man3/ttyslot.3

   1 .\" Copyright (C) 2002 Andries Brouwer <aeb@cwi.nl>
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .\" This replaces an earlier man page written by Walter Harms
   6 .\" <walter.harms@informatik.uni-oldenburg.de>.
   7 .\"
   8 .TH TTYSLOT 3 2022-09-09 "Linux man-pages (unreleased)"
   9 .SH NAME
  10 ttyslot \- find the slot of the current user's terminal in some file
  11 .SH LIBRARY
  12 Standard C library
  13 .RI ( libc ", " \-lc )
  14 .SH SYNOPSIS
  15 .nf
  16 .BR "#include <unistd.h>" "       /* See NOTES */"
  17 .PP
  18 .B "int ttyslot(void);"
  19 .fi
  20 .PP
  21 .RS -4
  22 Feature Test Macro Requirements for glibc (see
  23 .BR feature_test_macros (7)):
  24 .RE
  25 .PP
  26 .BR ttyslot ():
  27 .nf
  28     Since glibc 2.24:
  29         _DEFAULT_SOURCE
  30     From glibc 2.20 to 2.23:
  31         _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
  32     Glibc 2.19 and earlier:
  33         _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
  34 .fi
  35 .SH DESCRIPTION
  36 The legacy function
  37 .BR ttyslot ()
  38 returns the index of the current user's entry in some file.
  39 .PP
  40 Now "What file?" you ask.
  41 Well, let's first look at some history.
  42 .SS Ancient history
  43 There used to be a file
  44 .I /etc/ttys
  45 in UNIX\ V6, that was read by the
  46 .BR init (1)
  47 program to find out what to do with each terminal line.
  48 Each line consisted of three characters.
  49 The first character was either \(aq0\(aq or \(aq1\(aq,
  50 where \(aq0\(aq meant "ignore".
  51 The second character denoted the terminal: \(aq8\(aq stood for "/dev/tty8".
  52 The third character was an argument to
  53 .BR getty (8)
  54 indicating the sequence of line speeds to try (\(aq\-\(aq was: start trying
  55 110 baud).
  56 Thus a typical line was "18\-".
  57 A hang on some line was solved by changing the \(aq1\(aq to a \(aq0\(aq,
  58 signaling init, changing back again, and signaling init again.
  59 .PP
  60 In UNIX\ V7 the format was changed: here the second character
  61 was the argument to
  62 .BR getty (8)
  63 indicating the sequence of line speeds to try (\(aq0\(aq was: cycle through
  64 300-1200-150-110 baud; \(aq4\(aq was for the on-line console DECwriter)
  65 while the rest of the line contained the name of the tty.
  66 Thus a typical line was "14console".
  67 .PP
  68 Later systems have more elaborate syntax.
  69 System V-like systems have
  70 .I /etc/inittab
  71 instead.
  72 .SS Ancient history (2)
  73 On the other hand, there is the file
  74 .I /etc/utmp
  75 listing the people currently logged in.
  76 It is maintained by
  77 .BR login (1).
  78 It has a fixed size, and the appropriate index in the file was
  79 determined by
  80 .BR login (1)
  81 using the
  82 .BR ttyslot ()
  83 call to find the number of the line in
  84 .I /etc/ttys
  85 (counting from 1).
  86 .SS The semantics of ttyslot
  87 Thus, the function
  88 .BR ttyslot ()
  89 returns the index of the controlling terminal of the calling process
  90 in the file
  91 .IR /etc/ttys ,
  92 and that is (usually) the same as the index of the entry for the
  93 current user in the file
  94 .IR /etc/utmp .
  95 BSD still has the
  96 .I /etc/ttys
  97 file, but System V-like systems do not, and hence cannot refer to it.
  98 Thus, on such systems the documentation says that
  99 .BR ttyslot ()
 100 returns the current user's index in the user accounting data base.
 101 .SH RETURN VALUE
 102 If successful, this function returns the slot number.
 103 On error (e.g., if none of the file descriptors 0, 1, or 2 is
 104 associated with a terminal that occurs in this data base)
 105 it returns 0 on UNIX\ V6 and V7 and BSD-like systems,
 106 but \-1 on System V-like systems.
 107 .SH ATTRIBUTES
 108 For an explanation of the terms used in this section, see
 109 .BR attributes (7).
 110 .ad l
 111 .nh
 112 .TS
 113 allbox;
 114 lbx lb lb
 115 l l l.
 116 Interface       Attribute       Value
 117 T{
 118 .BR ttyslot ()
 119 T}      Thread safety   MT-Unsafe
 120 .TE
 121 .hy
 122 .ad
 123 .sp 1
 124 .SH STANDARDS
 125 SUSv1; marked as LEGACY in SUSv2; removed in POSIX.1-2001.
 126 SUSv2 requires \-1 on error.
 127 .SH NOTES
 128 The utmp file is found in various places on various systems, such as
 129 .IR /etc/utmp ,
 130 .IR /var/adm/utmp ,
 131 .IR /var/run/utmp .
 132 .PP
 133 The glibc2 implementation of this function reads the file
 134 .BR _PATH_TTYS ,
 135 defined in
 136 .I <ttyent.h>
 137 as "/etc/ttys".
 138 It returns 0 on error.
 139 Since Linux systems do not usually have "/etc/ttys", it will
 140 always return 0.
 141 .PP
 142 On BSD-like systems and Linux, the declaration of
 143 .BR ttyslot ()
 144 is provided by
 145 .IR <unistd.h> .
 146 On System V-like systems, the declaration is provided by
 147 .IR <stdlib.h> .
 148 Since glibc 2.24,
 149 .I <stdlib.h>
 150 also provides the declaration with the following
 151 feature test macro definitions:
 152 .PP
 153 .in +4n
 154 .EX
 155 (_XOPEN_SOURCE >= 500 ||
 156         (_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED))
 157     && ! (_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)
 158 .EE
 159 .in
 160 .PP
 161 Minix also has
 162 .IR fttyslot ( fd ).
 163 .\" .SH HISTORY
 164 .\" .BR ttyslot ()
 165 .\" appeared in UNIX V7.
 166 .SH SEE ALSO
 167 .BR getttyent (3),
 168 .BR ttyname (3),
 169 .BR utmp (5)