fileevent(3pm)

💖 fileevent(3pm) — the Tk::fileevent era 💖

okay bestie, welcome to the manual page for Tk::fileevent, straight out of the User Contributed Perl Documentation. Sit down, grab a strawberry 🍓, and let me tell you about the girl who tells you the instant your filehandle has something to say.

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✨ NAME ✨

Tk::fileevent — Execute a callback when a filehandle becomes readable or writable 🦋

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✨ SYNOPSIS ✨

$widget->fileevent(fileHandle,readable?,callback?)

$widget->fileevent(fileHandle,writable?,callback?)

no cap, that's the whole vibe 💅

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✨ DESCRIPTION ✨

okay so. This command is used to create file event handlers. And a file event handler is literally just a binding between a filehandle and a callback, such that the callback gets evaluated whenever that filehandle becomes readable or writable. That's it, that's the whole relationship 💖

File event handlers are most commonly used to let data be received from another process on an event-driven basis, so that the receiver can keep interacting with the user while it's waiting for the data to arrive. Because think about it: if an application invokes <>, sysread or read on a blocking filehandle when there's no input data available, the process will block. And until the input data arrives, it will not be able to service other events, so it will appear to the user to "freeze up" 😭. Not the vibe. With fileevent, the process can tell when data is present and only invoke gets or read when they won't block. main character energy honestly 👑

The fileHandle argument to fileevent refers to an open filehandle, like 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 — and this is important bestie — 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. periodt.

If the callback argument is not specified, fileevent returns the current callback for fileHandle, or an empty string if there is none. And 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 🫶

Now, when is a filehandle considered readable? 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. And 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. 😭💀 like girl, that's a spin cycle, not a program.

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 info on how they handle blocking and nonblocking filehandles. iykyk 📚

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 — and this is the dramatic part — 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. it's giving protective bestie behavior honestly 🙈

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🐛 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). so yeah, cross-platform girlies, check yourself before you wreck yourself ✨

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💐 CREDITS 💐

fileevent is based on the addinput command created by Mark Diekhans. thank you king 🫡👑

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👀 SEE ALSO 👀

• Tk::IO
• Tk::callbacks

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🔑 KEYWORDS 🔑

asynchronous I/O, blocking, filehandle, event handler, nonblocking, readable, callback, writable. 🌸

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and that's the whole page, bestie — written for perl v5.40.1, dated 2025-04-13, on the fileevent(3pm) page from the User Contributed Perl Documentation. go forth and never block your event loop again 💅💖✨