Using lengthgets() correctly (was SET_LENGTH)

If you use R not the S-compatibility macros you will be less likely to 
confuse yourself.  The definition is (analogous to realloc)

Rdefines.h:#define SET_LENGTH(x, n)  (x = lengthgets(x, n))

so x is changed and needs to be reprotected.  [I don't know that
SET_LENGTH *is* part of the API: it is not defined in `Writing R 
Extensions'.]

Your error will potentially occur at the first garbage collection.  Try using 
REPROTECT.

Note though that what you are doing is very inefficient, and re-setting the 
length without changing the dim is also an error.  What R's internal examples 
do is to allocate a modest vector, double in size when needed, call 
lengthgets to shorten at the end, then set the dim attribute.

On Sat, 16 Apr 2005, Larry Tordsen wrote:

Hello.

I'm trying to read a delimited file.  I'm a little new
to the R api.  In the program, I do something like
this...

/*****************************/
SEXP retval;
PROTECT(retval = allocMatrix(STRSXP,1,names.dim));
int r_eltctr = 0;
while(!myfile.eof())
{
 SET_LENGTH(retval,r_eltctr+num_of_fields);
 for(int i = 0;numofcolumns; ++i)
   {
     INTEGER(retval)[r_eltctr++] = 1;/*some
value...*/
   }
 ++r_eltctr;
}
UNPROTECT(1);
/***************************************/

There is a segmentation fault when SET_LENGTH() is
called several iterations into the program.  It seems
like it should not be a memory issue, unless a
constraint is set by R.  The vector only has about
3500 elements at the time it crashes.

Does anyone have any ideas what the problem could be?

The problem seems to be the assumption that R (and not the user) was at 
fault.

-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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