How do I represent C's "unsigned negative" values in Rust code?

Question

I'm calling the ResumeThread WinAPI function from Rust, using the winapi crate.

The documentation says:

If the function succeeds, the return value is the thread's previous suspend count.

If the function fails, the return value is (DWORD) -1.

How can I effectively check if there was an error?

In C:

if (ResumeThread(hMyThread) == (DWORD) -1) {
    // There was an error....
}

In Rust:

unsafe {
    if ResumeThread(my_thread) == -1 {
            // There was an error....
    }
}

the trait `std::ops::Neg` is not implemented for `u32`

I understand the error; but what is the best way to be semantically the same as the C code? Check against std::u32::MAX?

Answer 1

In C, (type) expression is called a type cast. In Rust, you can perform a type cast by using the as keyword. We also give the literal an explicit type:

if ResumeThread(my_thread) == -1i32 as u32 {
    // There was an error....
}

I would personally use std::u32::MAX, potentially renamed, as they are the same value:

use std::u32::MAX as ERROR_VAL;

if ResumeThread(my_thread) == ERROR_VAL {
    // There was an error....
}

See also:

• How do I convert between numeric types safely and idiomatically?
• Is casting between integers expensive?
• What is the difference between From::from and as in Rust?
• In Rust, is "as" an operator?
• How do I convert a boolean to an integer in Rust?

Answer 2

Yes, either std::u32::MAX or perhaps !0u32 (the equivalent of C ~0UL, which emphasizes the fact that it's the all-bits-set value for the given type).