---
name: "snp"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/snp"
---

# snp(4)

tty snoop interface

## NAME

**snp** – tty snoop interface

## SYNOPSIS

**#include \<sys/snoop.h\>**

*int*\

**ioctl**(*fd*, *SNPSTTY*, *&dev*);

*int*\

**ioctl**(*fd*, *SNPGTTY*, *&dev*);

*int*\

**ioctl**(*fd*, *FIONREAD*, *&result*);

## DESCRIPTION

*/dev/snp* is a snoop device which allows users to attach to any tty and watch activities on it. The kernel must be compiled with **device snp**, or the **snp** module must be loaded, for these devices to be available.

To associate a given **snp** device with a tty to be observed, open the **snp** device and a tty device, and then issue the SNPSTTY ioctl on **snp** device. The argument passed to the [ioctl(2)](/ioctl/2.md) is the address of a variable of type *int*, holding the file descriptor of a tty device. To detach the **snp** device from a tty use a pointer to a value of -1.

The SNPGTTY ioctl returns information about the current tty attached to the open **snp** device.

The FIONREAD ioctl returns a positive value equal to the number of characters in a read buffer. Special values defined are:

- SNP\_TTYCLOSE

  tty not attached.

- SNP\_DETACH

  **snp** device has been detached by user or tty device has been closed and detached.

## SEE ALSO

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

## HISTORY

The **snp** device first appeared in FreeBSD 2.1. In FreeBSD 8.0 the **snp** driver was rewritten to work with the replaced TTY subsystem.

## AUTHORS

The author of the current implementation is Ed Schouten ⟨*ed@FreeBSD.org*⟩. Previous versions of **snp** were based on code written by Ugen J.S. Antsilevich ⟨*ugen@NetVision.net.il*⟩.