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Tk Library Procedures Tk_CreateErrorHandler(3TK)

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NAME

Tk_CreateErrorHandler, Tk_DeleteErrorHandler - handle X pro-

tocol errors

SYNOPSIS

#include

Tk_ErrorHandler

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

Tk_DeleteErrorHandler(handler)

ARGUMENTS Display *display (in) Display whose errors are to be handled. int error (in) Match only error events with this value in the

error_code field.

If -1, then match

any error_code

value. int request (in) Match only error events with this value in the

request_code field.

If -1, then match

any request_code

value. int minor (in) Match only error events with this value in the

minor_code field.

If -1, then match

any minor_code

value.

Tk_ErrorProc *proc (in) Procedure to invoke

whenever an error event is received for display and matches error, request, and minor. NULL means ignore any matching errors. Tk Last change: 1

Tk Library Procedures Tk_CreateErrorHandler(3TK)

ClientData clientData (in) Arbitrary one-word

value to pass to proc.

Tk_ErrorHandler handler (in) Token for error

handler to delete (return value from a previous call to

Tk_CreateErrorHandler).

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DESCRIPTION

Tk_CreateErrorHandler arranges for a particular procedure

(proc) to be called whenever certain protocol errors occur on a particular display (display). Protocol errors occur when the X protocol is used incorrectly, such as attempting

to map a window that doesn't exist. See the Xlib documenta-

tion for XSetErrorHandler for more information on the kinds of errors that can occur. For proc to be invoked to handle a particular error, five things must occur: [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 (see below for more information).

Proc should have arguments and result that match the follow-

ing type:

typedef int Tk_ErrorProc(

ClientData clientData, XErrorEvent *errEventPtr); The clientData parameter to proc is a copy of the clientData

argument given to Tcl_CreateErrorHandler when the callback

was created. Typically, clientData points to a data struc-

ture containing application-specific information that is

needed to deal with the error. ErrEventPtr is a pointer to the X error event. The procedure proc should return an integer value. If it returns 0 it means that proc handled Tk Last change: 2

Tk Library Procedures Tk_CreateErrorHandler(3TK)

the error completely and there is no need to take any other

action for the error. If it returns non-zero it means proc

was unable to handle the error. If a value of NULL is specified for proc, all matching errors will be ignored: this will produce the same result as if a procedure had been specified that always returns 0. If more than more than one handler matches a particular error, then they are invoked in turn. The handlers will be invoked in reverse order of creation: most recently

declared handler first. If any handler returns 0, then sub-

sequent (older) handlers will not be invoked. If no handler returns 0, then Tk invokes X'es default error handler, which prints an error message and aborts the program. If you wish to have a default handler that deals with errors that no other handler can deal with, then declare it first. 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; disobey it at

your own risk.

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, the actual callback to proc may not occur until

after the Tk_DeleteErrorHandler call, due to buffering in

the client and server. If an error event pertains to a pro-

tocol 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. 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; this will flush

out any buffered requests and errors, but will result 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. Tk Last change: 3

Tk Library Procedures Tk_CreateErrorHandler(3TK)

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.

KEYWORDS callback, error, event, handler

ATTRIBUTES

See attributes(5) for descriptions of the following attri-

butes:

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| ATTRIBUTE TYPE | ATTRIBUTE VALUE|

|____________________|__________________|_

| Availability | runtime/tk-8 |

|____________________|__________________|_

| Interface Stability| Uncommitted |

|____________________|_________________|

NOTES Source for Tk is available on http://opensolaris.org. Tk Last change: 4