Re: [Rd] Interrupting C++ code execution

From: Simon Urbanek <simon.urbanek_at_r-project.org>
Date: Mon, 25 Apr 2011 09:34:45 -0400

On Apr 25, 2011, at 5:22 AM, schattenpflanze_at_arcor.de wrote:

> Hello,
>
> I am writing an R interface for one of my C++ programs. The computations in C++ are very time consuming (several hours), so the user needs to be able to interrupt them. Currently, the only way I found to do so is calling R_CheckUserInterrupt() frequently. Unfortunately, there are several problems with that:
>
> 1. Calling R_CheckUserInterrupt() interrupts immediately, so I have no possibility to exit my code gracefully. In particular, I suppose that objects created on the heap (e.g., STL containers) are not destructed properly.
>

In general, you're responsible for the cleanup. See R-devel archives for discussion on the interactions of C++ and R error handling. Generally, you should not use local objects and you should use on.exit to make sure you clean up.

> 2. Calling R_CheckUserInterrupt() within a parallel OpenMP loop causes memory corruptions. Even if I do so within a critical section, it usually results in segfaults, crashes, or invalid variable contents afterwards. I suppose this is due to the threads not being destroyed properly. Since most of the time critical computations are done in parallel, this means I can hardly interrupt anything.
>

As you know R is not thread-safe so you cannot call any R API from a thread - including OMP threads - so obviously you can't call R_CheckUserInterrupt(). Since you're using threads the safe way is to perform your computations on a separate thread and let R handle events so that you can abort your computation thread as part of on.exit.

> Having a function similar to R_CheckUserInterrupt() but returning a boolean variable (has an interrupt occurred or not?) would solve these problems. Is there a way to find out about user interrupt requests (the user pressing ctrl+c or maybe a different set of keys) without interrupting immediately?
>

Checking for interrupts may involve running the OS event loop (to allow the user to interact with R) and thus is not guaranteed to return. There is no general solution - if you're worried only about your, local code, then on unix, for example, you could use custom signal handlers to set a flag and co-operatively interrupt your program. On Windows there is the UserBreak flag which can be set by a separate thread and thus you may check on it. That said, all this is very much platform-specific.

Cheers,
Simon



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