I have a question about global in TCL.
In one tcl file tclone.tcl, I have a global variable: global SIGNAL
in another tcl file called tcltwo.tcl, I set the variable SIGNAL as: set SIGNAL 10
In tclone.tcl, I improted the tcltwo.tcl as following" package require tcltwo.tcl
will the variable SIGNAL in tclone.tcl will be set as 10 when I execute it? and what is the usage of gloable variable?
As stated in its manual page, the global command only has meaning inside proc bodies:
This command has no effect unless executed in the context of a proc body.
So the whole question is unclear. If you meant that you have a proc in the first file setting a global variable and another proc (in the second file) reading it, then the question makes sense and the answer is yes, the code from the second file will see the change made by the code from the first file provided the "setting" procedure runs before the "getting" one. To possibly make it more clear, a global variable is global with regard to an interpreter the code operating that variable runs. Hence no matter which way do you use to fetch the code into an interpreter (package require vs source vs eval etc), all that code will see the same set of globals.
But in any case you should probably abstrain from using globals and use namespaced variables: they are also global but you greatly reduce the risk of introducing some other code later which will inadvertently mess with that global variable it should not touch. Of course, as usually this depends on how complicated your application is expected to be.
Related
I have a namespace variable which is defined as below:
namespace eval ::SMB::{
variable SmbInfo
------
------
proc ::SMB::SmbCreate {name dutport} {
variable SmbInfo
global DutPorts DutPort_2 DutPorts_3 smb
------
------
if{"" != [info command SMB::$name]} {
return -code error "command name \"$name\" already exists"
}
set SmbInfo($name -DutPort) $dutport
I am new to Tcl and trying to understand the above piece of code. Few questions, please correct me if I am wrong at any point:
The variable SmbInfo defined on top in namespace is getting overridden by the one declared in the procedure SmbCreate. I am unable to figure out what is the objective of the line:
set SmbInfo($name -DutPort) $dutport
I can see that 'DutPorts' is defined as global but I could not find 'DutPort'. I have not executed the code yet. Could it be an error?
Is ($name - DutPort) creating an array index for the variable SmbInfo and the value of $dutport is being set to that particular array variable?
I have similar code structures in the file like below
set SmbInfo($name - SmbSetDmac) [BuildMac1 $SmbInfo($from_name-DutPort)]
Where BuildMac1 is a procedure. A bit explanation of the above code might also make the thing clear.
If anything I missed to post in the question, kindly point me, I will edit my question.
Thanks in advance.
The second declaration doesn't override, it's the same variable in both cases.
The command is a syntax error because of the space after $name. The intent seems to be to assign the value of $dutport to the member of SmbInfo that has the name "$name -DutPort" (where $name is replaced by the variable value).
A similar assignment, but here the value comes from the result of the command.
There are a few syntax errors in the code, too many or too few spaces here and there. It seems unlikely this code has ever been executed.
The purpose of the smb::SmbCreate command would seem to be to 1) create a new command in the SMB namespace named by the first parameter (unless such a command already exists) and 2) store metadata in the SmbInfo namespace variable, indexed by a) the name parameter and b) a keyword such as -DutPort or -SmbSetDmac.
Code like this essentially implements an ad-hoc object-oriented interface. If the whitespace issues are resolved, it should work fine.
You have many syntactic problems that are going to cause you much grief. Tcl cares very much about its syntax level, which includes exactly where the spaces and newlines are, and whether there are {braces} and [brackets] as expected. You must get these things right.
Looking at the specific code you're having problems with, this line:
set SmbInfo($name -DutPort) $dutport
would appear to be highly unlikely, as it is passing three arguments to the set command when that only takes one or two. I'd guess that you've got a command that you're calling to obtain a key for an array, and that the code therefore ought to be this:
set SmbInfo([$name -DutPort]) $dutport
See those [brackets]? They matter here, as they say “run my contents as a little subscript and use the result”. With that sorted out, there's also the question of whether $name -DutPort works at all, but you'll just have to be guided by the error messages there. Tcl usually gives very good error messages, though sometimes you have to think about why the code got in the state where it is giving that message in order to figure out what the actual problem is. You know, usual debugging…
I would expect similar problems with:
set SmbInfo($name - SmbSetDmac) [BuildMac1 $SmbInfo($from_name-DutPort)]
and would guess that it is actually supposed to be:
set SmbInfo([$name -SmbSetDmac]) [BuildMac1 $SmbInfo([$from_name -DutPort])]
Note again that I have modified the spaces to follow the existing pattern (which I'm guessing is a property access; it looks like it's OTcl or XOTcl) and added brackets.
Finally, this line:
if{"" != [info command SMB::$name]} {
is also syntactically wrong, and should instead be:
if {"" != [info command SMB::$name]} {
That extra space matters, because it separates the word that is the command name (if) from the word that is the condition expression. The remainder of the line is probably correct (the SMB::$name might be suspicious, except you're using it in info command, but then you probably only need info command $name as it already knows about what namespace you're working in and you're using the unqualified name elsewhere).
I am trying to use a global variable (gpio_out_set_3) by declaring it outside a function (because the variable might be used in other functions too in future). Inside the function, I have declared the same variable as 'global' and trying to access it through '$gpio_out_set_3'.
I am getting an error "can't read "gpio_out_set_3": no such variable"
set gpio_out_set_3 0x03
proc port2phy { device } {
global gpio_out_set_3
erf_wr devcpu_gcb $gpio_out_set_3 $phy_mdc_gate_en
}
Please help.
Declare all of your global variable at the beginning of your main file using variable command.
variable gpio_out_set_3 0x03
TIP: I dont like the global command. I always forget to use, and hard to see which variable is global and which variable is local. I prefer the $::<varname>, which points to the global namespace.
proc port2phy { device } {
erf_wr devcpu_gcb $::gpio_out_set_3 $phy_mdc_gate_en
}
My guess is that when you're creating variable gpio_out_set_3 you're not at top level. You are in some other procedure. So the gpio_out_set_3 in not really global, but instead local in some proc.
Your global variable is named gpio_out_set_0 (not the same as gpio_out_set_3).
Although newer versions of tcl allow for executable code to exist almost anywhere in your program, a good habit to form is to be sure that where you set the variable initially is in the main section of your program code. In order for your code to run properly on any version of tcl, there should be no executable code prior to, or in-between your procs (including set commands). All of your procs should go at the beginning of the program followed by your main code. Things that can/should go before your procs are comment lines and any necessary "package require ..." commands.
I wonder if it is right semantics to have a variable as an argument, something like this:
proc p1 {$aa} {}
I tried it on tclsh, there is no complaint, but the following experiment fails:
% set aa bb
bb
% set bb 200
200
% proc p1 {$aa} {puts $bb}
% p1 bb
can't read "bb": no such variable
Do you see what is wrong?
[UPDATE - after seeing Peter's answer]
I know the upvar semantic, thanks.
My main curiosity is still around using variable as proc argument. I know it is not common, but just cannot help musing what it really can do if the language syntax allows it.
Yes, your upvar example is exactly what I want to explore using a variable as proc argument, but my exploration so far tells me, really, there is no way we can do this because "$" is interpreted as a plain char.
Do debunk me please if I am wrong.
Tcl does not support reference arguments as such: the usual pass-by-reference semantics is too static for Tcl. Instead, the logic of the command can, by use of upvar, dynamically create reference parameters including indirect reference parameters and calculated reference parameters, and also retarget the local name to another external variable. The upvar mechanism may look ungainly, but is very powerful indeed.
(The (edited) remains of my original answer follows:)
The usual idiom for doing this is
proc p varName {
upvar 1 $varName var
puts $var
}
The upvar command looks into another stack frame (in this case 1, which is the caller's stack frame) and makes a variable named $varName (i.e. the variable's name is the value of varName) in that stack frame and a variable named "var" in the command's stack frame refer to the same data object.
I won't explain this further since this is not useful to the asker.
Documentation: proc, puts, upvar
There are cases where it makes sense for the arguments in a procedure creation call to be supplied from a variable. The main example is where you are creating procedures dynamically, calculating the arguments you want to use as you go along.
That's actually a use-case that isn't done very frequently! It's not particularly easy to use well. But I have done it. (OK, that's a method call, but the syntax of formal arguments is shared.)
The main reason that the capability is there is that it's part of Tcl's general syntax. Tcl tries very hard to not have special cases in how it parses things (other than in how a command parses the strings passed into it) and this includes in things that would be very special cases in the enormous majority of other programming languages, such as formal parameter lists to procedures. In Tcl, these are just ordinary values and can be produced using any technique that gives ordinary values.
The usual thing with putting them in braces is just how you do it reliably and is easy to teach. It's also overwhelmingly what people want to do.
All this is independent of the facts that Tcl's commands (including its procedures) can handle variable numbers of arguments (check out the special args parameter and how to specify default values) and that $aa is a legal (but strange) name for a local variable.
Sorry for the title couldn't think of anything better.
Heres my question:
Im trying to change a variable inside a proc only when the user hits space. the proc loops itself with after so if the user wish to hit space more then once the variable will increment.
Heres what I know:
There are many ways to go about this. You can pass variable inside the proc, you can link the variable with global or upvar and/or if your in a namespace then you can use variable. but the only one that seem to work with my is global. I'm getting a feeling it's because global makes a link but if thats true then variable should work too, right?
Here my test code:
proc test1 {} {
global testing
bind . <Key-a> {incr testing}
puts $testing
puts "test2"
after 100 test2
}
namespace eval test2 {
variable testing 0
namespace export {[a-z]*}
proc of1 {} {
variable testing
bind . <Key-a> {incr testing}
puts $testing
after 100 test3::of1
}
}
proc test3 {testing} {
bind . <Key-a> {incr testing}
puts $testing
puts "test4"
after 100 test4 $testing
}
set testing 0
#test1
test2::of1
#test3 0
grid .c
side question:
Why is it that in the global namespace we use set and global while in namespace we use variable (that seem to set and do global in one command). they seem to do the same job in different namespaces?
Variables in Callbacks
The scripts registered by the bind command — also in things like after events and fileevent callbacks — are evaluated in the global scope because they may be called long after the procedure that defined them returns; Tcl does not do scope capture (and that's actually a really complicated feature, so it isn't likely to come soon unless someone writes lots of code). This means that the variable that you want your procedure to notice changes to must also have global scope.
However, namespaced variables count just fine as global variables for the purpose of this discussion as they're nameable from a global context (real local variables are not). That means that we can do several ways to build a script that accesses a namespace variable from a bind-defined callback. Here's one of the nicer ones:
bind . <Key-a> [namespace code {incr testing}]
That's effectively the same as this:
bind . <Key-a> [list namespace eval [namespace current] {incr testing}]
(There are some strict differences that don't matter in this example.)
Another way to do the callback is this:
bind . <Key-a> [list incr [namespace which -variable testing]]
Which in this case is going to be much like:
bind . <Key-a> [list incr [namespace current]::testing]
If things are getting any more complicated than this toy example, it's time to stop updating variables directly in a binding script and instead write a helper procedure. That always simplifies things a lot. Or to use a class/object to encapsulate the details.
The variable Command: Why and Where to Use It
Why is it that in the global namespace we use set and global while in namespace we use variable (that seem to set and do global in one command). they seem to do the same job in different namespaces?
That's a good question. In fact, what global does is very much like upvar #0 (with the variable names doubled up), and set is a fundamental variable access command. They're commands that you can use regularly wherever you want their behaviour.
The variable command is rather stranger. What it does is three-fold:
If called in a namespace context and the variable does not exist in that namespace, it creates that variable in a present-but-unset state.
If called in a context with local variables, it links a local variable with the name (after stripping everything up to the last namespace separator) to a namespace variable with the name (using the whole supplied name if there are qualifiers, and resolving non-absolute names with respect to the current context namespace). This also forces the namespace variable to exist in the present-but-unset state.
If a value is given, the namespace variable is set to the value. This gets rid of the present-but-unset-ness.
The important behaviour is actually the creating of that present-but-unset state, since otherwise you can end up with a set (or array set) in the namespace escaping that namespace and instead working with a global variable, but not always. It all depends on the exact behaviour of the code that resolves variables, and that's deeply tricky. This is hard to explain properly, and ridiculously hard to justify. It's been the cause of quite a few outright bugs, and is definitely no better than a horrible misfeature.
The setting of the initial value is little more than a lollipop; you could instead put set straight afterwards without ill-effect. What's more, it inhibits using variable to pull in multiple variables from a namespace except in the case where you're setting them to a known value; great for initialisation, but sucky for other uses. (In case you hadn't guessed, I think this is an area of Tcl where the interface was got rather badly wrong back when it was introduced, back in Tcl 8.0. Not one bit of this is nice.)
The key take-away is this though:
Always use variable in your namespaces to declare variables as this is the only way you can be sure that the semantics is predictable. You can then initialise them any way you want. (You have to do it this way if you're creating arrays.)
Fully-qualified variable names have no craziness associated with them. Tcl always knows exactly how to look up the thing you're naming in that case.
I have a curious case of Tcl that perhaps I just don't understand.
The following code is done at the top level (not inside of any procedure):
if {![info exists g_log_file_name]} {
set g_log_file_name "default.txt"
}
What I hope it would do is to declare a global variable with some value if it wasn't declared yet (which can be done at some other script or C application). However, the if statement always false. I ran on Tcl 7.4.
What may be the problem?
Thank you so much.
% info level
0
% info exists g_log_file_name
0
% set g_log_file_name whatever
whatever
% info exists g_log_file_name
1
Hence the reason you observe is probably because the variable is really always unset at the time your if command is executed.
Possible reasons for this I can imagine are:
It's just not set: literally, no code attempt to do this;
The external code sets some other variable: name mismatch;
The external code sets a variable in some other interpreter: in a C code embedding Tcl, there can be any number of Tcl interpreters active at any moment (and those are free to create child interpreters as well);
I'm not sure abot the long forgotten version of Tcl you have at hand, but 8.x has the trace command which can be used to log accesses to a particular variable—you could try to use it to see what happens.