# Target Metadata (`OS_PLATFORM`, `OS_ARCH`, `OS_IS_UNIX`, `OS_IS_POSIX`)

Silk exposes a small set of compiler-provided **target metadata** values to both
runtime code and Formal Silk (compile-time verification).

These values let programs adapt to the compilation target (platform/OS and CPU
architecture) without requiring environment-specific runtime queries.

## Notes

- target metadata is available as built-in compile-time constants
 in every module:
 - `OS_PLATFORM`, `OS_ARCH`, `OS_IS_UNIX`, `OS_IS_POSIX`.
- the standard library re-exports these via [`std::os`](/silk/docs/std/os/)
 ([os](/silk/docs/std/os/)).

## Built-In Constants

The compiler provides the following built-in constants in every module:

- `OS_PLATFORM: string`
- `OS_ARCH: string`
- `OS_IS_UNIX: bool`
- `OS_IS_POSIX: bool`

These behave like normal `const` values:

- They do not require an import.
- They may be used anywhere an expression of the corresponding type is allowed.
- They are compile-time constants (their values are fixed at compile time and
 are embedded into the output artifact).

### `OS_PLATFORM`

A canonical target platform/OS name string.

Current compiler target set and values:

- `linux-x86_64`, `linux-x86_64-musl`, `linux-aarch64`, and
 `linux-aarch64-musl`:
 - `OS_PLATFORM == "linux"`
- `macos-x86_64` and `macos-aarch64`:
 - `OS_PLATFORM == "macos"`
- `ios-aarch64`, `ios-simulator-aarch64`, and `ios-simulator-x86_64`:
 - `OS_PLATFORM == "ios"`
- `android-aarch64`:
 - `OS_PLATFORM == "android"`
- `windows-x86_64` and `windows-aarch64`:
 - `OS_PLATFORM == "windows"`
- `wasm32-unknown-unknown`:
 - `OS_PLATFORM == "unknown"`
- `wasm32-wasi`:
 - `OS_PLATFORM == "wasi"`
- `amdgcn-amd-amdhsa-gfx942`, `amdgcn-amd-amdhsa-gfx1100`, and
 `amdgcn-amd-amdhsa-gfx1151`:
 - `OS_PLATFORM == "amdhsa"`

### `OS_ARCH`

A canonical target CPU architecture name string.

Current compiler target set and values:

- `linux-x86_64`, `linux-x86_64-musl`, `macos-x86_64`,
 `ios-simulator-x86_64`, and `windows-x86_64`:
 - `OS_ARCH == "x86_64"`
- `linux-aarch64`, `linux-aarch64-musl`, `macos-aarch64`, `ios-aarch64`,
 `ios-simulator-aarch64`, `android-aarch64`, and `windows-aarch64`:
 - `OS_ARCH == "aarch64"`
 - Formal Silk comparisons also accept the ARM64 aliases `"arm64"` and `"aarch"` in any letter case, including through compile-time string constants, even though the canonical `OS_ARCH` value remains `"aarch64"`.
- `wasm32-unknown-unknown` and `wasm32-wasi`:
 - `OS_ARCH == "wasm32"`
- `amdgcn-amd-amdhsa-gfx942`, `amdgcn-amd-amdhsa-gfx1100`, and
 `amdgcn-amd-amdhsa-gfx1151`:
 - `OS_ARCH == "amdgcn"`

### `OS_IS_UNIX`

Whether the compilation target is a UNIX family target.

Current compiler target set:

- `linux-x86_64`: `true`
- `linux-x86_64-musl`: `true`
- `linux-aarch64`: `true`
- `linux-aarch64-musl`: `true`
- `macos-x86_64`: `true`
- `macos-aarch64`: `true`
- `ios-aarch64`: `true`
- `ios-simulator-aarch64`: `true`
- `ios-simulator-x86_64`: `true`
- `android-aarch64`: `true`
- `windows-x86_64`: `false`
- `windows-aarch64`: `false`
- `wasm32-unknown-unknown`: `false`
- `wasm32-wasi`: `false`
- `amdgcn-amd-amdhsa-gfx942`: `false`
- `amdgcn-amd-amdhsa-gfx1100`: `false`
- `amdgcn-amd-amdhsa-gfx1151`: `false`

### `OS_IS_POSIX`

Whether the compilation target is a POSIX target.

Current compiler target set:

- `linux-x86_64`: `true`
- `linux-x86_64-musl`: `true`
- `linux-aarch64`: `true`
- `linux-aarch64-musl`: `true`
- `macos-x86_64`: `true`
- `macos-aarch64`: `true`
- `ios-aarch64`: `true`
- `ios-simulator-aarch64`: `true`
- `ios-simulator-x86_64`: `true`
- `android-aarch64`: `true`
- `windows-x86_64`: `false`
- `windows-aarch64`: `false`
- `wasm32-unknown-unknown`: `false`
- `wasm32-wasi`: `false`
- `amdgcn-amd-amdhsa-gfx942`: `false`
- `amdgcn-amd-amdhsa-gfx1100`: `false`
- `amdgcn-amd-amdhsa-gfx1151`: `false`

## Relationship to [`std::os`](/silk/docs/std/os/)

The standard library provides [`std::os`](/silk/docs/std/os/) helpers that expose the same metadata
in a namespaced form and additionally map these strings into enums for use with
[`match`](/silk/wiki/language/flow-match/) (see [os](/silk/docs/std/os/); targets not covered by the current enum set map
to `Unknown`).

## Examples

### Target-gated behavior

```silk
import std::os;
import { println } from "std/io";

fn main () -> int {
  if OS_IS_POSIX {
    println("posix");
  }

  match (std::os::platform()) {
    std::os::Platform::Linux => println("linux"),
    std::os::Platform::WASI => println("wasi"),
    std::os::Platform::Unknown => println("unknown"),
  };

  return 0;
}
```

### Formal Silk requirements

```silk
#require OS_IS_POSIX;
```