Is $! thread safe?

Ruby
1

From the code below, it appears that $! is thread safe.

Is it?

Thanks.

gegroet,
Erik V. - http://www.erikveen.dds.nl/

OUTPUT:

T=1 → [1, :a, 1]

T=2 → [2, :a, 2]

T=3 → [3, :b, 2]

T=4 → [4, :b, 1]

Thread.abort_on_exception = true

dt = lambda{t0=Time.now ; lambda{|*a| p [(Time.now-t0).to_i,
*a]}}[]

class MyException < StandardError
def initialize(n)
@n = n
end

def inspect
@n.to_s
end
end

(1…2).each do |m|
Thread.new(m) do |n|
begin
sleep(n) # t1 → sleep(1) ; t2 → sleep(2)

   raise MyException.new(n)
 rescue
   dt[:a, $!]                # t1 -> T=1      ; t2 -> T=2

   sleep(5-2*n)              # t1 -> sleep(3) ; t2 -> sleep(1)

   dt[:b, $!]                # t1 -> T=4      ; t2 -> T=3
 end

end
end

sleep 5

2

On Sat, 23 Jun 2007 07:26:19 +0900, Erik V. [email protected]
wrote:

From the code below, it appears that $! is thread safe.

Is it?

Each thread gets its own private $!, so yes.

-mental

3

Erik V. wrote:

From the code below, it appears that $! is thread safe.

Is it?

Yes, according to Pickaxe v2, p.319 (the “[thread]” is what signifies
this):

Exception Information
$! The exception object passed to raise. [thread]
Exception
$@ Array The stack backtrace generated by the last exception.
See Kernel#caller
on page 497 for details. [thread]

4

Gregory B. wrote:

curious.
No, only for a subset of them, including $SAFE and the pattern matching
vars.

5

On 6/22/07, MenTaLguY [email protected] wrote:

On Sat, 23 Jun 2007 07:26:19 +0900, Erik V. [email protected] wrote:

From the code below, it appears that $! is thread safe.

Is it?

Each thread gets its own private $!, so yes.

Is that a special case for built in globals? I’m assuming so, but just
curious.

6

On 6/22/07, Joel VanderWerf [email protected] wrote:

Gregory B. wrote:

On 6/22/07, MenTaLguY [email protected] wrote:

Each thread gets its own private $!, so yes.

Is that a special case for built in globals? I’m assuming so, but just
curious.

No, only for a subset of them, including $SAFE and the pattern matching
vars.

Ah, the pattern matching vars were actually the ones I was interested
in… Thanks.

7

From the code below, it appears that $! is thread safe.

Is it?

Yes, according to Pickaxe v2, p.319 (the “[thread]” is what
signifies this):

Exception Information
$! Exception The exception object passed to raise. [thread]

Page 319 of Pickaxe v2 doesn’t tell me anything about
exceptions or global variables. But I found the same text on
page 334. (It’s not easy to search in a printed book… :})

Page 108 of the same book (at least of my instance) says
“Ruby places a reference to the associated Exception object
into the global variables $!.”

I suppose “global” in this sentence means “context of current
thread” instead of “context of current program”. It was a bit
confusing to me…

Thanks!

gegroet,
Erik V. - http://www.erikveen.dds.nl/

8

Erik V. wrote:

Page 319 of Pickaxe v2 doesn’t tell me anything about
exceptions or global variables. But I found the same text on
page 334. (It’s not easy to search in a printed book… :})

Sorry about that. I didn’t expect that the PDF and the printed copy
would have different pagination…