> 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/snp.4.md).

# snp.4

`snp` — tty 窥探接口

## 名称

`snp`

## 概要

`#include <sys/snoop.h>`

`Ft int Fn ioctl fd SNPSTTY &dev Ft int Fn ioctl fd SNPGTTY &dev Ft int Fn ioctl fd FIONREAD &result`

## 描述

`/dev/snp` 是一个窥探设备，允许用户附加到任何 tty 并观察其上的活动。内核必须以 `device snp` 编译，或加载 `snp` 模块，这些设备才可用。

要将给定的 `snp` 设备与要观察的 tty 关联起来，请打开 `snp` 设备和一个 tty 设备，然后在 `snp` 设备上发起 `SNPSTTY` ioctl。传递给 ioctl(2) 的参数是一个 `int` 类型变量的地址，该变量持有 tty 设备的文件描述符。要将 `snp` 设备从 tty 分离，使用指向值为 -1 的指针。

`SNPGTTY` ioctl 返回关于附加到所打开 `snp` 设备的当前 tty 的信息。

`FIONREAD` ioctl 返回一个等于读缓冲区中字符数的正值。已定义的特殊值有：

**`SNP_TTYCLOSE`** 未附加 tty。

**`SNP_DETACH`** `snp` 设备已被用户分离，或 tty 设备已关闭并分离。

## 参见

[pty(4)](/man/man4/pty.4.md), [kldload(8)](/man/man8/kldload.8.md), watch(8)

## 历史

`snp` 设备首次出现于 FreeBSD 2.1。在 FreeBSD 8.0 中，`snp` 驱动被重写以与替换后的 TTY 子系统协同工作。

## 作者

当前实现的作者是 Ed Schouten <ed@FreeBSD.org>。早期版本的 `snp` 基于 Ugen J.S. Antsilevich <ugen@NetVision.net.il> 编写的代码。


---

# 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/snp.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.