From: "Suraj N. Kurapati" <skurapat@...>
Date: 2006-04-16T10:58:04+09:00
Subject: Re: understanding "[BUG] cross-thread violation on rb_gc()"

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Suraj N. Kurapati wrote:
> In particular, the
> C program creates some Ruby strings from C strings using
> StringValueCStr() and puts them inside a Ruby array.

Whoops, my mistake. The C program creates Ruby strings from C
strings using rb_str_new2(), not by using StringValueCStr().

> 
> So far so good. Now suppose that the C program wants to run an
> embedded Ruby interpreter in the "background" while it is doing
> other things. The C program does this by wrapping the ruby_run()
> function (which never returns) inside a pthread.
> 
> 
> Let's review the situation so far with a diagram:
> 
> {a process:
> 
>    main()
> 
>    {a pthread:
> 
>       ruby_run()
>    }
> }
> 
> Here we have the C program's main() function running in the
> foreground and the embedded Ruby interpreter running in the background.
> 
> 
> Now, suppose the embedded Ruby interpreter inside ruby_run() invokes
> the garbage collector by calling the rb_gc() function. What happens?
> 
> 
> There are two situations:
> 
> 1. The garbage collector collects garbage created *only* by the
> embedded Ruby interpreter inside ruby_run(). It ignores the garbage
> created by the C program (remember that it created a Ruby array of
> Ruby strings).
> 
> No problem here. The garbage collector frees up space for the
> embedded Ruby interpreter and the interpreter merrily resumes
> whatever it was doing.
> 
> 
> 2. The garbage collector tries to collect the Ruby array of Ruby
> strings created by the C program. But wait a minute... the C program
> which created that garbage is running in a *different* thread than
> the embedded Ruby interpreter inside ruby_run()!
> 
> So, the garbage collector emits this error message and terminates
> the Ruby interpreter (ruby_run): "[BUG] cross-thread violation on
> rb_gc()".
> 
> 
> That's my understanding of the error message. If I am correct, then
> a solution to this problem would be to prohibit the C program from
> using Ruby's C API functions. That way, the C program never creates
> any Ruby objects which can be garbage collected by the embedded Ruby
> interpreter that runs inside another thread.
> 
> What do you think?
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