[Rd] How allocate STRSXP outside of gc

[Vadim Ogranovich]

mkChar is a rather expensive call since it allocates a new R object. For
example in reading char data from a file it is often advantageous to
first try to look up an already made R string and only then use mkChar.
That is, the overhead of the lookup is usually smaller than that of
mkChar.

> -----Original Message-----
> From: r-devel-bounces at stat.math.ethz.ch 
> [mailto:r-devel-bounces at stat.math.ethz.ch] On Behalf Of Jan T. Kim
> Sent: Wednesday, April 13, 2005 3:44 AM
> To: r-devel at stat.math.ethz.ch
> Subject: Re: [Rd] How allocate STRSXP outside of gc
> 
> On Tue, Apr 12, 2005 at 12:31:03PM -0700, Vadim Ogranovich wrote:
> > Hi,
> >  
> > I am trying to figure a way to allocate a string SEXP so that gc() 
> > won't ever collect it.
> >  
> > Here is a little bit of a background. Suppose I want to write a 
> > .Call-callable function that upon each call returns the same value, 
> > say
> > mkChar("foo"):
> >  
> > SEXP getFoo() {
> >    return mkChar("foo");
> > }
> >  
> > The above implementation doesn't take advantage of the fact that
> > mkChar("foo") could be pre-computed only once, and then the 
> function 
> > would return the pre-computed value. So the question is how 
> to create 
> > this precomputed value.
> >  
> >  
> > The closest thing I could find in the sources is 
> R_NaString, but I was 
> > not able to trace down how it comes about.
> 
> For being unaffected by R's memory management, it may be the 
> best to not use a SEXP for storing the pre-computed result at 
> all. Rather, use a static variable "private" to your code, as in
> 
>     SEXP getFoo()
>     {
>       static char *foo = NULL;
> 
>       if (foo == NULL)
>       {
>         foo = the_difficult_to_compute_value_of_foo();
>       }
>       return mkChar(foo);
>     }
> 
> This way, getFoo indeed invokes mkChar each time, but in your 
> scenario, that might be an overhead which is negligible 
> compared to the actual computation of the foo value.
> 
> Best regards, Jan
> --
>  +- Jan T. Kim 
> -------------------------------------------------------+
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