1 | '\" |
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2 | '\" Copyright (c) 1990-1994 The Regents of the University of California. |
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3 | '\" Copyright (c) 1994-1996 Sun Microsystems, Inc. |
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4 | '\" |
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5 | '\" See the file "license.terms" for information on usage and redistribution |
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6 | '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. |
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7 | '\" |
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8 | '\" RCS: @(#) $Id: bgerror.n,v 1.13 2007/12/13 15:22:32 dgp Exp $ |
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9 | '\" |
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10 | .so man.macros |
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11 | .TH bgerror n 7.5 Tcl "Tcl Built-In Commands" |
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12 | .BS |
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13 | '\" Note: do not modify the .SH NAME line immediately below! |
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14 | .SH NAME |
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15 | bgerror \- Command invoked to process background errors |
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16 | .SH SYNOPSIS |
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17 | \fBbgerror \fImessage\fR |
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18 | .BE |
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19 | |
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20 | .SH DESCRIPTION |
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21 | .VS 8.5 |
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22 | Release 8.5 of Tcl supports the \fBinterp bgerror\fR command, |
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23 | which allows applications to register in an interpreter the command |
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24 | that will handle background errors in that interpreter. In older |
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25 | releases of Tcl, this level of control was not available, and applications |
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26 | could control the handling of background errors only by creating |
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27 | a command with the particular command name \fBbgerror\fR in the |
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28 | global namespace of an interpreter. The following documentation |
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29 | describes the interface requirements of the \fBbgerror\fR command |
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30 | an application might define to retain compatibility with pre-8.5 |
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31 | releases of Tcl. Applications intending to support only |
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32 | Tcl releases 8.5 and later should simply make use of \fBinterp bgerror\fR. |
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33 | .VE 8.5 |
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34 | .PP |
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35 | The \fBbgerror\fR command does not exist as built-in part of Tcl. Instead, |
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36 | individual applications or users can define a \fBbgerror\fR |
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37 | command (e.g. as a Tcl procedure) if they wish to handle background |
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38 | errors. |
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39 | .PP |
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40 | A background error is one that occurs in an event handler or some |
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41 | other command that did not originate with the application. |
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42 | For example, if an error occurs while executing a command specified |
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43 | with the \fBafter\fR command, then it is a background error. |
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44 | For a non-background error, the error can simply be returned up |
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45 | through nested Tcl command evaluations until it reaches the top-level |
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46 | code in the application; then the application can report the error |
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47 | in whatever way it wishes. When a background error occurs, the |
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48 | unwinding ends in the Tcl library and there is no obvious way for Tcl |
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49 | to report the error. |
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50 | .PP |
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51 | When Tcl detects a background error, it saves information about the |
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52 | error and invokes a handler command registered by \fBinterp bgerror\fR |
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53 | later as an idle event handler. The default handler command in turn |
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54 | calls the \fBbgerror\fR command . |
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55 | Before invoking \fBbgerror\fR, Tcl restores the |
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56 | \fBerrorInfo\fR and \fBerrorCode\fR variables to their values at the |
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57 | time the error occurred, then it invokes \fBbgerror\fR with the error |
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58 | message as its only argument. Tcl assumes that the application has |
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59 | implemented the \fBbgerror\fR command, and that the command will |
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60 | report the error in a way that makes sense for the application. Tcl |
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61 | will ignore any result returned by the \fBbgerror\fR command as long |
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62 | as no error is generated. |
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63 | .PP |
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64 | If another Tcl error occurs within the \fBbgerror\fR command (for |
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65 | example, because no \fBbgerror\fR command has been defined) then Tcl |
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66 | reports the error itself by writing a message to stderr. |
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67 | .PP |
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68 | If several background errors accumulate before \fBbgerror\fR is |
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69 | invoked to process them, \fBbgerror\fR will be invoked once for each |
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70 | error, in the order they occurred. However, if \fBbgerror\fR returns |
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71 | with a break exception, then any remaining errors are skipped without |
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72 | calling \fBbgerror\fR. |
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73 | .PP |
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74 | If you are writing code that will be used by others as part of a |
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75 | package or other kind of library, consider avoiding \fBbgerror\fR. |
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76 | The reason for this is that the application programmer may also want |
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77 | to define a \fBbgerror\fR, or use other code that does and thus will |
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78 | have trouble integrating your code. |
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79 | .SH "EXAMPLE" |
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80 | This \fBbgerror\fR procedure appends errors to a file, with a timestamp. |
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81 | .CS |
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82 | proc bgerror {message} { |
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83 | set timestamp [clock format [clock seconds]] |
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84 | set fl [open mylog.txt {WRONLY CREAT APPEND}] |
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85 | puts $fl "$timestamp: bgerror in $::argv '$message'" |
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86 | close $fl |
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87 | } |
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88 | .CE |
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89 | |
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90 | .SH "SEE ALSO" |
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91 | after(n), interp(n), tclvars(n) |
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92 | |
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93 | .SH KEYWORDS |
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94 | background error, reporting |
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