oops..here is the manpage

svn path=/head/; revision=6533

usr.bin/watch/snp.4

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+.Dd Februar 17, 1995
+.Dt SNP(4)
+.Os
+.Sh NAME
+
+ snp  - tty snoop interface.
+
+.Sh SYNOPSIS
+#include <sys/snoop.h>
+
+.Sh DESCRIPTION
+ /dev/snp? are snoop devices which allow user to attach to any tty
+and watch activities on it. Device can be used by root only.
+ To connect device to tty use special iocontrol SNPSTTY.Argument to
+it is a structure 'snptty'. 
+              struct snptty {
+                     int st_type;
+                     int st_unit;
+              }
+
+st_type - type of tty to attach. Currently supported are :
+       ST_PTY - pseudo-tty devices.
+       ST_VTY - virtual ttys (syscons).
+       ST_SIO - serial ttys.
+
+st_unit - number of tty unit.
+By specifying ever st_type or st_unit -1 user can detach snp 
+device from tty.
+
+ SNPGTTY  - iocontrol returns current tty attached.
+ FIONREAD - iocontrol returns ever positive value equal to number of
+            characters in read buffer. Also special values defined:
+             SNP_OFLOW    - device overflow occured,device automatically
+                            detached.
+             SNP_TTYCLOSE - tty still has not been attached.       
+             SNP_DETACH   - snp device has been detached by user or
+                            tty device has been closed and detached
+                            automatically.
+  
+.Sh SEE ALSO
+
+watch(8), pty(4), sio(4)
+.Sh BUGS
+
+While in line mode,user input can't be seen.
+Has no ability to write to attached tty device.
+
+.Sh HISTORY
+FreeBSD 2.1 by Ugen J.S.Antsilevich <ugen@NetVision.net.il>