TCL global stumping me

Question

I have a script where I am doing what appears to be the exact same thing, but it works in that one and not this one. I'm hoping that a few more pairs of eyes will be able to help me find out the issue, I've already spent an hour and a half on it.

I have a proc that edits the global var, it's more complex that this but I'll use this for simplicity's sake:

proc myCustomProc { var } {

global __myGlobal
set __myGlobal [ expr $__myGlobal + 1 ]
}

I have defined a variable globally in my "main" proc:

proc FOO {} {
global __myGlobal
...
...
myCustomProc 5
puts $__myGlobal

Then I get can't read "__myGlobal": no such variable

I have the exact code with a different varname working in a different script, so I'm stumped. Obviously it's NOT identical, I just cannot find the issue.

Edit: both procs are in the same .tcl file

Can you show us actual code that demonstrates this problem? There are some oddities in your example that may simply be obfuscation/transcription errors. — larsks
Sure you can. Write some short stubs that demonstrate the problem, and post that. If you give us code that we can dump into a file and run, your chances of getting an answer quickly go way up. For example, in your second example above, you've defined a proc named FOO, but you're never calling it...that looks like a bug to me, but it could simply be that you didn't finish search/replace on the second block of code. — larsks
"FOO" is the main proc that is being run, maybe I should have been more specific than, "I have defined a variable globally in my "main" proc:" my apologies — Joshua
did you set the variable before you access it first? set __myGlobal [ expr $__myGlobal + 1 ] might throw an error (if __myGlobal is not set), incr __myGlobal will not. — Johannes Kuhn

Donal Fellows Donal Fellows · Accepted Answer · 2012-08-09T12:37:44

You can't read from a variable that is unset, and that's true whether that variable is global or not. Thus, in the code:

set __myGlobal [ expr $__myGlobal + 1 ]

It first reads the value from the global variable, then adds one to that value, then writes the result back to the global variable. (Actually, it is interpreting the contents of the variable as an expression fragment, which I'd lay good money on being something you don't want as it is slow and unsafe; put the whole expression in braces please.)

For adding one to an integer (and from Tcl 8.5 onwards) you should just use the incr command instead, as that interprets a non-existent value as if it was zero:

incr __myGlobal

But if you're doing something more complex (or working in 8.4 or before), you should instead put a check with info exists like this in front:

if {![info exists __myGlobal]} {
    set __myGlobal "the default value";    # Or whatever default you want
}

You could also use a more complex expression like this:

set __myGlobal [expr {[info exists __myGlobal] ? $__myGlobal+1 : 1}]

But I usually try to avoid the ternary operator; it's often not that readable.

TCL global stumping me

2 Answers