ttyslot — find the slot of the current user's terminal in some file
#include <unistd.h> /* on BSD-like systems */ #include <stdlib.h> /* on System V-like systems */
int
ttyslot( |
void) ; |
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(8) 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',
signalling init, changing back again, and signalling 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(8). It has a fixed
size, and the appropriate index in the file was determined
by login(8) 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 current 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.
SUSv1; marked as LECACY 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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