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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15 Release 8.5 of Tcl supports the interp bgerror command, which allows
16 applications to register in an interpreter the command that will handle
17 background errors in that interpreter. In older releases of Tcl, this
18 level of control was not available, and applications could control the
19 handling of background errors only by creating a command with the par‐
20 ticular command name bgerror in the global namespace of an interpreter.
21 The following documentation describes the interface requirements of the
22 bgerror command an application might define to retain compatibility
23 with pre-8.5 releases of Tcl. Applications intending to support only
24 Tcl releases 8.5 and later should simply make use of interp bgerror.
25 The bgerror command does not exist as built-in part of Tcl. Instead,
27 individual applications or users can define a bgerror command (e.g. as
28 a Tcl procedure) if they wish to handle background errors.
29 A background error is one that occurs in an event handler or some other
31 command that did not originate with the application. For example, if
32 an error occurs while executing a command specified with the after com‐
33 mand, then it is a background error. For a non-background error, the
34 error can simply be returned up through nested Tcl command evaluations
35 until it reaches the top-level code in the application; then the appli‐
36 cation can report the error in whatever way it wishes. When a back‐
37 ground error occurs, the unwinding ends in the Tcl library and there is
38 no obvious way for Tcl to report the error.
39 When Tcl detects a background error, it saves information about the
41 error and invokes a handler command registered by interp bgerror later
42 as an idle event handler. The default handler command in turn calls
43 the bgerror command . Before invoking bgerror, Tcl restores the error‐
44 Info and errorCode variables to their values at the time the error
45 occurred, then it invokes bgerror with the error message as its only
46 argument. Tcl assumes that the application has implemented the bgerror
47 command, and that the command will report the error in a way that makes
48 sense for the application. Tcl will ignore any result returned by the
49 bgerror command as long as no error is generated.
50 If another Tcl error occurs within the bgerror command (for example,
52 because no bgerror command has been defined) then Tcl reports the error
53 itself by writing a message to stderr.
54 If several background errors accumulate before bgerror is invoked to
56 process them, bgerror will be invoked once for each error, in the order
57 they occurred. However, if bgerror returns with a break exception,
58 then any remaining errors are skipped without calling bgerror.
59 If you are writing code that will be used by others as part of a pack‐
61 age or other kind of library, consider avoiding bgerror. The reason
62 for this is that the application programmer may also want to define a
63 bgerror, or use other code that does and thus will have trouble inte‐
64 grating your code.
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67 This bgerror procedure appends errors to a file, with a timestamp.
68 proc bgerror {message} {
70 set timestamp [clock format [clock seconds]]
71 set fl [open mylog.txt {WRONLY CREAT APPEND}]
72 puts $fl "$timestamp: bgerror in $::argv '$message'"
73 close $fl
74 }
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77 after(n), errorCode(n), errorInfo(n), interp(n)
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80 background error, reporting
81Tcl 7.5 bgerror(n)