Ancient history

There used to be a file /etc/ttys in UNIX V6, that was read by the init(8) program to find out what to do with each terminal line. Each line consisted of three characters. The first character was either '0' or '1', where '0' meant "ignore". The second character denoted the terminal: '8' stood for "/dev/tty8". The third character was an argument to getty(8) indicating the sequence of line speeds to try ('−' was: start trying 110 baud). Thus a typical line was "18−". A hang on some line was solved by changing the '1' to a '0', signaling init, changing back again, and signaling init again.

In UNIX V7 the format was changed: here the second character was the argument to getty(8) indicating the sequence of line speeds to try ('0' was: cycle through 300-1200-150-110 baud; '4' was for the on-line console DECwriter) while the rest of the line contained the name of the tty. Thus a typical line was "14console".

Later systems have more elaborate syntax. System V-like systems have /etc/inittab instead.

Ancient history (2)

On the other hand, there is the file /etc/utmp listing the people currently logged in. It is maintained by login(1). It has a fixed size, and the appropriate index in the file was determined by login(1) using the ttyslot() call to find the number of the line in /etc/ttys (counting from 1).

The semantics of ttyslot

Thus, the function ttyslot() returns the index of the controlling terminal of the calling process in the file /etc/ttys, and that is (usually) the same as the index of the entry for the current user in the file /etc/utmp. BSD still has the /etc/ttys file, but System V-like systems do not, and hence cannot refer to it. Thus, on such systems the documentation says that ttyslot() returns the current user's index in the user accounting data base.

Multithreading (see pthreads(7))

The ttyslot() function calls thread-unsafe function getttyent(), so it is not thread-safe.