TTYSLOT(3)                (2017-09-15)                 TTYSLOT(3)

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

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

          int ttyslot(void);

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

          ttyslot():
              Since glibc 2.24:
                  _DEFAULT_SOURCE
              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

     DESCRIPTION
          The legacy function ttyslot() returns the index of the  cur-
          rent 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 ter-
          minal line.  Each line consisted of three  characters.   The
          first  character  was  either  aq0aq  or  aq1aq, where aq0aq meant
          "ignore".  The second character denoted  the  terminal:  aq8aq
          stood  for "/dev/tty8".  The third character was an argument
          to getty(8) indicating the sequence of line  speeds  to  try
          (aq-aq  was:  start trying 110 baud).  Thus a typical line was
          "18-".  A hang on some line was solved by changing  the  aq1aq
          to a aq0aq, 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 (aq0aq was: cycle through 300-1200-150-110 baud;
          aq4aq 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

     Page 1                         GNU              (printed 5/17/22)

     TTYSLOT(3)                (2017-09-15)                 TTYSLOT(3)

          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  con-
          trolling  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.

     RETURN VALUE
          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.

     ATTRIBUTES
          For an explanation of the terms used in  this  section,  see
          attributes(7).     allbox;    lb    lb    lb    l    l    l.
          Interface Attribute Value    T{    ttyslot()     T}   Thread
          safety  MT-Unsafe

     CONFORMING TO
          SUSv1; marked as LEGACY in SUSv2; removed  in  POSIX.1-2001.
          SUSv2 requires -1 on error.

     NOTES
          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 declaration with the following
          feature test macro definitions:

              (_XOPEN_SOURCE >= 500 ||

     Page 2                         GNU              (printed 5/17/22)

     TTYSLOT(3)                (2017-09-15)                 TTYSLOT(3)

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

          Minix also has fttyslot(fd).

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

     COLOPHON
          This page is part of release 5.10  of  the  Linux  man-pages
          project.   A  description  of the project, information about
          reporting bugs, and the latest version of this page, can  be
          found at https://www.kernel.org/doc/man-pages/.

     Page 3                         GNU              (printed 5/17/22)