# How-To: Use a Custom Stdlib Root (and Runtime)

The `std::...` modules are loaded from a *stdlib root* (a directory containing a
[`std/`](https://github.com/oro-computer/silk/tree/master/std/) tree). Because [`std::runtime`](/silk/docs/std/runtime/) is part of that source tree, selecting a
different stdlib root is also how you select a different runtime implementation
underneath [`std::fs`](/silk/docs/std/fs/), [`std::io`](/silk/docs/std/io/), [`std::task`](/silk/docs/std/task/), and [`std::sync`](/silk/docs/std/sync/).

Reference: [`std::runtime`](/silk/docs/std/runtime/) (see the sidebar under “Standard library”).

## Use a Custom Std Root (CLI)

Point the compiler at an alternate stdlib root:

```sh
silk build --std-root /path/to/my-stdlib-root my_program.slk -o my_program
```

You can also set an environment variable:

```sh
export SILK_STD_ROOT=/path/to/my-stdlib-root
silk build my_program.slk -o my_program
```

Notes:

- `--nostd` disables auto-loading of std modules, but it does not prevent
 `from "std/..."` file imports from resolving relative to the selected std
 root. See: [CLI and toolchain](/silk/docs/guides/cli/).

## Provide a Prebuilt Std Archive

On hosted targets where std archives are supported, you can also provide a prebuilt std archive:

```sh
silk build --std-root /path/to/my-stdlib-root --std-lib /path/to/libsilk_std.a my_program.slk -o my_program
```

Or via environment variable:

```sh
export SILK_STD_LIB=/path/to/libsilk_std.a
```

If no suitable archive is provided, the compiler may fall back to compiling
reachable std sources as part of the build on supported targets.

## Implementing a Custom Runtime

To reuse the shipped high-level `std::...` modules while changing the OS
integration, provide compatible [`std::runtime::...`](/silk/docs/std/runtime/) modules in your stdlib root.

Example (hosted POSIX baseline):

- [`std/runtime/fs.slk`](https://github.com/oro-computer/silk/blob/master/std/runtime/fs.slk) delegates to [`std/runtime/posix/fs.slk`](https://github.com/oro-computer/silk/blob/master/std/runtime/posix/fs.slk),
- [`std/runtime/posix/fs.slk`](https://github.com/oro-computer/silk/blob/master/std/runtime/posix/fs.slk) implements primitives using `ext` (e.g. `open`,
 `read`, `close`).

For a new environment (for example Windows), the intent is to provide
[`std/runtime/windows/fs.slk`](https://github.com/oro-computer/silk/blob/master/std/runtime/windows/fs.slk) and have [`std/runtime/fs.slk`](https://github.com/oro-computer/silk/blob/master/std/runtime/fs.slk) delegate to it in that
stdlib distribution.

Keep the stable contract at the [`std::runtime::...`](/silk/docs/std/runtime/) signature level, not at the
`ext` spelling level. Higher-level ergonomics belong in `std::...` modules.