[Rd] must .Call C functions return SEXP?
atp at piskorski.com
Thu Oct 28 01:43:47 CEST 2010
For using R's .Call interface to C functions, all the examples I've
seen have the C function return type SEXP. Why? What does R actually
do with this return type? What happens if I *don't* return a SEXP?
Reason I ask, is I've written some R code which allocates two long
lists, and then calls a C function with .Call. My C code writes to
those two pre-allocated lists, thus, I thought I should NOT need to
return any SEXP from my C function. However, if I do that, it
segfaults somewhere in "src/main/memory.c".
If I instead return the SEXP for one of my two result lists it appears
to work fine... But of course I don't understand what's going on here
and so don't trust it. The list SEXP I am returning was allocated by
R, not by my C code, so why does it make any difference whether my C
function returns it or not?
>From "src/main/names.c" I see that .Call is implemented by do_dotcall
in "src/main/dotcode.c". I don't necessarily understand what that
really does, but AFAICT if my C function returns nothing, the
do_dotcall's retval should simply remain set to R_NilValue, which
should be fine.
I must be misunderstanding something here, but I don't know what, and
would definitely appreciate any help. Thanks!
My R code looks like this:
result.1 <- vector("list" ,1e6)
result.2 <- vector("list" ,1e6)
.Call("my_C_function", result.1, result.2, other.input)
My C code looks like this:
result_v = Rf_allocVector(REALSXP, 5);
SET_VECTOR_ELT(result_list_1, k1, result_v);
REAL(result_v) = some_number;
REAL(result_v) = another_number;
/* Also do the same sort of thing for result_list_2. */
return(result_list_1); /* Appears to work ok. */
/* return; */ /* Segfaults. */
Andrew Piskorski <atp at piskorski.com>
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