# XO1001: std/sys/<os> used outside the matching target arm

Platform specific APIs live in `std/sys/<os>` and may be referenced only inside the arm of a `match target.os` for that OS, so code for one platform cannot leak into another's build while every arm is still type checked on every host. The prototype does not emit this code yet: no `std/sys/<os>` modules exist, and the example below names a hypothetical `std/sys/linux` function. See core 12.2.

## Example

```xo
use std/sys/linux

pub fn page_size() -> Int {
  linux.page_size()
}
```

## Fix

```xo
use std/sys/linux

pub fn page_size() -> Int {
  match target.os {
    Linux => linux.page_size()
    Darwin => 16384
    Windows => 4096
    Wasi => 65536
  }
}
```

Run `xo explain XO1001` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).