In Tcl, variables are "scoped" to the procedure in which they're used. That's a sensible default, since you want to be able to write main applications without the risk of interfering with variables within your procs.

A global command in a proc allows you to refer to a variable in the main code from within a proc. (No need to declare it global at the top level too)

An upvar command allows you to reference a variable in your calling code from within your proc, and is commonly used for "pass by name" requirements where you're writing a proc that must alter an incoming variable.

The uplevel command allows you to run a piece of code as if it was in the namespace of your calling code and not in your proc - and you can even specify a number of levels of code up or an absolute level number.

Suggestion. Use upvar whenever you need to call by name. Use global sparingly. And think very carefully before you ever use uplevel!

Example:

proc demo {what where} {
global counter
upvar $where hereitis
uplevel incr dc
append hereitis ", UK"
incr counter
puts "Demo run"
return "$what $hereitis"
}
#
set dc 0
set counter 0
set place Melksham
set rz [demo "eat in" place]
puts "function returns: $rz"
puts "place will have UK on the end: $place"
puts "counter is number of calls to demo: $counter"
puts "variable dc has become: $dc"

Runs as

Dorothy:~/oct07 grahamellis$ tclsh uld
Demo run
function returns: eat in Melksham, UK
place will have UK on the end: Melksham, UK
counter is number of calls to demo: 1
variable dc has become: 1
Dorothy:~/oct07 grahamellis$

(written 2007-10-24, updated 2011-03-01)

 T207 - Tcl/Tk - Procedures and Variable Scope
  [96] Variable Scope - (2004-10-22)
  [122] Passing arrays to procs in Tcl - (2004-11-18)
  [308] Call by name v call by value - (2005-05-11)
  [409] Functions and commands with dangerous names - (2005-08-11)
  [775] Do not duplicate your code - (2006-06-23)
  [1163] A better alternative to cutting and pasting code - (2007-04-26)
  [1782] Calling procs in Tcl and how it compares to Perl - (2008-09-02)
  [2466] Tcl - passing arrays and strings in and back out of procs - (2009-10-22)
  [2476] Tcl - uplevel to run code at calling level - (2009-10-24)
  [2520] Global and Enable - two misused words! - (2009-11-30)
  [2929] Passing a variable number of parameters in to a function / method - (2010-08-20)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [3414] Passing back multiple results in Tcl - upvar and uplevel - (2011-09-01)
  [3629] Sharing lots of values in Tcl without having lots of global declarations - (2012-02-28)
  [4398] Accessing variables across subroutine boundaries - Perl, Python, Java and Tcl - (2015-01-18)

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Away or home - which do I prefer?
Sorting in Tcl - lists and arrays
Tcl - global, upvar and uplevel.
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