[Rd] CHAR () and Rmpi

[Hao Yu]

Hi. I am the maintainer of Rmpi package. Now I have a problem regarding
the change of CHAR () in R 2.6.0. According to R 2.6.0 NEWS:
*******
CHAR() now returns (const char *) since CHARSXPs should no
        longer be modified in place.  This change allows compilers to
        warn or error about improper modification.  Thanks to Herve
        Pages for the suggestion.
*******
Unfortunately this causes Rmpi to fail since MPI requires char pointers
rather than const char pointers. Normally I use
    CHAR(STRING_ELT((sexp_rdata),0))
to get the pointer to MPI where a R character vector (C sense) is stored.
Because of the change, all character messengers fail. Is there an easy way
to get a char pointer to STRING_ELT((sexp_rdata),0) and is also backward
compatible to old R versions. BTW Rmpi does not do any modification of
characters at C level.

Thanks
Hao Yu

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