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man pages section 1: User Commands

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Updated: Wednesday, February 9, 2022

upvar (1t)


upvar - Create link to variable in a different stack frame


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


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


       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 (it defaults to 1).  For each  oth-
       erVar  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 proce-
       dure 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 repre-
       sent the namespace context.  This means  each  namespace  eval  command
       counts as another call level for uplevel and upvar commands.  For exam-
       ple, 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  out-
       ermost namespace (the global namespace).

       If  an upvar variable is unset (e.g. x in add2 above), the unset opera-
       tion affects the variable it is linked  to,  not  the  upvar  variable.
       There is no way to unset an upvar variable except by exiting the proce-
       dure in which it is defined.  However, it is possible  to  retarget  an
       upvar variable by executing another upvar command.

       Upvar  interacts  with  traces  in a straightforward but possibly unex-
       pected manner.  If a variable trace is defined on otherVar, that  trace
       will  be triggered by actions involving myVar.  However, the trace pro-
       cedure will be passed the name of myVar, rather than the name of other-
       Var.   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  particu-
       lar, if the array is env, then changes made to myVar will not be passed
       to subprocesses correctly.

       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 attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

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

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

       Source  code  for open source software components in Oracle Solaris can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       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-

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

Tcl                                                                  upvar(1t)