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

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

       #include <unistd.h>       /See NOTES */

       int ttyslot(void);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           Since glibc 2.24:
           From glibc 2.20 to 2.23:
               _DEFAULT_SOURCE ||
               _XOPEN_SOURCE && _XOPEN_SOURCE < 500
           Glibc 2.19 and earlier:
               _BSD_SOURCE ||
               _XOPEN_SOURCE && _XOPEN_SOURCE < 500

       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(1)  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  argu-
       ment  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  con-
       sole  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 cur-
       rently 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  (count-
       ing from 1).

   The semantics of ttyslot
       Thus,  the function ttyslot() returns the index of the controlling ter-
       minal of the calling process in the file /etc/ttys, and that  is  (usu-
       ally)  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.

       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 System V-like systems.

       For an  explanation  of  the  terms  used  in  this  section,  see  at-

       |Interface | Attribute     | Value     |
       |ttyslot() | Thread safety | MT-Unsafe |
       SUSv1;  marked  as LEGACY in SUSv2; removed in POSIX.1-2001.  SUSv2 re-
       quires -1 on error.

       The utmp file is found in 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.

       On BSD-like systems and Linux, the declaration of ttyslot() is provided
       by <unistd.h>.  On System V-like systems, the declaration  is  provided
       by <stdlib.h>.  Since glibc 2.24, <stdlib.h> also provides the declara-
       tion with the following feature test macro definitions:

           (_XOPEN_SOURCE >= 500 ||
                   (_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED))
               && ! (_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)

       Minix also has fttyslot(fd).

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

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GNU                               2017-09-15                        TTYSLOT(3)
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