ttyslot(3) - Linux manual page (original) (raw)

ttyslot(3) Library Functions Manual ttyslot(3)

NAME top

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

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#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 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 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
   [attributes(7)](../man7/attributes.7.html).
   ┌────────────────────────────────────┬───────────────┬───────────┐
   │ **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

   None.

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_).

COLOPHON top

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Linux man-pages 6.10 2024-07-23 ttyslot(3)

Pages that refer to this page:getttyent(3)