Provided by: tcl8.5-doc_8.5.19-4_all 
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 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 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. 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 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 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 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.
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(3tcl), namespace(3tcl), uplevel(3tcl), variable(3tcl)
KEYWORDS
context, frame, global, level, namespace, procedure, variable