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

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NAME

       upvar - Create link to variable in a different stack frame

SYNOPSIS

       upvar ?level? otherVar myVar ?otherVar myVar ...?
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DESCRIPTION

       This  command  arranges for one or more local variables in the current procedure to refer to variables in an enclosing 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  is not # 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  argu-
       ment.   The variable named by otherVar need not exist at the time of the call;  it will be created the first time myVar is referenced, 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 variable, not an array element.  An error is returned if the name looks like an array element, such as a(b).	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 example, consider the following procedure:
	      proc add2 name {
		 upvar $name x
		 set x [expr {$x + 2}]
	      }
       If  add2  is  invoked  with an argument giving the name of a variable, 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.

       namespace eval is another way (besides procedure calls) that the Tcl naming context can change.	It adds a call frame to the stack to  rep-
       resent  the  namespace  context.   This means each namespace eval command counts as another call level for uplevel and upvar commands.  For
       example, info level 1 will return a list describing a command that is either the outermost procedure call or the outermost  namespace  eval
       command.  Also, uplevel #0 evaluates a script at top-level in the outermost namespace (the global namespace).

       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.

TRACES AND UPVAR

       Upvar  interacts  with  traces in a straightforward but possibly unexpected manner.  If a variable trace is defined on otherVar, that trace
       will be triggered by actions involving myVar.  However, the trace procedure will be passed the name of myVar, rather than the name of  oth-
       erVar.  Thus, the output of the following code will be "localVar" rather than "originalVar":
	      proc traceproc { name index op } {
		 puts $name
	      }
	      proc setByUpvar { name value } {
		 upvar $name localVar
		 set localVar $value
	      }
	      set originalVar 1
	      trace variable originalVar w traceproc
	      setByUpvar originalVar 2

       If otherVar refers to an element of an array, then variable 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.

EXAMPLE

       A decr command that works like incr except it subtracts the value from the variable instead of adding it:
	      proc decr {varName {decrement 1}} {
		  upvar 1 $varName var
		  incr var [expr {-$decrement}]
	      }

SEE ALSO

       global(n), namespace(n), uplevel(n), variable(n)

KEYWORDS

       context, frame, global, level, namespace, procedure, variable

Tcl																	  upvar(n)