Unexpected value when using remainder operation.

[lukyxu]

I tried this code:

extern "C" {
    fn printf(s: *const i8, ...);
}

fn print_int(value: i8) {
    let s = "%d\n\0";
    let s_p = s as *const str;
    let c_p = s_p as *const i8;
    unsafe {
        printf(c_p, value);
    }
}

fn main() {
    let mut checksum: i8 = -30_i8 % -55_i8;
    print_int(checksum);
}

I expected to see this happen: -30

Instead, this happened: 226

Meta

• What version of Rust GCC were you using, git sha if possible. gccrs (GCC) 12.0.1 20220118 (experimental) git sha: 9d81164aa0447c738fe0435de14ec9666a03d5da

[nirmal-j-patel]

I don't think the error is related to remainder operation. If you change the printf line to printf(c_p, value as isize);, the output will be -30. My understanding is that printf treats value as a normal integer. -30 and 226 have the same binary representation (0xe2). If you try to compile this program with rustc, you get this error:

error[E0617]: can't pass `i8` to variadic function
  --> remainder.rs:10:21
   |
10 |         printf(c_p, value);
   |                     ^^^^^ help: cast the value to `c_int`: `value as c_int`

error: aborting due to previous error

So it does appear that the issue is not remainder operation but actually displaying the number using printf.

[powerboat9]

This looks like a duplicate of #1495 @philberty

[powerboat9]

The given code now produces this error:

<source>:10:21: error: expected 'c_int' variadic argument
   10 |         printf(c_p, value);
      |                     ^~~~~

https://godbolt.org/z/nv9YqnGcP