FFI call to C adds an extra argument before the specified argument list

[kevinmehall]

I'm trying to call a function in a C library with two args, a pointer and an int, and was seeing the C function segfault. When I break on the C function in GDB, I see that the pointer I passed in the first argument from Rust shows up in the second argument, and there's another address added as the first argument.

Simplified to a minimal example by passing a constant for the pointer to show that it's shifted into the second argument:

libusb_set_debug(10 as *mut libusb_context, 4);
/* Breakpoint 1, libusb_set_debug (ctx=0x804e738, level=10) at core.c:1600 */

I'm using Fedora 18, 32-bit
rustc 0.7 (3e933b1 2013-07-06 08:32:10 -0700)
host: i686-unknown-linux-gnu

Here's the original code and GDB output:

use std::cast;
use std::libc::{c_void, c_int, size_t, malloc, free};
use std::ptr;
use std::unstable::intrinsics;

struct libusb_context;

#[link_args = "-lusb-1.0"]
extern{
    fn libusb_init(ctx: **mut libusb_context) -> c_int;
    fn libusb_exit(ctx: *mut libusb_context) -> c_void;
    fn libusb_set_debug(ctx: *mut libusb_context, level: c_int) -> c_void;
}

fn main() {
    unsafe{
        let mut ctx: *mut libusb_context = intrinsics::init();
        let r = libusb_init(&ctx);
        println(fmt!("r=%?, ctx=0x%x", r, ctx as uint));
        libusb_set_debug(ctx, 4);
        println("set debug");
    }
}

/*
r=0, ctx=0xb7a00468
[Switching to Thread 0xb7bffb40 (LWP 32527)]

Breakpoint 1, libusb_set_debug (ctx=0x805b690, level=-1214249880)
    at core.c:1600
1600    {
(gdb) p/x level
$1 = 0xb7a00468
*/

I also tried calling libusb_exit(ctx), and it also receives the wrong pointer and segfaults.

[jdm]

You probably need to use extern "C" instead of plain extern.

[kevinmehall]

I had the functions returning c_void, thinking that mapped to C functions that return void. However, c_void is just an empty enum in libc.rs:

pub enum c_void {}

It looks like Rust enums are passed as C structs. The cdecl calling convention specifies that to return a structure, the caller passes a hidden first argument. I fixed it by removing the return type from my extern fn:

fn libusb_set_debug(ctx: *mut libusb_context, level: c_int);

I suppose c_void is only intended to be used for void pointers.