[R] mkChar can be interrupted

[Luke Tierney]

On Mon, 14 Jun 2004, Vadim Ogranovich wrote:

> I am confused. Here is an excerpt from R-exts:
> 
> "As from R 1.8.0 no port of R can be interrupted whilst running long
> computations in
> compiled code,..."
> 
> Doesn't it imply that the primitive functions like allocVector, mkChar,
> etc., which are likely to occur in any compiled code called via .Call,
> are not supposed to handle interrupts in any way?

No it does not.  Read the full context.  It says that if you wite a
piece of C code that may run a long time and you want to guarantee
that users will be able to interrupt your code then you should insure
that R_CheckUserInterrupt is called periodically.  If your code
already periodically calls other R code that checks for interrupts
then you may not need to do this yourself, but in general you do.

Prior to 1.8.0 on Unix-like systems the asynchronous signal handler
for SIGINT would longjmp to the nearest top level or browser context,
which meant that on these sytems any code was interruptible at any
point unless it was explicitly protected by a construct that suspended
interrupts.  Allowing interrupts at any point meant that inopportune
interrupts could and did crash R, which is why this was changed.

Unless there is explicit documentation to the contrary you should
assume that every function in the R API might allocate and might cause
a non-local exit (i.e. a longjmp) when an exception is raised (and an
interrupt is one of, but only one of, the exceptions that might
occur).

luke

> Thanks,
> Vadim
> 
> 
> > From: Luke Tierney [mailto:luke at stat.uiowa.edu] 
> > 
> > On Mon, 14 Jun 2004, Vadim Ogranovich wrote:
> > 
> > > > From: Luke Tierney [mailto:luke at stat.uiowa.edu]
> ...
> > > > 
> > > > Not sure why you think this suggest mkChar can be interrupted.
> > > > 
> ...
> > > > by calls to this function.  I don't believe there are any 
> > such safe 
> > > > points in mkChar, but there are several potential ones 
> > within your 
> > > > example.
> > > 
> > > Apart from mkChar I am only calling SET_STRING_ELT. Is this 
> > what you 
> > > mean?
> > 
> > You are printing, you have an assignment expression, all of 
> > those contain points where an interrupt could be checked for.
> 
> These are not relevant since Ctrl-C is pressed when the code is inside
>   for (i=0; i<n; ++i) {
>     SET_STRING_ELT(resSexp, i, mkChar("foo"));
>   }
> 
> Just look at the way I deliver the signal.
> 
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-- 
Luke Tierney
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
   Actuarial Science
241 Schaeffer Hall                  email:      luke at stat.uiowa.edu
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