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


       #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


       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

   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.


       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.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue     │
       │ttyslot() │ Thread safety │ MT-Unsafe │


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


       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

           (_XOPEN_SOURCE >= 500 ||
                   (_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED))
               && ! (_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)

       Minix also has fttyslot(fd).


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


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