Tk_CreateErrorHandler(3tk)

💖 Tk_CreateErrorHandler — handle X protocol errors 💖

okay so bestie, this is a page from Tk Library Procedures, and it's literally all about Tk_CreateErrorHandler and Tk_DeleteErrorHandler, the two functions that catch X protocol errors for you. welcome to the group chat, grab a snack, we're doing error handling 🦋

---

✨ NAME ✨

Tk_CreateErrorHandler, Tk_DeleteErrorHandler — handle X protocol errors 💅

---

✨ SYNOPSIS ✨

#include <tk.h>

Tk_ErrorHandler
Tk_CreateErrorHandler(display, error, request, minor, proc, clientData)

Tk_DeleteErrorHandler(handler)

---

✨ ARGUMENTS ✨

• Display *display (in) — the display whose errors are gonna get handled. this is your display, bestie, we're not doing anybody else's 💖
• int error (in) — only match error events whose error_code field is this value. if it's -1, then it matches any error_code value. no filter, no problem ✨
• int request (in) — only match error events whose request_code field is this value. if it's -1, then it matches any request_code value. slay 🎀
• int minor (in) — only match error events whose minor_code field is this value. if it's -1, then it matches any minor_code value. iconic 💅
• Tk_ErrorProc *proc (in) — the procedure that gets invoked whenever an error event comes in for display and matches error, request, and minor. and if it's NULL? that means ignore any matching errors, periodt 🙈
• void *clientData (in) — an arbitrary one-word value that gets passed to proc. your little bag of snacks to hand over 🌸
• Tk_ErrorHandler handler (in) — the token for the error handler you wanna delete. this is the return value from a previous call to Tk_CreateErrorHandler 👑

---

✨ DESCRIPTION ✨

okay so Tk_CreateErrorHandler arranges for a particular procedure (proc) to be called whenever certain protocol errors happen on a particular display (display). protocol errors are what happens when the X protocol gets used incorrectly — like, imagine trying to map a window that doesn't even exist. rude. 😭 if you want more info on the kinds of errors that can occur, go read the Xlib documentation for XSetErrorHandler.

now here's the vibe check. for proc to actually get invoked to handle a particular error, five things have to happen:

1. the error must pertain to display.
2. either the error argument to Tk_CreateErrorHandler must have been -1, or the error argument must match the error_code field from the error event.
3. either the request argument to Tk_CreateErrorHandler must have been -1, or the request argument must match the request_code field from the error event.
4. either the minor argument to Tk_CreateErrorHandler must have been -1, or the minor argument must match the minor_code field from the error event.
5. the protocol request to which the error pertains must have been made when the handler was active (peep below for more on that one 👀).

proc should have arguments and a result that match this type, exactly:

typedef int Tk_ErrorProc(
        void *clientData,
        XErrorEvent *errEventPtr);

the clientData parameter to proc is a copy of the clientData argument that was given to Tcl_CreateErrorHandler when the callback was created. typically, clientData points to a data structure holding application-specific info that's needed to deal with the error. and errEventPtr is a pointer to the X error event. 💫

proc should return an integer value. if it returns 0, that means proc handled the error completely and there's no need to take any other action for that error. if it returns non-zero, that means proc was unable to handle the error. no shade, it happens 😔

if you specify a value of NULL for proc, all matching errors will be ignored — which produces the same result as if a procedure had been specified that always returns 0. same energy, less typing 💅

bestie listen — if more than more than one handler matches a particular error, then they get invoked in turn. and they're invoked in reverse order of creation: most recently declared handler first. if any handler returns 0, then the subsequent (older) handlers will not be invoked. and if no handler returns 0, then Tk invokes X's default error handler, which prints an error message and aborts the program. 💀 so if you want a default handler that deals with errors no other handler can deal with, declare it first. put your safety net down before you start doing tricks, fr fr.

the X documentation states that “the error handler should not call any functions (directly or indirectly) on the display that will generate protocol requests or that will look for input events.” this restriction applies to handlers declared by Tk_CreateErrorHandler — so disobey it at your own risk. not me warning you twice 🫶

Tk_DeleteErrorHandler may be called to delete a previously-created error handler. the handler argument identifies the error handler, and should be a value returned by a previous call to Tk_CreateEventHandler. 👑

a particular error handler applies to errors resulting from protocol requests generated between the call to Tk_CreateErrorHandler and the call to Tk_DeleteErrorHandler. however — and this is the plot twist — the actual callback to proc may not occur until after the Tk_DeleteErrorHandler call, because of buffering in the client and server. so if an error event pertains to a protocol request made just before calling Tk_DeleteErrorHandler, then the error event may not have been processed before the Tk_DeleteErrorHandler call. when this situation arises, Tk will save information about the handler and invoke the handler's proc later when the error event finally arrives. 🦋

so if an application wishes to delete an error handler and know for certain that all relevant errors have been processed, it should first call Tk_DeleteErrorHandler and then call XSync. that flushes out any buffered requests and errors — but heads up, it results in a performance penalty because it requires communication to and from the X server. after the XSync call, Tk is guaranteed not to call any error handlers deleted before the XSync call. worth it for the peace of mind, honestly 💖

and for the Tk error handling mechanism to work properly, it is essential that application code never calls XSetErrorHandler directly. applications should use only Tk_CreateErrorHandler. like, do not. i'm begging. 🙏

---

✨ KEYWORDS ✨

callback, error, event, handler 🌈

---

aaaand that's the whole page, bestie 🎀 tysm for reading — this one came from the Tk crew, and it's Tk_CreateErrorHandler(3tk), so you know it's the real deal. go forth and handle your errors responsibly. mwah 💜💅✨