Got a process struct / need a tty.

[Brandon S. Allbery]

As quoted from <598 at uncw.UUCP> by english at uncw.UUCP (Warren R. English Jr.):
+---------------
| 	How would one go about finding the control terminal
| of a process? After reading all the process structures from /dev/kmem
| into any array, I can't seem to figure out how to get the filename
| of the controlling tty. It seemed to me that after you have
| access to each and every process on the machine, it would be easy
| to find the tty. So far, I have tried looking at the user structure and
| the vnode structure that the process structure points to, but still,
| no /dev/ttyxx can I find. The machine is a Sequent Balance Series,
| running Dynix. 
+---------------

Unix kernels do not use pathnames; they use <device, inumber> pairs.  System
calls like open(), creat(), chdir(), chroot(), etc. spend most of their time
(modulo slow device I/O or block on no carrier, of course) decoding a pathname
into the much faster <device, inumber> format.

In the case of controlling ttys, it goes on step farther:  since a controlling
tty is *always* a device, the <device, inumber> is further decoded into a
device number by looking in the *node (vnode, I assume, in your case) table.
This produces a <major, minor> pair; the major device number specifies the
driver, the minor number specifies the specific device handled by the driver.
The only way to go from these back to pathnames is to stat() every entry in
the /dev directory (convention) for character devices with the correct major
and minor device numbers.

The major/minor device numbers are stored in the user struct, in u.u_ttyd on
USG Unix; I don't know if this was changed in BSD-based Unixes like Dynix.
They are stored together; macros exist to extract them, but you want to
compare both.  The stat struct has the device major/minor numbers in the
st_rdev element; it is only valid if, for a (struct stat statbuf) set by a
successful call to stat(), (statbuf.st_mode & S_IFMT) is equal to S_IFCHR or
S_IFBLK.  (TTY devices are always S_IFCHR.)  (The S_* macros are defined in
<sys/stat.h> on both USG and AT&T.)

++Brandon
-- 
Brandon S. Allbery (human), allbery at NCoast.ORG (Inet), BALLBERY (MCI Mail)
ALLBERY (Delphi), uunet!cwjcc.cwru.edu!ncoast!allbery (UUCP), B.ALLBERY (GEnie)
BrandonA (A-Online) ("...and a partridge in a pear tree!" ;-)