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man pages section 1: User Commands

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Updated: Wednesday, February 9, 2022

bgerror (1t)


bgerror - Command invoked to process background errors


bgerror message


bgerror(1t)                  Tcl Built-In Commands                 bgerror(1t)


       bgerror - Command invoked to process background errors

       bgerror message

       Release  8.5  of  Tcl supports the interp bgerror command, which allows
       applications to register in an interpreter the command that will handle
       background  errors in that interpreter.  In older releases of Tcl, this
       level of control was not available, and applications could control  the
       handling  of background errors only by creating a command with the par-
       ticular 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 com-
       mand,  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 appli-
       cation can report the error in whatever way it wishes.   When  a  back-
       ground 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 error-
       Info 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  pack-
       age  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.

       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 attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

       after(n), errorCode(n), errorInfo(n), interp(n)

       background error, reporting

       Source code for open source software components in Oracle  Solaris  can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                   7.5                          bgerror(1t)