.\"
.\" $FreeBSD: head/share/man/man4/snp.4 101811 2002-08-13 14:12:31Z ru $
.\"
.Dd February 24, 1995
.Dt SNP 4
.Os
.Sh NAME
.Nm snp
.Nd tty snoop interface
.Sh SYNOPSIS
.In sys/snoop.h
.Ft int
.Fn ioctl fd SNPSTTY &dev
.Ft int
.Fn ioctl fd SNPGTTY &dev
.Ft int
.Fn ioctl fd FIONREAD &result
.Sh DESCRIPTION
.Pa /dev/snp?
are snoop devices which allow users to attach to any tty
and watch activities on it.
The kernel must be compiled with
.Cd "device snp" ,
or the
.Nm
module must be loaded,
for these devices to be available.
.Pp
To associate a given
.Nm
device with a tty to be observed, open the
.Nm
device and then use the
.Dv SNPSTTY
ioctl.
The argument passed to the
.Xr ioctl 2
is the address of a variable of type
.Vt dev_t .
To detach the
.Nm
device from a tty use a pointer to a value of
.Po Vt dev_t Pc Ns \-1 .
.Pp
The
.Dv SNPGTTY
ioctl returns information about the current tty attached to
the open
.Nm
device.
.Pp
The
.Dv FIONREAD
ioctl returns a positive value equal to the number of characters
in a read buffer.
Special values defined are:
.Bl -tag -width ".Dv SNP_TTYCLOSE"
.It Dv SNP_OFLOW
device overflow occurred, device detached.
.It Dv SNP_TTYCLOSE
tty not attached.
.It Dv SNP_DETACH
.Nm
device has been detached by user or tty device has been closed
and detached.
.El
.Sh SEE ALSO
.Xr pty 4 ,
.Xr sio 4 ,
.Xr kldload 8 ,
.Xr watch 8
.Sh BUGS
Caveat emptor!
This manual page is horribly stale and wildly inaccurate in some places.
.Pp
While in line mode, user input cannot be seen.
No signals may be sent to the observed tty.
.Sh AUTHORS
.An Ugen J.S. Antsilevich Aq ugen@NetVision.net.il
.Sh HISTORY
The
.Nm
device first appeared in
.Fx 2.1 .