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SNP(4) FreeBSD Kernel Interfaces Manual SNP(4)

tty snoop interface

#include <sys/snoop.h>
ioctl(fd, SNPSTTY, &dev);
ioctl(fd, SNPGTTY, &dev);
ioctl(fd, FIONREAD, &result);

/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) 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:
device overflow occurred, device detached.
tty not attached.
snp device has been detached by user or tty device has been closed and detached.

pty(4), sio(4), kldload(8), watch(8)

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.

The author of the current implementation is Ed Schouten <>. Previous versions of snp were based on code written by Ugen J.S. Antsilevich <>.

This version of snp does not return proper error codes when calling FIONREAD. It also does not allow SNPSTTY to detach itself from the TTY.
November 5, 2008 FreeBSD 12.0-RELEASE

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