man3/ttyslot.3

.\" Copyright (C) 2002 Andries Brouwer <aeb@cwi.nl>
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.\" This replaces an earlier man page written by Walter Harms
.\" <walter.harms@informatik.uni-oldenburg.de>.
.\"
.TH TTYSLOT 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
ttyslot \- find the slot of the current user's terminal in some file
.SH SYNOPSIS
.nf
.BR "#include <unistd.h>" "       /See NOTES */"
.PP
.B "int ttyslot(void);"
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR ttyslot ():
.nf
    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)
.fi
.SH DESCRIPTION
The legacy function
.BR ttyslot ()
returns the index of the current user's entry in some file.
.PP
Now "What file?" you ask.
Well, let's first look at some history.
.SS Ancient history
There used to be a file
.I /etc/ttys
in UNIX\ V6, that was read by the
.BR init (1)
program to find out what to do with each terminal line.
Each line consisted of three characters.
The first character was either \(aq0\(aq or \(aq1\(aq,
where \(aq0\(aq meant "ignore".
The second character denoted the terminal: \(aq8\(aq stood for "/dev/tty8".
The third character was an argument to
.BR getty (8)
indicating the sequence of line speeds to try (\(aq\-\(aq was: start trying
110 baud).
Thus a typical line was "18\-".
A hang on some line was solved by changing the \(aq1\(aq to a \(aq0\(aq,
signaling init, changing back again, and signaling init again.
.PP
In UNIX\ V7 the format was changed: here the second character
was the argument to
.BR getty (8)
indicating the sequence of line speeds to try (\(aq0\(aq was: cycle through
300-1200-150-110 baud; \(aq4\(aq 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".
.PP
Later systems have more elaborate syntax.
System V-like systems have
.I /etc/inittab
instead.
.SS Ancient history (2)
On the other hand, there is the file
.I /etc/utmp
listing the people currently logged in.
It is maintained by
.BR login (1).
It has a fixed size, and the appropriate index in the file was
determined by
.BR login (1)
using the
.BR ttyslot ()
call to find the number of the line in
.I /etc/ttys
(counting from 1).
.SS The semantics of ttyslot
Thus, the function
.BR ttyslot ()
returns the index of the controlling terminal of the calling process
in the file
.IR /etc/ttys ,
and that is (usually) the same as the index of the entry for the
current user in the file
.IR /etc/utmp .
BSD still has the
.I /etc/ttys
file, but System V-like systems do not, and hence cannot refer to it.
Thus, on such systems the documentation says that
.BR ttyslot ()
returns the current user's index in the user accounting data base.
.SH 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.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR ttyslot ()
T}	Thread safety	MT-Unsafe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
SUSv1; marked as LEGACY in SUSv2; removed in POSIX.1-2001.
SUSv2 requires \-1 on error.
.SH NOTES
The utmp file is found in various places on various systems, such as
.IR /etc/utmp ,
.IR /var/adm/utmp ,
.IR /var/run/utmp .
.PP
The glibc2 implementation of this function reads the file
.BR _PATH_TTYS ,
defined in
.I <ttyent.h>
as "/etc/ttys".
It returns 0 on error.
Since Linux systems do not usually have "/etc/ttys", it will
always return 0.
.PP
On BSD-like systems and Linux, the declaration of
.BR ttyslot ()
is provided by
.IR <unistd.h> .
On System V-like systems, the declaration is provided by
.IR <stdlib.h> .
Since glibc 2.24,
.I <stdlib.h>
also provides the declaration with the following
feature test macro definitions:
.PP
.in +4n
.EX
(_XOPEN_SOURCE >= 500 ||
        (_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED))
    && ! (_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)
.EE
.in
.PP
Minix also has
.IR fttyslot ( fd ).
.\" .SH HISTORY
.\" .BR ttyslot ()
.\" appeared in UNIX V7.
.SH SEE ALSO
.BR getttyent (3),
.BR ttyname (3),
.BR utmp (5)