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)
- 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.
- 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).
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 does not exist. See the
Xlib documentation 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 following 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 structure
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 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
subsequent (older) handlers will not be invoked. If no handler returns 0, then
Tk invokes X's 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 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. 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.
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