[Rd] "warning: assignment discards qualifiers from pointer target type"

Prof Brian Ripley ripley at stats.ox.ac.uk
Wed Jun 8 13:22:10 CEST 2011

On Tue, 7 Jun 2011, Duncan Murdoch wrote:

> On 07/06/2011 9:08 AM, oliver wrote:
>> Hello,
>> following an advice here from the list I looked into sources of other
>> packages (xts) and found the TYPEOF() macro/function, which really is
>> helpful.

It is documented, of course, but actually better alternatives are 
described in 'Writing R Extensions'.

We would urge you to study the R sources rather than copy bad habits 
from randomly chosen package sources.

>> I iused the follwong code snippet:
>>    switch( TYPEOF( filename_sexp ) )
>>    {
>>          case STRSXP: filename = CHAR( STRING_ELT(filename_sexp, 0) );
>>                       break;
>>          default:     error("filename argument must be a string");
>>                       break;
>>    }
>> Here, filename is of type char*
>> and one function opens a file with that name.
>> So it is purely intended to just grab out the char* from the
>> String-Expression.
>> Am I doing something wrong here, or is it ok, but I have somehow
>> to say the extracting macros/functions, that it is really intended
>> to throw away information and that a warning is not necessary?
> The result of calling CHAR should be a "const char *".  You are not allowed 
> to touch the string it points to.

Note too that isString() exists for this purpose, and there is no 
check in that code that LENGTH(filename_sexp) > 0 (or == 1).  In the R 
sources you will often see things like

     if(!isString(sfile) || LENGTH(sfile) < 1)
         error("invalid '%s' argument", "description");

Then, reading on,

     file = translateChar(STRING_ELT(sfile, 0));

for you cannot (in general) assume that the character vector passed is 
in the native encoding.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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