ttyslot(3) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       #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

DESCRIPTION         top

       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
       (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         top

       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         top

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

       │Interface Attribute     Value     │
       │ttyslot() │ Thread safety │ MT-Unsafe │

CONFORMING TO         top

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

NOTES         top

       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

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

       Minix also has fttyslot(fd).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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

GNU                              2017-09-15                       TTYSLOT(3)

Pages that refer to this page: endttyent(3)getttyent(3)getttynam(3)setttyent(3)