TTYSLOT(3)                                                   Linux Programmer's Manual                                                  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 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 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 (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.

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  ter-
       minal 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).

       +----------+---------------+-----------+
       |Interface | Attribute     | Value     |
       +----------+---------------+-----------+
       |ttyslot() | 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  pro-
       vided by <stdlib.h>.  Since glibc 2.24, <stdlib.h> also provides the declaration 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).

SEE ALSO

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

COLOPHON

       This  page is part of release 4.15 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/.

GNU                                                                 2017-09-15                                                          TTYSLOT(3)