LinuxQuestions.org - tcl procedures and changing variable values inside a procedure

I am trying to avoid use of global variables but I feel like having two left hands with Tcl.

I now tried a simple procedure, hacked it from some examples - it does not work...

Code:

proc incr_a_b_upvar { a b c} {

 puts "a = $a"

 puts "b = $b"

 puts "c = $c"

 upvar 1 $a a_sub

 upvar 1 $b b_sub

 set a_sub $a

 set b_sub $b

 

 puts "a_sub = $a_sub"

 puts "b_sub = $b_sub"

 

 set a [expr {$a + $c}]

 set b [expr {$b + $c}]

}

set a 2

set b 3

set c 1

incr_a_b_upvar $a $b $c

puts "a = $a"

puts "b = $b"

puts "c = $c"

In similar examples it should actually work without those
two set commands inside the procedure

Code:

 set a_sub $a

 set b_sub $b

but without them I get:

Quote:

a = 2
b = 3
c = 1
can't read "a_sub": no such variable
while executing
"puts "a_sub = $a_sub""
(procedure "incr_a_b_upvar" line 10)
invoked from within
"incr_a_b_upvar $a $b $c"
(file "incr_a_b.tcl" line 30)

So what gives?