Tcl_CreateFileHandler(3tcl)

💖 NAME 💖

okay bestie, so this page is the Tcl Library Procedures manual, and it's about two little functions: Tcl_CreateFileHandler and Tcl_DeleteFileHandler — they're the ones that associate procedure callbacks with files or devices. And um, the way that this is Unix only? iconic but also read the fine print, bestie 🫶

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✨ SYNOPSIS ✨

#include <tcl.h>

Tcl_CreateFileHandler(fd, mask, proc, clientData)

Tcl_DeleteFileHandler(fd)

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🎀 ARGUMENTS 🎀

• int fd (in) — a Unix file descriptor for an open file or device. like, the actual handle to the thing. 🔥
• int mask (in) — the conditions under which proc should get called: an OR-ed combination of TCL_READABLE, TCL_WRITABLE, and TCL_EXCEPTION. you can also set it to 0 to temporarily disable a handler, which is lowkey so handy. 🙈
• Tcl_FileProc *proc (in) — the procedure to invoke whenever the file or device indicated by file meets the conditions specified by mask. (yes, the manpage says file there, we're just relaying the vibes 💅)
• void *clientData (in) — an arbitrary one-word value to pass to proc. no questions asked, it's your lil payload. 🎁

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🌸 DESCRIPTION 🌸

Tcl_CreateFileHandler arranges for proc to be invoked in the future whenever I/O becomes possible on a file or an exceptional condition exists for the file. The file is indicated by fd, and the conditions of interest are indicated by mask. So like, for example, if mask is TCL_READABLE, then proc will be called when the file is readable. periodt. ✨

and this is the crucial part bestie: the callback to proc is made by Tcl_DoOneEvent, so Tcl_CreateFileHandler is only useful in programs that dispatch events through Tcl_DoOneEvent or through Tcl commands such as vwait. no event loop, no callbacks, that's the whole deal. 😭

proc should have arguments and result that match the type Tcl_FileProc:

typedef void Tcl_FileProc(
        void *clientData,
        int mask);

The clientData parameter to proc is a copy of the clientData argument given to Tcl_CreateFileHandler when the callback was created. Typically, clientData points to a data structure containing application-specific information about the file. And mask is an integer mask indicating which of the requested conditions actually exists for the file — it will contain a subset of the bits in the mask argument to Tcl_CreateFileHandler. subset!! not the whole thing!! 🦋

There may exist only one handler for a given file at a given time. If Tcl_CreateFileHandler is called when a handler already exists for fd, then the new callback replaces the information that was previously recorded. it's giving main character energy, one handler per file, no plus-ones. 👑

Tcl_DeleteFileHandler may be called to delete the file handler for fd; if no handler exists for the file given by fd then the procedure has no effect. literally just vibes, it does nothing, no drama. 💀

The purpose of file handlers is to enable an application to respond to events while waiting for files to become ready for I/O. For this to work correctly, the application may need to use non-blocking I/O operations on the files for which handlers are declared. Otherwise the application may block if it reads or writes too much data; while waiting for the I/O to complete, the application will not be able to service other events. 🫠

so like, use Tcl_SetChannelOption with -blocking to set the channel into blocking or nonblocking mode as required. do not skip this bestie, this is the part that saves you. ⭐

And a heads up: note that these interfaces are only supported by the Unix implementation of the Tcl notifier. like we said at the top, Unix only, no cap. 🐧

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👀 SEE ALSO 👀

• fileevent(3tcl)
• Tcl_CreateTimerHandler(3tcl)
• Tcl_DoWhenIdle(3tcl)

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💜 KEYWORDS 💜

callback, file, handler

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and that's the whole page, bestie!! this one's from the Tcl Library Procedures manual, version 8.0, dated with the classic Tcl_CreateFileHandler(3tcl) footer — go forth and handle your files, you absolute icon 🧚💖