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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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.so man.macros

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

.BS

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

.SH NAME

uplevel \- Execute a script in a different stack frame

.SH SYNOPSIS

\fBuplevel \fR?\fIlevel\fR?\fI arg \fR?\fIarg ...\fR?

.BE

.SH DESCRIPTION

.PP

All of the \fIarg\fR arguments are concatenated as if they had

been passed to \fBconcat\fR; the result is then evaluated in the

variable context indicated by \fIlevel\fR.  \fBUplevel\fR returns

the result of that evaluation.

.PP

If \fIlevel\fR is an integer then

it gives a distance (up the procedure calling stack) to move before

executing the command.  If \fIlevel\fR consists of \fB#\fR followed by

a number then the number gives an absolute level number.  If \fIlevel\fR

is omitted then it defaults to \fB1\fR.  \fILevel\fR cannot be

defaulted if the first \fIcommand\fR argument starts with a digit or \fB#\fR.

.PP

For example, suppose that procedure \fBa\fR was invoked

from top-level, and that it called \fBb\fR, and that \fBb\fR called \fBc\fR.

Suppose that \fBc\fR invokes the \fBuplevel\fR command.  If \fIlevel\fR

is \fB1\fR or \fB#2\fR  or omitted, then the command will be executed

in the variable context of \fBb\fR.  If \fIlevel\fR is \fB2\fR or \fB#1\fR

then the command will be executed in the variable context of \fBa\fR.

If \fIlevel\fR is \fB3\fR or \fB#0\fR then the command will be executed

at top-level (only global variables will be visible).

.PP

The \fBuplevel\fR command causes the invoking procedure to disappear

from the procedure calling stack while the command is being executed.

In the above example, suppose \fBc\fR invokes the command

.CS

\fBuplevel 1 {set x 43; d}\fR

.CE

where \fBd\fR is another Tcl procedure.  The \fBset\fR command will

modify the variable \fBx\fR in \fBb\fR's context, and \fBd\fR will execute

at level 3, as if called from \fBb\fR.  If it in turn executes

the command

.CS

\fBuplevel {set x 42}\fR

.CE

then the \fBset\fR command will modify the same variable \fBx\fR in \fBb\fR's

context:  the procedure \fBc\fR does not appear to be on the call stack

when \fBd\fR is executing.  The command ``\fBinfo level\fR'' may

be used to obtain the level of the current procedure.

.PP

\fBUplevel\fR makes it possible to implement new control

constructs as Tcl procedures (for example, \fBuplevel\fR could

be used to implement the \fBwhile\fR construct as a Tcl procedure).

.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).

.SH "SEE ALSO"

namespace(n)

.SH KEYWORDS

context, level, namespace, stack frame, variables