1bgerror(n) Tcl Built-In Commands bgerror(n)
2______________________________________________________________________________
6
8 bgerror - Command invoked to process background errors
9
11 bgerror message
12_________________________________________________________________
13
16 Release 8.5 of Tcl supports the interp bgerror command, which allows │
17 applications to register in an interpreter the command that will handle │
18 background errors in that interpreter. In older releases of Tcl, this │
19 level of control was not available, and applications could control the │
20 handling of background errors only by creating a command with the par‐ │
21 ticular command name bgerror in the global namespace of an interpreter. │
22 The following documentation describes the interface requirements of the │
23 bgerror command an application might define to retain compatibility │
24 with pre-8.5 releases of Tcl. Applications intending to support only │
25 Tcl releases 8.5 and later should simply make use of interp bgerror.
26 The bgerror command does not exist as built-in part of Tcl. Instead,
28 individual applications or users can define a bgerror command (e.g. as
29 a Tcl procedure) if they wish to handle background errors.
30 A background error is one that occurs in an event handler or some other
32 command that did not originate with the application. For example, if
33 an error occurs while executing a command specified with the after com‐
34 mand, then it is a background error. For a non-background error, the
35 error can simply be returned up through nested Tcl command evaluations
36 until it reaches the top-level code in the application; then the appli‐
37 cation can report the error in whatever way it wishes. When a back‐
38 ground error occurs, the unwinding ends in the Tcl library and there is
39 no obvious way for Tcl to report the error.
40 When Tcl detects a background error, it saves information about the
42 error and invokes a handler command registered by interp bgerror later
43 as an idle event handler. The default handler command in turn calls
44 the bgerror command . Before invoking bgerror, Tcl restores the error‐
45 Info and errorCode variables to their values at the time the error
46 occurred, then it invokes bgerror with the error message as its only
47 argument. Tcl assumes that the application has implemented the bgerror
48 command, and that the command will report the error in a way that makes
49 sense for the application. Tcl will ignore any result returned by the
50 bgerror command as long as no error is generated.
51 If another Tcl error occurs within the bgerror command (for example,
53 because no bgerror command has been defined) then Tcl reports the error
54 itself by writing a message to stderr.
55 If several background errors accumulate before bgerror is invoked to
57 process them, bgerror will be invoked once for each error, in the order
58 they occurred. However, if bgerror returns with a break exception,
59 then any remaining errors are skipped without calling bgerror.
60 If you are writing code that will be used by others as part of a pack‐
62 age or other kind of library, consider avoiding bgerror. The reason
63 for this is that the application programmer may also want to define a
64 bgerror, or use other code that does and thus will have trouble inte‐
65 grating your code.
66
68 This bgerror procedure appends errors to a file, with a timestamp.
69 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 }
75
78 after(n), interp(n), tclvars(n)
79
82 background error, reporting
83Tcl 7.5 bgerror(n)