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

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) 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.

SEE ALSO
     pty(4), kldload(8), watch(8)

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>.

BUGS
     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.

FreeBSD 13.1-RELEASE-p6        November 5, 2008        FreeBSD 13.1-RELEASE-p6