I'm trying to push a pointer back to dst in the base64_decode function.

confused.rs

extern crate libc;
extern crate rustc_serialize;

use libc::{size_t, c_void, c_char};
use std::{slice, ptr, iter, str};
use std::ffi::{CString, CStr};

use rustc_serialize::base64::{ToBase64, FromBase64, STANDARD};

//to copy
//size_t base64_decode(void *dst, size_t sizeof_dst, const void *src, size_t sizeof_src);
#[no_mangle]
pub extern "C" fn base64_decode(dst: *mut c_void, dst_len: size_t, src: *const c_void, src_len: size_t) -> size_t {
    let source  = unsafe {
        assert!(!src.is_null());
        slice::from_raw_parts(src as *const u8, src_len as usize)
    };

    let source_text = match str::from_utf8(source) {
        Err(e) => panic!("Error while ingesting the input string."),
        Ok(o) => o,
    };

    let encode_text: String = source_text.as_bytes().to_base64(STANDARD);
    let encode_text_size = encode_text.len() as usize;
    let mut dst_pointer = CString::new(encode_text).unwrap().into_raw() as *mut _ as *mut c_void;
    unsafe { (*dst) = dst_pointer};

    return encode_text_size;
}
//size_t base64_encode(void *dst, size_t sizeof_dst, const void *src, size_t sizeof_src);
#[no_mangle]
pub extern fn base64_encode(dst: *mut c_void, dst_len: size_t, src: *const c_void, src_len: size_t) -> size_t {
    let source = unsafe {
        assert!(!src.is_null());
        slice::from_raw_parts(src, src_len as usize)
    };
    src_len as usize
}

#[cfg(test)]
mod tests {
    use super::{base64_decode, base64_encode};
    use libc::{c_void};
    use std::ffi::CString;
    #[test]
    fn test_decode() {
        let text = "this is a test.";
        let mut dest_ptr = CString::new(text).unwrap().into_raw() as *mut c_void;
        let text_ptr = text.as_ptr() as *const c_void;
        let retu: usize = base64_decode(dest_ptr, text.len(),text_ptr, text.len());
        assert_eq!(retu, text.len());

    }
}