Is bool FFI compatible, and if so, what is it defined to be?

[carllerche]

I am unsure how bool is represented in memory and how it works with fns that have that are C ABI compatible.

Perhaps it would be worth matching what stdbool does?

[huonw]

I would think that we should define it to be equivalent to the following:

#[repr(u8)]
enum Bool { False = 0, True = 1 }

(I don't know if this is what stdbool is.)

[alexcrichton]

One option is to leave Rust's bool as an undefined representation, but adding a define for c_bool in liblibc. I'm not sure if c_bool has a portable definition, however.

[thestinger]

@huonw: stdbool.h is a header defining bool as _Bool, false as 0 and true as 1. C uses keywords like _Bool, _Thread_local (threads.h defines it as thread_local) and _Noreturn (stdnoreturn.h defines it as noreturn) for backwards compatibility.

[SiegeLord]

This is somewhat of a blocker for bindgen's https://github.com/crabtw/rust-bindgen/issues/98. I'd prefer there to be a c_bool for this purpose.

[mahkoh]

Some C programs depend on _Bool containing only 0 and 1 and in C converting anything to _Bool produces either 0 or 1. Consider the following code (where sizeof(_Bool) = 1):

#include <stdbool.h>
#include <stdio.h>

void f(bool b) {
    printf("%d\n", (int)b);
}

extern {
    fn f(x: u8);
}

fn main() {
    unsafe { f(2); }
}

This will print 2.

[thestinger]

It's undefined behaviour to stick a value other than 0 or 1 into _Bool, which is why -fsanitize=bool exists.

[mahkoh]

Are you sure that it's UB? I checked the standard but couldn't find it. It does say that

An object declared as type _Bool is large enough to store the values 0 and 1.

[mahkoh]

Either way, what the program above should show is that having c_bool = u8 or similar is maybe not a good idea.

[mahkoh]

This is still an issue. At the very least, there has to be a c_bool in libc.

[tomjakubowski]

Any update on this?

[eefriedman]

It should be easy to file a PR to add the line pub type c_bool = bool; at

rust/src/liblibc/lib.rs

Line 209 in 0f1e514

pub type int8_t = i8;

. (Rust's builtin bool should behave like a C99 bool as far as I know, and I can't see any reason why that would change.)

[steveklabnik]

Closing in favor of rust-lang/rfcs#992