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

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 (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).

       ┌──────────┬───────────────┬───────────┐
       │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  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

       This  page  is  part of release 5.05 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/.