NAME
       upvar - Create link to variable in a different stack frame

SYNOPSIS
       upvar ?level? otherVar myVar ?otherVar myVar ...?


DESCRIPTION
       This command arranges for one or more local  variables  in
       the  current procedure to refer to variables in an enclos-
       ing procedure call or to global variables.  Level may have
       any  of  the  forms permitted for the uplevel command, and
       may be omitted if the first letter of the  first  otherVar
       isn't  # or a digit (it defaults to 1).  For each otherVar
       argument, upvar makes the variable by  that  name  in  the
       procedure  frame  given  by  level (or at global level, if
       level is #0) accessible in the current  procedure  by  the
       name given in the corresponding myVar argument.  The vari-
       able named by otherVar need not exist at the time  of  the
       call;   it  will be created the first time myVar is refer-
       enced, just like an ordinary  variable.   There  must  not
       exist  a  variable  by the name myVar at the time upvar is
       invoked.  MyVar is always treated as the name of  a  vari-
       able,  not  an array element.  Even if the name looks like
       an array element, such as a(b), a regular variable is cre-
       ated.   OtherVar may refer to a scalar variable, an array,
       or an array element.  Upvar returns an empty string.

       The upvar command simplifies the implementation  of  call-
       by-name  procedure  calling  and  also  makes it easier to
       build new control constructs as Tcl procedures.  For exam-
       ple, consider the following procedure:
              proc add2 name {
                upvar $name x
                set x [expr $x+2]
              }
       Add2  is  invoked  with  an  argument giving the name of a
       variable, and it adds two to the value of  that  variable.
       Although  add2  could  have been implemented using uplevel
       instead of upvar, upvar  makes  it  simpler  for  add2  to
       access the variable in the caller's procedure frame.

       If  an upvar variable is unset (e.g. x in add2 above), the
       unset operation affects the variable it is linked to,  not
       the  upvar  variable.   There  is no way to unset an upvar
       variable except by exiting the procedure in  which  it  is
       defined.   However,  it  is  possible to retarget an upvar
       variable by executing another upvar command.


BUGS
       If otherVar refers to an element of an array,  then  vari-
       able  traces  set for the entire array will not be invoked
       when myVar is  accessed  (but  traces  on  the  particular
       element  will  still  be  invoked).  In particular, if the
       array is env, then changes  made  to  myVar  will  not  be
       passed to subprocesses correctly.


KEYWORDS
       context, frame, global, level, procedure, variable
