> For the complete documentation index, see [llms.txt](https://book.bsdcn.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.bsdcn.org/man/man4/nullfs.4.md).

# nullfs.4

`nullfs` — 空文件系统

## 名称

`nullfs`

## 概要

`要启用对此驱动的支持，请在内核配置文件中加入以下行：`

> options NULLFS

`或者，要在引导时以模块方式加载该驱动，请在 loader.conf(5) 中加入以下行：`

```sh
nullfs_load="YES"
```

## 描述

`nullfs` 驱动允许 FreeBSD 内核挂载环回文件系统子树。

## 实例

挂载 `nullfs` 文件系统：

```sh
mount_nullfs /usr/ports /home/devel/ports
```

也可以在 [fstab(5)](/man/man5/fstab.5.md) 中定义类似以下的条目：

```sh
/usr/ports	/home/devel/ports	nullfs		rw	0	0
```

## 参见

[fstab(5)](/man/man5/fstab.5.md), mount\_nullfs(8)

## 历史

`nullfs` 层首次出现于 4.4BSD。

## 作者

`nullfs` 内核实现由 John Heideman 编写。

本手册页由 Daniel Gerzo <danger@FreeBSD.org> 编写。


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://book.bsdcn.org/man/man4/nullfs.4.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.