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man pages section 1: User Commands

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Updated: Thursday, June 13, 2019
 
 

uplevel (1t)

Name

uplevel - Execute a script in a different stack frame

Synopsis

uplevel ?level? arg ?arg ...?

Description

uplevel(1t)                  Tcl Built-In Commands                 uplevel(1t)



______________________________________________________________________________

NAME
       uplevel - Execute a script in a different stack frame

SYNOPSIS
       uplevel ?level? arg ?arg ...?
______________________________________________________________________________

DESCRIPTION
       All of the arg arguments are concatenated as if they had been passed to
       concat; the result is then evaluated in the variable context  indicated
       by level.  Uplevel returns the result of that evaluation.

       If level is an integer then it gives a distance (up the procedure call-
       ing stack) to move before executing the command.  If level consists  of
       #  followed by a number then the number gives an absolute level number.
       If level is omitted then it defaults to 1.  Level cannot  be  defaulted
       if the first command argument starts with a digit or #.

       For  example,  suppose that procedure a was invoked from top-level, and
       that it called b, and that b called c.   Suppose  that  c  invokes  the
       uplevel  command.   If  level  is 1 or #2  or omitted, then the command
       will be executed in the variable context of b.  If level  is  2  or  #1
       then  the  command  will  be executed in the variable context of a.  If
       level is 3 or #0 then the command will be executed at  top-level  (only
       global variables will be visible).

       The uplevel command causes the invoking procedure to disappear from the
       procedure calling stack while the command is being  executed.   In  the
       above example, suppose c invokes the command

              uplevel 1 {set x 43; d}

       where  d  is  another  Tcl  procedure.  The set command will modify the
       variable x in b's context, and d will execute at level 3, as if  called
       from b.  If it in turn executes the command

              uplevel {set x 42}

       then  the  set  command will modify the same variable x in b's context:
       the procedure c does not appear to be on the call stack when d is  exe-
       cuting.   The info level command may be used to obtain the level of the
       current procedure.

       Uplevel makes it possible to implement new control  constructs  as  Tcl
       procedures  (for  example, uplevel could be used to implement the while
       construct as a Tcl procedure).

       The namespace eval and apply commands offer other ways (besides  proce-
       dure  calls)  that  the Tcl naming context can change.  They add 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  describ-
       ing a command that is either the outermost procedure call or the outer-
       most namespace eval command.  Also, uplevel #0 evaluates  a  script  at
       top-level in the outermost namespace (the global namespace).

EXAMPLE
       As stated above, the uplevel command is useful for creating new control
       constructs.  This example shows how (without error handling) it can  be
       used to create a do command that is the counterpart of while except for
       always performing the test after running the loop body:

              proc do {body while condition} {
                  if {$while ne "while"} {
                      error "required word missing"
                  }
                  set conditionCmd [list expr $condition]
                  while {1} {
                      uplevel 1 $body
                      if {![uplevel 1 $conditionCmd]} {
                          break
                      }
                  }
              }


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+------------------+
       |ATTRIBUTE TYPE | ATTRIBUTE VALUE  |
       +---------------+------------------+
       |Availability   | runtime/tcl-8    |
       +---------------+------------------+
       |Stability      | Uncommitted      |
       +---------------+------------------+
SEE ALSO
       apply(n), namespace(n), upvar(n)

KEYWORDS
       context, level, namespace, stack frame, variable



NOTES
       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-
       core8.6.7-src.tar.gz

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.



Tcl                                                                uplevel(1t)