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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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ttyslot(3) Library Functions Manual ttyslot(3)
ttyslot - find the slot of the current user's terminal in some
file
Standard C library (libc, -lc)
#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 glibc 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 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.
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).
┌────────────────────────────────────┬───────────────┬───────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────┼───────────────┼───────────┤
│ ttyslot() │ Thread safety │ MT-Unsafe │
└────────────────────────────────────┴───────────────┴───────────┘
The utmp file is found in various places on various systems, such
as /etc/utmp, /var/adm/utmp, /var/run/utmp.
None.
SUSv1; marked as LEGACY in SUSv2; removed in POSIX.1-2001. SUSv2
requires -1 on error.
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).
getttyent(3), ttyname(3), utmp(5)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 ttyslot(3)
Pages that refer to this page: getttyent(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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