Rust 1.78 Does Not Return c_void Pointer Correctly about rust HOT 8 CLOSED

Since passing an int* and returning a void* still works, our current fix is to write a wrapper similar to this:

 extern {
     fn initialize_wrapper(result: *mut i32) -> *mut c_void;
}
...
let mut result: i32 = 0;
let context = unsafe { initialize_wrapper(&mut result) };
// check result
...
// pass context to another C function

from rust.

extern {
    fn initialize(ctx: &*mut c_void) -> i32;
}

That looks like UB because your passing a shared reference rather than a mutable one. I.e. it should be defined as:

extern {
    fn initialize(ctx: &mut *mut c_void) -> i32;
}

from rust.

It's just odd that it worked up to 1.77.0 and also with 1.78.0, there are still some weird conditions where it still works (see description).

from rust.

I believe that what you're trying to do is UB, by taking an immutable reference and trying to assign to it you're violating the immutability requirements of that reference.

At least that's what I think is happening when I try to recreate your sample code in pure Rust, playground.

Using a mutable reference should already be a step in the right direction.

extern {
    fn initialize(ctx: &mut *mut c_void) -> i32;
}
// ...
let mut context = ptr::null_mut();
let result = unsafe { initialize(&mut context) };

from rust.

It's just odd that it worked up to 1.77.0 and also with 1.78.0, there are still some weird conditions where it still works (see description).

Optimization around aliasing are (if I remember correctly) only done at higher level of optimization.

from rust.

Thanks, using the &mut option works. But I'm surprised that the 1.78.0 compiler did not complain about passing immutable reference since it behaves differently than previous versions.

from rust.

the compiler usually doesn't complain about doing undefined behavior because it doesn't know that you're doing undefined behavior.

from rust.

here the compiler just assumed that you wouldn't modify it, and it has no way of knowing that you are modifying it. you just have to be careful. i don't think there's a tool that could have helped in this case, but in general, make sure to use tools like address sanitizer to check your unsafe code (and for unsafe code without FFI, Miri).

closing as not a bug

from rust.