How do you use DynamicLibrary with a DLL on windows in rust?

Question

I've been trying to get a basic proof of concept up and running using cmake and rust here https://github.com/shadowmint/rust-dl-example, but to no avail.

The basic idea is to build a DLL and then load symbols like this:

  let dl = DynamicLibrary::open(Some(dl_path));
  match dl {
    Ok(dll) => {
      // Try loading symbols. Note that because of the api we can't
      // directly pass T as fn(c_int) -> c_int, because the return is
      // a pointer, not a pointer to a pointer.
      unsafe{
        rtn.foo = match dll.symbol::<c_void>("foo") {
          Ok(symbol) => Some(transmute(symbol)),
          Err(_) => None
        };
        rtn.bar = match dll.symbol::<c_void>("bar") {
          Ok(symbol) => Some(transmute(symbol)),
          Err(_) => None
        };
        trace!("Read: {}", dll.symbol::<c_void>("foo"));
        trace!("Read: {}", dll.symbol::<c_void>("bar"));
        rtn.lib = Some(dll);
      }
    }

Which works a charm on linux and osx, but sadly fails on windows:

Compiling dltest v0.1.0 (file:///C:/projects/rust-all/rust-dl-example)
Running target\dltest-3ed01b3dac66913e.exe

running 1 test
Pattern: Some(C:\projects\rust-all\rust-dl-example\build\*foo*.dll)
Some(C:\projects\rust-all\rust-dl-example\build\libfoo.dll)
Read: Err(Error code 127)
Read: Err(Error code 127)
Successfully loaded table
Done1
Done2
stack backtrace:
   1:           0x5452c8 - main
   2:           0x549525 - main
   3:           0x54effd - main
   4:           0x54ecc2 - main
   5:           0x4d8e8d - main
   6:           0x402411
   7:           0x4023e3
   8:           0x40238d
   9:           0x40ccbc
  10:           0x43d438 - main
  11:           0x52e277 - main
  12:           0x54d4cc - main
  13:           0x54fc0f - main
  14:           0x54fbe9 - main
  15:           0x54d55e - main
  16:           0x54d309 - main
  17:           0x54d116 - main
  18:           0x54e64d - main
  19:         0x76fc652d - BaseThreadInitThunk
task failed during unwinding. aborting.

Error code 127 is windows magic for 'no symbol found', but first of all it isn't returning as an error correctly and secondly, I can't see anything wrong with my DLL. It works fine from a c program... and it doesn't have any weird linkages:

What's going on? Anyone got a working example of a DLL with windows & rust?

Answer 1

My guess is that you are not correctly matching the calling convention of your DLL. I used this code to create a DLL:

#[no_mangle]
// Note explicit calling convention
pub extern "C" fn foo_add_2(a: u32) -> u32 {
  a + 2
}

Compiled that as:

rustc --crate-type=dylib

And accessed it as:

use std::dynamic_lib::DynamicLibrary;
use std::mem::transmute;

fn main() {

  let lib = match DynamicLibrary::open(Some("lib")) {
    Ok(x) => x,
    Err(x) => panic!("{}", x),
    };

  // Note explicit calling convention
  let meth: extern "C" fn(u32) -> u32 = unsafe {
    match lib.symbol::<u8>("foo_add_2") {
      Ok(x) => transmute(x),
      Err(x) => panic!("{}", x),
    }
  };

  let input = 5;
  let res = meth(input);

  println!("Hello, library! ({} + 2 => {})", input, res);
}

This worked fine on a 32-bit VM with Windows 7 and a Rust nightly from 2014-12-08.

However, if I change the calling conventions to not match, or leave it off, I see your error 127.