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TTYSLOT(3)			    Linux Programmer's Manual			       TTYSLOT(3)

       ttyslot - find the slot of the current user's terminal in some file

       #include <unistd.h>    /* on BSD-like systems  */
       #include <stdlib.h>    /* on SYSV-like systems */

       int ttyslot(void);

       The legacy function ttyslot() returns the index of the current user's entry in some file.

       Now "What file?" you ask. Well, let's first look at some history.

   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', signalling init, changing back again, and signalling 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.  SYSV-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(8).  It has a fixed size, and the appropriate index in the file was
       determined by login(8) 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 current
       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 SYSV-
       like systems do not, and hence cannot refer to it.  Thus, on such systems  the  documenta-
       tion  says  that  ttyslot()  returns  the current user's index in the user accounting data

       If successful, this function returns the slot number.  On error (e.g., if none of the file
       descriptors  0,	1  or  2  is associated with a terminal that occurs in this data base) it
       returns 0 on Unix V6 and V7 and BSD-like systems, but -1 on SYSV-like systems.

       The  utmp  file	is  found  various  places  on	various  systems,  such   as   /etc/utmp,
       /var/adm/utmp, /var/run/utmp.

       The  glibc2  implementation  of	this  function	reads  the  file  _PATH_TTYS,  defined in
       <ttyent.h> as "/etc/ttys". It returns 0 on error.  Since Linux systems do not usually have
       "/etc/ttys", it will always return 0.

       Minix also has fttyslot(fd).

       ttyslot() appeared in Unix V7.

       XPG2. Legacy in SUSv2. Deleted in SUSv3.  SUSv2 requires -1 on error.

       getttyent(3), ttyname(3), utmp(5)

GNU					    2002-07-20				       TTYSLOT(3)
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