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tcl_backgrounderror(3) [opendarwin man page]

Tcl_BackgroundError(3)					      Tcl Library Procedures					    Tcl_BackgroundError(3)


Tcl_BackgroundError - report Tcl error that occurred in background processing SYNOPSIS
#include <tcl.h> Tcl_BackgroundError(interp) ARGUMENTS
Tcl_Interp *interp (in) Interpreter in which the error occurred. _________________________________________________________________ DESCRIPTION
This procedure is typically invoked when a Tcl error occurs during ``background processing'' such as executing an event handler. When such an error occurs, the error condition is reported to Tcl or to a widget or some other C code, and there is not usually any obvious way for that code to report the error to the user. In these cases the code calls Tcl_BackgroundError with an interp argument identifying the interpreter in which the error occurred. At the time Tcl_BackgroundError is invoked, the interpreter's result is expected to contain an error message. Tcl_BackgroundError will invoke the bgerror Tcl command to report the error in an application-specific fashion. If no bgerror command exists, or if it returns with an error condition, then Tcl_BackgroundError reports the error itself by printing a message on the standard error file. Tcl_BackgroundError does not invoke bgerror immediately because this could potentially interfere with scripts that are in process at the time the error occurred. Instead, it invokes bgerror later as an idle callback. Tcl_BackgroundError saves the values of the errorInfo and errorCode variables and restores these values just before invoking bgerror. It is possible for many background errors to accumulate before bgerror is invoked. When this happens, each of the errors is processed in order. However, if bgerror returns a break exception, then all remaining error reports for the interpreter are skipped. KEYWORDS
background, bgerror, error Tcl 7.5 Tcl_BackgroundError(3)

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bgerror(n)						       Tcl Built-In Commands							bgerror(n)


bgerror - Command invoked to process background errors SYNOPSIS
bgerror message _________________________________________________________________ DESCRIPTION
Release 8.5 of Tcl supports the interp bgerror command, which allows applications to register in an interpreter the command that will han- | dle background errors in that interpreter. In older releases of Tcl, this level of control was not available, and applications could con- | trol the handling of background errors only by creating a command with the particular command name bgerror in the global namespace of an | interpreter. The following documentation describes the interface requirements of the bgerror command an application might define to retain | compatibility with pre-8.5 releases of Tcl. Applications intending to support only Tcl releases 8.5 and later should simply make use of | interp bgerror. The bgerror command does not exist as built-in part of Tcl. Instead, individual applications or users can define a bgerror command (e.g. as a Tcl procedure) if they wish to handle background errors. A background error is one that occurs in an event handler or some other command that did not originate with the application. For example, if an error occurs while executing a command specified with the after command, then it is a background error. For a non-background error, the error can simply be returned up through nested Tcl command evaluations until it reaches the top-level code in the application; then the application can report the error in whatever way it wishes. When a background error occurs, the unwinding ends in the Tcl library and there is no obvious way for Tcl to report the error. When Tcl detects a background error, it saves information about the error and invokes a handler command registered by interp bgerror later as an idle event handler. The default handler command in turn calls the bgerror command . Before invoking bgerror, Tcl restores the errorInfo and errorCode variables to their values at the time the error occurred, then it invokes bgerror with the error message as its only argument. Tcl assumes that the application has implemented the bgerror command, and that the command will report the error in a way that makes sense for the application. Tcl will ignore any result returned by the bgerror command as long as no error is generated. If another Tcl error occurs within the bgerror command (for example, because no bgerror command has been defined) then Tcl reports the error itself by writing a message to stderr. If several background errors accumulate before bgerror is invoked to process them, bgerror will be invoked once for each error, in the order they occurred. However, if bgerror returns with a break exception, then any remaining errors are skipped without calling bgerror. If you are writing code that will be used by others as part of a package or other kind of library, consider avoiding bgerror. The reason for this is that the application programmer may also want to define a bgerror, or use other code that does and thus will have trouble inte- grating your code. EXAMPLE
This bgerror procedure appends errors to a file, with a timestamp. proc bgerror {message} { set timestamp [clock format [clock seconds]] set fl [open mylog.txt {WRONLY CREAT APPEND}] puts $fl "$timestamp: bgerror in $::argv '$message'" close $fl } SEE ALSO
after(n), interp(n), tclvars(n) KEYWORDS
background error, reporting Tcl 7.5 bgerror(n)
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