Tcl_BackgroundError(3) Tcl Library Procedures Tcl_BackgroundError(3)

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NAME

Tcl_BackgroundException, Tcl_BackgroundError - report Tcl exception
that occurred in background processing

SYNOPSIS

#include <tcl.h>

Tcl_BackgroundException(interp, code)

Tcl_BackgroundError(interp)

ARGUMENTS

Tcl_Interp *interp (in) Interpreter in which the exception
occurred.

int code (in) The exceptional return code to be
reported.
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DESCRIPTION

This procedure is typically invoked when a Tcl exception (any return
code other than TCL_OK) occurs during "background processing" such as
executing an event handler. When such an exception occurs, the
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
exception to the user. In these cases the code calls
Tcl_BackgroundException with an interp argument identifying the
interpreter in which the exception occurred, and a code argument
holding the return code value of the exception. The state of the
interpreter, including any error message in the interpreter result,
and the values of any entries in the return options dictionary, is
captured and saved. Tcl_BackgroundException then arranges for the
event loop to invoke at some later time the command registered in
that interpreter to handle background errors by the interp bgerror
command, passing the captured values as arguments. The registered
handler command is meant to report the exception in an application-
specific fashion. The handler command receives two arguments, the
result of the interp, and the return options of the interp at the
time the error occurred. If the application registers no handler
command, the default handler command will attempt to call bgerror to
report the error. If an error condition arises while invoking the
handler command, then Tcl_BackgroundException reports the error
itself by printing a message on the standard error file.

Tcl_BackgroundException does not invoke the handler command
immediately because this could potentially interfere with scripts
that are in process at the time the error occurred. Instead, it
invokes the handler command later as an idle callback.

It is possible for many background exceptions to accumulate before
the handler command is invoked. When this happens, each of the
exceptions is processed in order. However, if the handler command
returns a break exception, then all remaining error reports for the
interpreter are skipped.

The Tcl_BackgroundError routine is an older and simpler interface
useful when the exception code reported is TCL_ERROR. It is
equivalent to:

Tcl_BackgroundException(interp, TCL_ERROR);

KEYWORDS

background, bgerror, error, interp

Tcl 7.5 Tcl_BackgroundError(3)