Retrieving a pointer to float from Rust FFI

Question

Ubuntu 13.04, Rust 0.6. I'm trying to use the Rust FFI to operate with openblas. The following code doesn't work as expected.

use core::io::println;
use core::libc::{c_int, c_float};
use core::vec::raw::to_ptr;

extern mod openblas {
    /*  Single precision dot product.
        sdot takes 5 args: number of elements,
                           pointer to a,
                           storage spacing for elements of a,
                           pointer to b,
                           storage spacing for elements of b
        returns a^T . b
    */
    fn cblas_sdot(N: c_int, x: *c_float, incx: c_int,
                  y: *c_float, incy: c_int) -> *c_float;
}

pub fn sdot(a: &[c_float], b: &[c_float]) -> *c_float {
    unsafe {
        openblas::cblas_sdot(3 as c_int, to_ptr(a), 1 as c_int,
                             to_ptr(b), 1 as c_int)
    }
}

fn main() {
    let a = [1. as c_float, 2. as c_float, 4. as c_float];
    let b = [1. as c_float, 2. as c_float, 5. as c_float];
    let x = sdot(a, b);
    println(fmt!("%?", x));

}

edit: I expected it to print 25, as a borrowed pointer. I got 140125321300160, which might be the address of the float.

Answer 1

The cblas_sdot function is not supposed to return a pointer.

From BLAS Reference:

cblas_sdot

Computes the dot product of two vectors (single-precision).

float cblas_sdot(const int N, const float *X, const int incX, const float *Y, const int incY);