1bgerror(n) Tcl Built-In Commands bgerror(n)
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8 bgerror - Command invoked to process background errors
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11 bgerror message
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16 The bgerror command doesn't exist as built-in part of Tcl. Instead,
17 individual applications or users can define a bgerror command (e.g. as
18 a Tcl procedure) if they wish to handle background errors.
19 A background error is one that occurs in an event handler or some other
21 command that didn't originate with the application. For example, if an
22 error occurs while executing a command specified with the after com‐
23 mand, then it is a background error. For a non-background error, the
24 error can simply be returned up through nested Tcl command evaluations
25 until it reaches the top-level code in the application; then the appli‐
26 cation can report the error in whatever way it wishes. When a back‐
27 ground error occurs, the unwinding ends in the Tcl library and there is
28 no obvious way for Tcl to report the error.
29 When Tcl detects a background error, it saves information about the
31 error and invokes the bgerror command later as an idle event handler.
32 Before invoking bgerror, Tcl restores the errorInfo and errorCode vari‐
33 ables to their values at the time the error occurred, then it invokes
34 bgerror with the error message as its only argument. Tcl assumes that
35 the application has implemented the bgerror command, and that the com‐
36 mand will report the error in a way that makes sense for the applica‐
37 tion. Tcl will ignore any result returned by the bgerror command as
38 long as no error is generated.
39 If another Tcl error occurs within the bgerror command (for example,
41 because no bgerror command has been defined) then Tcl reports the error
42 itself by writing a message to stderr.
43 If several background errors accumulate before bgerror is invoked to
45 process them, bgerror will be invoked once for each error, in the order
46 they occurred. However, if bgerror returns with a break exception,
47 then any remaining errors are skipped without calling bgerror.
48 Tcl has no default implementation for bgerror. However, in applications
50 using Tk there is a default bgerror procedure which posts a dialog box
51 containing the error message and offers the user a chance to see a
52 stack trace showing where the error occurred. In addition to allowing
53 the user to view the stack trace, the dialog provides an additional
54 application configurable button which may be used, for example, to save
55 the stack trace to a file. By default, this is the behavior associated
56 with that button. This behavior can be redefined by setting the option
57 database values *ErrorDialog.function.text, to specify the caption for
58 the function button, and *ErrorDialog.function.command, to specify the
59 command to be run. The text of the stack trace is appended to the com‐
60 mand when it is evaluated. If either of these options is set to the
61 empty string, then the additional button will not be displayed in the
62 dialog.
63 If you are writing code that will be used by others as part of a pack‐
65 age or other kind of library, consider avoiding bgerror. The reason
66 for this is that the application programmer may also want to define a
67 bgerror, or use other code that does and thus will have trouble inte‐
68 grating your code.
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71 This bgerror procedure appends errors to a file, with a timestamp.
72 proc bgerror {message} {
73 set timestamp [clock format [clock seconds]]
74 set fl [open mylog.txt {WRONLY CREAT APPEND}]
75 puts $fl "$timestamp: bgerror in $::argv '$message'"
76 close $fl
77 }
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81 after(n), tclvars(n)
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85 background error, reporting
86Tcl 7.5 bgerror(n)