read
more data than is ready; no events will be processed while the
commands block.
In nonblocking mode <A HREF="../../../lib/Pod/perlfunc.html#item_print"><CODE>print</CODE></A>, <CODE><></CODE>, <A HREF="../../../lib/Pod/perlfunc.html#item_sysread"><CODE>sysread</CODE></A> and <A HREF="../../../lib/Pod/perlfunc.html#item_read"><CODE>read</CODE></A> never block.
See the documentation for the individual commands for information
on how they handle blocking and nonblocking filehandles.</P>
<P>The callback for a file event is executed in the context of <EM>$widget</EM>
with which <STRONG>fileevent</STRONG> was invoked.
If an error occurs while executing the callback then the
<A HREF="../../../site/lib/Tk/Error.html">the Tk::Error manpage</A> mechanism is used to report the error.
In addition, the file event handler is deleted if it ever returns
an error; this is done in order to prevent infinite loops due to
buggy handlers.</P>
<P>
<HR>
<H1><A NAME="bugs">BUGS</A></H1>
<P>On windows platforms <STRONG>fileevent</STRONG> is limited in the types of filehandles
that behave correctly. Making filefhandles non-blocking is only implemented
on a subset of UNIX platforms (see <A HREF="../../../site/lib/Tk/IO.html">the Tk::IO manpage</A>).</P>
<P>
<HR>
<H1><A NAME="credits">CREDITS</A></H1>
<P><STRONG>fileevent</STRONG> is based on the <STRONG>addinput</STRONG> command created
