Crate types not the same..which one should I use?

These are not the same:

use libc::c_char;
use std::os::raw::c_char;

Looking at the definitions, std::os::raw::c_char is defined as a u8 while libc::c_char is defined as i8.

If I have FFI functions, which would I want to use? I’m thinking std::os::raw::c_char but I wanted clarity before I make changes…

Thnx
Matt

It depends on the platform whether C’s char is signed or unsigned. You can see the giant #[cfg] on std's c_char here, whereas libc has the definitions spread throughout different modules – but hopefully they do agree for any given platform!

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That make sense.

What triggered my curiosity is Pycharm (using rust plugin) triggered an error on passing a c_char from one function to another in a separate module. In one module, the c_char was from libc and in the other module used std.

It seems like they should both be resolving to the same type. So, could this just be a pycharm + rust plugin problem?

Thnx
Matt

To the compiler, type aliases are considered identical to the assigned type. So as long as both c_char really are defined the same way, there shouldn’t be any problem.

Maybe Pycharm doesn’t realize this, or maybe it’s conservatively supposing that they could be different in some configurations – I’m not sure. Is the error coming from Pycharm itself, or are they just presenting an error that was captured from rustc?

Here's the error:

The consumer function got (as input defined as libc::c_char) config_str and passed it to deserialize_arguments which uses std::os::raw::c_char

Matt

That does look like a real rustc error – what is the target platform?

MacOS. No specific targets are defined in the toml.

[lib]
name = "mylib"
path = "src/lib.rs"
crate-type = ["staticlib", "rlib", "cdylib"]

The code looks like this
in mod.rs

use libc::c_char;
#[no_mangle]
pub extern "C" fn address_handler(
    config_str: *const c_char
) -> i32 {

   deserialize_arguments(config_str)

}

in utils.rs

use std::os::raw::c_char;
pub fn deserialize_arguments(
    json_config: *const c_char
)

Thnx
Matt

I’m not sure what’s going on. AFAICT, they should both get type c_char = i8; on all apple targets.

1 Like

Yeah, nah. :slight_smile: