TTYSLOT(3) Linux Programmer's Manual TTYSLOT(3)
ttyslot - find the slot of the current user's terminal in some file
#include <unistd.h> /* on BSD-like systems, and Linux */
#include <stdlib.h> /* on System V-like systems */
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_XOPEN_SOURCE && _XOPEN_SOURCE_ < 500 && _XOPEN_SOURCE_EXTENDED
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.
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 argu-
ment 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 con-
sole 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
Ancient history (2)
On the other hand, there is the file /etc/utmp listing the people cur-
rently 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 (count-
ing from 1).
The semantics of ttyslot
Thus, the function ttyslot() returns the index of the controlling ter-
minal of the calling process in the file /etc/ttys, and that is (usu-
ally) 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
|Interface | Attribute | Value |
|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 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.
Minix also has fttyslot(fd).
getttyent(3), ttyname(3), utmp(5)
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