raw/mann/bgerror.n

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  10 '\" The definitions below are for supplemental macros used in Tcl/Tk
  11 '\" manual entries.
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  71 '\" RCS: @(#) $Id: bgerror.n,v 1.1 2003/12/20 03:31:54 bbbush Exp $
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  73 '\"     # Set up traps and other miscellaneous stuff for Tcl/Tk man pages.
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 208 .SH "STANDARD OPTIONS"
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 219 See the \\fBoptions\\fR manual entry for details on the standard options.
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 226 Command-Line Name:      \\fB\\$1\\fR
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 246 .TH bgerror n 7.5 Tcl "Tcl Built-In Commands"
 247 .BS
 248 '\" Note:  do not modify the .SH NAME line immediately below!
 249 .SH NAME
 250 bgerror \- Command invoked to process background errors
 251 .SH SYNOPSIS
 252 \fBbgerror \fImessage\fR
 253 .BE
 254
 255 .SH DESCRIPTION
 256 .PP
 257 The \fBbgerror\fR command doesn't exist as built-in part of Tcl.  Instead,
 258 individual applications or users can define a \fBbgerror\fR
 259 command (e.g. as a Tcl procedure) if they wish to handle background
 260 errors.
 261 .PP
 262 A background error is one that occurs in an event handler or some
 263 other command that didn't originate with the application.
 264 For example, if an error occurs while executing a command specified
 265 with the \fBafter\fR command, then it is a background error.
 266 For a non-background error, the error can simply be returned up
 267 through nested Tcl command evaluations until it reaches the top-level
 268 code in the application; then the application can report the error
 269 in whatever way it wishes.  When a background error occurs, the
 270 unwinding ends in the Tcl library and there is no obvious way for Tcl
 271 to report the error.
 272 .PP
 273 When Tcl detects a background error, it saves information about the
 274 error and invokes the \fBbgerror\fR command later as an idle event
 275 handler. Before invoking \fBbgerror\fR, Tcl restores the
 276 \fBerrorInfo\fR and \fBerrorCode\fR variables to their values at the
 277 time the error occurred, then it invokes \fBbgerror\fR with the error
 278 message as its only argument.  Tcl assumes that the application has
 279 implemented the \fBbgerror\fR command, and that the command will
 280 report the error in a way that makes sense for the application.  Tcl
 281 will ignore any result returned by the \fBbgerror\fR command as long
 282 as no error is generated.
 283 .PP
 284 If another Tcl error occurs within the \fBbgerror\fR command (for
 285 example, because no \fBbgerror\fR command has been defined) then Tcl
 286 reports the error itself by writing a message to stderr.
 287 .PP
 288 If several background errors accumulate before \fBbgerror\fR is
 289 invoked to process them, \fBbgerror\fR will be invoked once for each
 290 error, in the order they occurred.  However, if \fBbgerror\fR returns
 291 with a break exception, then any remaining errors are skipped without
 292 calling \fBbgerror\fR.
 293 .PP
 294 Tcl has no default implementation for \fBbgerror\fR. However, in
 295 applications using Tk there is a default \fBbgerror\fR procedure which
 296 posts a dialog box containing the error message and offers the user a
 297 chance to see a stack trace showing where the error occurred.  In
 298 addition to allowing the user to view the stack trace, the dialog
 299 provides an additional application configurable button which may be
 300 used, for example, to save the stack trace to a file.  By default,
 301 this is the behavior associated with that button.  This behavior can
 302 be redefined by setting the option database values
 303 \fB*ErrorDialog.function.text\fR, to specify the caption for the
 304 function button, and \fB*ErrorDialog.function.command\fR, to specify
 305 the command to be run.  The text of the stack trace is appended to the
 306 command when it is evaluated.  If either of these options is set to
 307 the empty string, then the additional button will not be displayed in
 308 the dialog.
 309
 310 .SH "SEE ALSO"
 311 after(n), tclvars(n)
 312
 313 .SH KEYWORDS
 314 background error, reporting