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'\" Copyright (c) 1993 The Regents of the University of California.

'\" Copyright (c) 1994-1997 Sun Microsystems, Inc.

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'\" RCS: @(#) $Id: upvar.n,v 1.1.1.1 2002-01-16 10:25:25 markom Exp $

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.so man.macros

.TH upvar n "" Tcl "Tcl Built-In Commands"

.BS

'\" Note:  do not modify the .SH NAME line immediately below!

.SH NAME

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

.SH SYNOPSIS

\fBupvar \fR?\fIlevel\fR? \fIotherVar myVar \fR?\fIotherVar myVar \fR...?

.BE

.SH DESCRIPTION

.PP

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.

\fILevel\fR may have any of the forms permitted for the \fBuplevel\fR

command, and may be omitted if the first letter of the first \fIotherVar\fR

isn't \fB#\fR or a digit (it defaults to \fB1\fR).

For each \fIotherVar\fR argument, \fBupvar\fR makes the variable

by that name in the procedure frame given by \fIlevel\fR (or at

global level, if \fIlevel\fR is \fB#0\fR) accessible

in the current procedure by the name given in the corresponding

\fImyVar\fR argument.

The variable named by \fIotherVar\fR need not exist at the time of the

call;  it will be created the first time \fImyVar\fR is referenced, just like

an ordinary variable.  There must not exist a variable by the

name \fImyVar\fR at the time \fBupvar\fR is invoked.

\fIMyVar\fR is always treated as the name of a variable, not an

array element.  Even if the name looks like an array element,

such as \fBa(b)\fR, a regular variable is created.

\fIOtherVar\fR may refer to a scalar variable, an array,

or an array element.

\fBUpvar\fR returns an empty string.

.PP

The \fBupvar\fR 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:

.CS

\fBproc add2 name {

        upvar $name x

        set x [expr $x+2]

}\fR

.CE

\fBAdd2\fR is invoked with an argument giving the name of a variable,

and it adds two to the value of that variable.

Although \fBadd2\fR could have been implemented using \fBuplevel\fR

instead of \fBupvar\fR, \fBupvar\fR makes it simpler for \fBadd2\fR

to access the variable in the caller's procedure frame.

.PP

\fBnamespace eval\fR 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 \fBnamespace eval\fR command

counts as another call level for \fBuplevel\fR and \fBupvar\fR commands.

For example, \fBinfo level 1\fR will return a list

describing a command that is either

the outermost procedure call or the outermost \fBnamespace eval\fR command.

Also, \fBuplevel #0\fR evaluates a script

at top-level in the outermost namespace (the global namespace).

.PP

.VS

If an upvar variable is unset (e.g. \fBx\fR in \fBadd2\fR above), the

\fBunset\fR 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 \fBupvar\fR

command.

.SH BUGS

.PP

If \fIotherVar\fR refers to an element of an array, then variable

traces set for the entire array will not be invoked when \fImyVar\fR

is accessed (but traces on the particular element will still be

invoked).  In particular, if the array is \fBenv\fR, then changes

made to \fImyVar\fR will not be passed to subprocesses correctly.

.VE

.SH "SEE ALSO"

namespace(n)

.SH KEYWORDS

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