Can't use lazy static integer constant for the size of an array

Question

The following code gets the page size from sysconf to optimally read files from the filesystem. I am unable to create the buffer with the size:

lazy_static! {
    static ref PAGE_SIZE: i64 = sysconf(SysconfVariable::ScPagesize).unwrap();
}

fn main() {
    let mut buffer = [0; *PAGE_SIZE as usize];
}

Gives me:

error[E0080]: constant evaluation error
  --> src/main.rs:6:30
   |
6  |         let mut buffer = [0; *PAGE_SIZE as usize];
   |                              ^^^^^^^^^^ unimplemented constant expression: deref operation

I thought it is a operator priority problem and tried to wrap with it braces but the result still the same:

error[E0080]: constant evaluation error
  --> src/main.rs:6:30
   |
6  |         let mut buffer = [0; (*(PAGE_SIZE)) as usize];
   |                              ^^^^^^^^^^^^^^ unimplemented constant expression: deref operation

How to use the constant above for allocating a buffer?

Tatsuyuki Ishi · Accepted Answer · 2017-04-16 08:34:32Z

2

This is not compile-time constant at all. Use vec! for anything that can't be determined compile-time:

let mut buffer = vec![0; *PAGE_SIZE as usize];

Currently there's no compiler level support for Variable-Length Arrays (VLA).

answered Apr 16, 2017 at 8:34

Tatsuyuki Ishi

4,0714 gold badges31 silver badges42 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

VP. Over a year ago

Okay, this is not a compile-time constant. The docs also say so. Is there any other way to read page size once and to use it everywhere? Is there any way to write page size into a real compile-time constant? static PAGE_SIZE: i64 = ... maybe? Anything?

Tatsuyuki Ishi Over a year ago

You can achieve that via a compiler plugin, or build script combined with env!. I cannot recommend allocating on stack, because allocating a page on stack may cause overflow.

Matthieu M. Over a year ago

@TatsuyukiIshi: I would like to note that using a build script and env! should be done carefully in the event of cross-compiling. The target and host systems may have different page sizes. Thus, I would instead suggest using cfg.

Collectives™ on Stack Overflow

Can't use lazy static integer constant for the size of an array

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related