ttyslot(3) — Linux manual page


ttyslot(3)              Library Functions Manual              ttyslot(3)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

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

       int ttyslot(void);

   Feature Test Macro Requirements for glibc (see

           Since glibc 2.24:
           From glibc 2.20 to glibc 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

   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

       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

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                         Attribute     Value     │
       │ ttyslot()                         │ Thread safety │ MT-Unsafe │

VERSIONS         top

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

STANDARDS         top


HISTORY         top

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

       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 ||
                   (_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)

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Pages that refer to this page: getttyent(3)