using SETLENGTH to fool R about the size of a vector

output.txt

x = <0x1038ff8a0>, dataptr = <0x1038ff8c8>
x = <0x1038ff8a0>, dataptr = <0x1038ff8c8>
x = <0x1038ff8a0>, dataptr = <0x1038ff8c8>
x = <0x1038ff8a0>, dataptr = <0x1038ff8c8>
x = <0x1038ff8a0>, dataptr = <0x1038ff8c8>
x = <0x1038ff8a0>, dataptr = <0x1038ff8c8>
x = <0x1038ff8a0>, dataptr = <0x1038ff8c8>
x = <0x1038ff8a0>, dataptr = <0x1038ff8c8>
x = <0x1038ff8a0>, dataptr = <0x1038ff8c8>
x = <0x1038ff8a0>, dataptr = <0x1038ff8c8>
[[1]]
[1] 1

[[2]]
[1] 1.5

[[3]]
[1] 2

[[4]]
[1] 2.5

[[5]]
[1] 3

[[6]]
[1] 3.5

[[7]]
[1] 4

[[8]]
[1] 4.5

[[9]]
[1] 5

[[10]]
[1] 5.5

vectors.c

#define USE_RINTERNALS
#define R_NO_REMAP

#include <R.h>
#include <Rinternals.h>

SEXP means( SEXP x ){
    int n = Rf_length(x) ;
    SEXP res = PROTECT( Rf_allocVector( VECSXP, n ) ) ;
    SEXP call = PROTECT( Rf_lang2( Rf_install( "mean"), x ) ) ; 
    for( int i=0; i<n; i++){
        SETLENGTH(x, i+1 ) ;
        SET_VECTOR_ELT( res, i, Rf_eval( call, R_GlobalEnv ) ) ;
        Rprintf( "x = <%p>, dataptr = <%p> \n", x, DATAPTR(x) ) ;
    }
    UNPROTECT(2) ;
    return res ;
}

vectors.R

system( "R CMD SHLIB vectors.c" )
dyn.load( "vectors.so") 
x <- as.numeric(1:10)
means(x)