.\"
.\"  $FreeBSD$
.\"  $Source: /home/ncvs/src/share/man/man4/snp.4,v $
.\"
.Dd February 24, 1995
.Dt SNP 4
.Os FreeBSD
.Sh NAME
.Nm snp
.Nd tty snoop interface
.Sh SYNOPSIS
.Fd #include <sys/ioctl.h>
.Fd #include <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
/dev/snp? are snoop devices which allow user to attach to any tty
and watch activities on it.

To associate a given
.Nm snp
device with a tty to be observed,  open the
.Nm snp
device and then use the SNPSTTY ioctl.
The argument passed to the IOCTL is the address of a variable of type
dev_t.
By setting the value of the variable pointed to -1, the user may detach
the snp device from a tty.
.Pp
The SNPGTTY ioctl returns information about the current tty attached to
the open
.Nm snp
device.
.Pp
The FIONREAD ioctl returns a positive value equal to the number of characters
in a read buffer.
Special values defined are:
.Bl -tag -width SNP_TTYCLOSE
.It Dv SNP_OFLOW
device overflow occured, device detached.
.It Dv SNP_TTYCLOSE
tty not attached.
.It Dv SNP_DETACH
.Nm snp
device has been detached by user or tty device has been closed
and detached.
.Sh SEE ALSO
.Xr pty 4 ,
.Xr sio 4 ,
.Xr watch 8
.Sh RESTRICTIONS
Only the superuser may access the
.Nm snp
devices.
.Sh BUGS
While in line mode, user input can't be seen.
No signals may be sent to the observed tty.
.Sh AUTHOR
Ugen J.S. Antsilevich <ugen@NetVision.net.il>
.Sh HISTORY
The
.Nm snp
device first appeared in FreeBSD 2.1.