snp — tty snoop interface

       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>.

       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.

Debian                                          November 5, 2008                                          SNP(4)

/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 devicesnp, 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:

       SNP_OFLOW     device overflow occurred, device detached.

       SNP_TTYCLOSE  tty not attached.

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

       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.

       snp — tty snoop interface

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

#include<sys/snoop.h>intioctl(fd, SNPSTTY, &dev);

       intioctl(fd, SNPGTTY, &dev);

       intioctl(fd, FIONREAD, &result);