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)):
ttyslot():
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
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 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.
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).
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 |
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
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)