>> Users can (and do) write open code in modules.
> I don't understand. Do you think that needs to be prevented?
No, I just want to know what happens when they do it.
Let's look at an example.
1. Non-threaded
#!/usr/local/bin/perl
sub foo { print 1 }
foo();
eval 'sub foo { print 2 }';
foo();
When the compiler sees
sub foo { print 1 }
it
1. builds an op tree for the block `{ print 1 }'
2. creates a code ref that points to the op tree
3. creates a glob that points to the code ref
4. sets $main::{foo} to the glob
Graphically:
%main::
key value
foo *main::foo
*main::foo
SCALAR -> undef
ARRAY -> undef
HASH -> undef
CODE -> &main::foo
&main::foo
OPTREE -> { print 1 }
When the interpreter sees
eval 'sub foo { print 2 }';
it calls the compiler, which
1. builds a new op tree for the block `{ print 2 }'
2. creates a new code ref that points to the new op tree
3. sets the CODE pointer in the *main::foo glob to the new code ref
4. decrements the reference count on the old code ref.
If the ref count goes to zero,
it deletes the old code ref and its op tree
Both calls to
foo();
are bound to the *main::foo glob at compile time. When the interpreter
executes a call, it follows the CODE pointer from the glob to the code
ref, follows the OPTREE pointer from the code ref to the op tree,
and finally walks the op tree.
This program outputs
12
2. Threaded
I will write `global::' vice `main::' for the global stash,
and write `thread::' for the thread local stash.
#!/usr/local/bin/perl
use Threads;
sub foo { print 1 }
foo();
Threads->new(\&hack_foo)->join;
foo();
sub hack_foo { eval 'sub foo { print 2 }' }
At (initial) compile time, the compiler sets
STASH GLOB CODE REF OP TREE
$global::{foo} -> *global::foo -> &global::foo -> { print 1 }
The question is where { print 2 } goes. Do we get
$global::{foo} -> *global::foo -> &global::foo -> { print 2 }
as in the non-threaded case, or do we get
$global::{foo} -> *global::foo -> &global::foo -> { print 1 }
$thread::{foo} -> *thread::foo -> &thread::foo -> { print 2 }
Does this program output
12
or
11
- SWM
