fileevent(3pm) User Contributed Perl Documentation fileevent(3pm)
NAME
Tk::fileevent - Execute a callback when a filehandle becomes readable or
writable
SYNOPSIS
$widget->fileevent(fileHandle,readable?,callback?)
$widget->fileevent(fileHandle,writable?,callback?)
DESCRIPTION
This command is used to create file event handlers. 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 "<>", "sysread" or "read" 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
fileevent, the process can tell when data is present and only invoke
gets or read when they won't block.
The fileHandle argument to fileevent refers to an open filehandle, such
as the return value from a previous open or socket command. If the
callback argument is specified, then fileevent creates a new event
handler: callback will be evaluated whenever the filehandle becomes
readable or writable (depending on the argument to fileevent). In this
case fileevent returns an empty string. The readable and writable event
handlers for a file are independent, and may be created and deleted
separately. However, there may be at most one readable and one writable
handler for a file at a given time in a given interpreter. If fileevent
is called when the specified handler already exists in the invoking
interpreter, the new callback replaces the old one.
If the callback argument is not specified, fileevent returns the current
callback for fileHandle, or an empty string if there is none. If the
callback 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.
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 callback 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
callback reads no data, returns, and is immediately invoked again.
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.
Event-driven I/O works best for filehandles that have been placed into
nonblocking mode. In blocking mode, a "print" command may block if you
give it more data than the underlying file or device can accept, and a
"<>", "sysread" or "read" 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 "print", "<>", "sysread" and "read" never
block. See the documentation for the individual commands for
information on how they handle blocking and nonblocking filehandles.
The callback for a file event is executed in the context of $widget with
which fileevent was invoked. If an error occurs while executing the
callback then the Tk::Error 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.
BUGS
On windows platforms fileevent is limited in the types of filehandles
that behave correctly. Making filehandles non-blocking is only
implemented on a subset of UNIX platforms (see Tk::IO).
CREDITS
fileevent is based on the addinput command created by Mark Diekhans.
SEE ALSO
Tk::IO Tk::callbacks
KEYWORDS
asynchronous I/O, blocking, filehandle, event handler, nonblocking,
readable, callback, writable.
perl v5.40.1 2025-04-13 fileevent(3pm)
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