cannot apply unary operator `-` to type `u8` on arm64

[matwey]

When building 0.195.1 on aarch64 architecture the following issue happens:

[  209s] error[E0600]: cannot apply unary operator `-` to type `u8`
[  209s]     --> flux/src/cffi.rs:1152:35
[  209s]      |
[  209s] 1152 |         let v: Vec<c_char> = vec![-61, 0];
[  209s]      |                                   ^^^ cannot apply unary operator `-`
[  209s]      |
[  209s]      = note: unsigned values cannot be negated

[lmangani]

Interesting. I'm not 100% sure but did some quick research and it seems only valid signed 8-bit characters should be used in Vec<c_char>. I guess x86, the code might be lenient and allow negative values for u8, but on ARM64 it seems using signed values might be required. We could change the code to use valid signed 8-bit characters by specifying the type as i8

let v: Vec<c_char> = vec![-61i8, 0i8];

Would you be willing to try and confirm if this approach works?

[matwey]

I cannot check right now, but I know for sure that char type in C has different signedness on different platforms.

[andreas-schwab]

If you want to process an UTF-8 string you should use unsigned char, not char.