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OpenDarwin 7.2.1 - man page for upvar (opendarwin section n)

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


       upvar - Create link to variable in a different stack frame

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

       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 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  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.	Even if  the  name  looks
       like an array element, such as a(b), a regular variable is created.  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]
       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

       namespace  eval	is  another way (besides procedure calls) that the Tcl naming context can
       change.	It adds a call frame to the stack to represent 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  out-
       ermost 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 vari- |
       able 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 otherVar.  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.

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

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

Tcl											 upvar(n)

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