UTMP
NAME
utmp, wtmp - login records
SYNOPSIS
#include <utmp.h>
DESCRIPTION
The
utmp
file allows one to discover information about who is currently using the
system.
There may be more users currently using the system, because not
all programs use utmp logging.
Warning:
utmp
must not be writable, because many system programs (foolishly)
depend on its integrity.
You risk faked system logfiles and
modifications of system files if you leave
utmp
writable to any user.
The file is a sequence of entries with the following structure declared
in the include file (note that this is only one of several definitions
around; details depend on the version of libc):
#define UT_UNKNOWN 0
#define RUN_LVL 1
#define BOOT_TIME 2
#define NEW_TIME 3
#define OLD_TIME 4
#define INIT_PROCESS 5
#define LOGIN_PROCESS 6
#define USER_PROCESS 7
#define DEAD_PROCESS 8
#define ACCOUNTING 9
#define UT_LINESIZE 12
#define UT_NAMESIZE 32
#define UT_HOSTSIZE 256
struct exit_status {
short int e_termination; /* process termination status */
short int e_exit; /* process exit status */
};
struct utmp {
short ut_type; /* type of login */
pid_t ut_pid; /* PID of login process */
char ut_line[UT_LINESIZE]; /* device name of tty - "/dev/" */
char ut_id[4]; /* init id or abbrev. ttyname */
char ut_user[UT_NAMESIZE]; /* user name */
char ut_host[UT_HOSTSIZE]; /* hostname for remote login */
struct exit_status ut_exit; /* The exit status of a process
marked as DEAD_PROCESS */
/* The ut_session and ut_tv fields must be the same size when
compiled 32- and 64-bit. This allows data files and shared
memory to be shared between 32- and 64-bit applications */
#if __WORDSIZE == 64 && defined __WORDSIZE_COMPAT32
int32_t ut_session; /* Session ID, used for windowing */
struct {
int32_t tv_sec; /* Seconds */
int32_t tv_usec; /* Microseconds */
} ut_tv; /* Time entry was made */
#else
long int ut_session; /* Session ID, used for windowing */
struct timeval ut_tv; /* Time entry was made */
#endif
int32_t ut_addr_v6[4]; /* IP address of remote host */
char __unused[20]; /* Reserved for future use */
};
/* Backwards compatibility hacks. */
#define ut_name ut_user
#ifndef _NO_UT_TIME
#define ut_time ut_tv.tv_sec
#endif
#define ut_xtime ut_tv.tv_sec
#define ut_addr ut_addr_v6[0]
This structure gives the name of the special file associated with the
user's terminal, the user's login name, and the time of login in the form
of
time(2).
String fields are terminated by
'\0' if they are shorter than the size
of the field.
The first entries ever created result from
init(8)
processing
inittab(5).
Before an entry is processed, though,
init(8)
cleans up utmp by setting
ut_type to
DEAD_PROCESS, clearing
ut_user,
ut_host, and
ut_time with null bytes for each
record which
ut_type is not
DEAD_PROCESS or
RUN_LVL
and where no process with PID
ut_pid exists.
If no empty record
with the needed
ut_id can be found, init creates a new one.
It sets
ut_id from the inittab,
ut_pid and
ut_time to the
current values, and
ut_type to
INIT_PROCESS.
getty(8)
locates the entry by the PID, changes
ut_type to
LOGIN_PROCESS, changes
ut_time, sets
ut_line, and waits
for connection to be established.
login(8),
after a user has been
authenticated, changes
ut_type to
USER_PROCESS, changes
ut_time, and sets
ut_host and
ut_addr.
Depending on
getty(8)
and
login(8),
records may be located by
ut_line instead of the preferable
ut_pid.
When
init(8)
finds that a process has exited, it locates its utmp
entry by
ut_pid, sets
ut_type to
DEAD_PROCESS, and
clears
ut_user,
ut_host and
ut_time with null bytes.
xterm(1)
and other terminal emulators directly create a
USER_PROCESS record and generate the
ut_id by using the last
two letters of
/dev/ttyp%c or by using
p%d for
/dev/pts/%d.
If they find a
DEAD_PROCESS for this ID,
they recycle it, otherwise they create a new entry.
If they can, they
will mark it as
DEAD_PROCESS on exiting and it is advised that
they null
ut_line,
ut_time,
ut_user, and
ut_host
as well.
telnetd(8)
sets up a
LOGIN_PROCESS entry and leaves the rest to
login(8)
as usual.
After the telnet session ends,
telnetd(8)
cleans up utmp in the described way.
The
wtmp file records all logins and logouts.
Its format is exactly like
utmp except that a null user name
indicates a logout
on the associated terminal.
Furthermore, the terminal name
~
with user name
shutdown or
reboot indicates a system
shutdown or reboot and the pair of terminal names
|/
}
logs the old/new system time when
date(1)
changes it.
wtmp is maintained by
login(1),
init(1),
and some versions of
getty(8).
Neither of these programs creates the file, so if it is
removed, record-keeping is turned off.
Note that on
biarch platforms, that is, systems which can run both
32-bit and 64-bit applications (x86-64, ppc64, s390x, etc.),
ut_tv is the same size in 32-bit mode as in 64-bit mode.
The same goes for
ut_session and
ut_time if they are present.
This allows data files and shared memory to be shared between
32-bit and 64-bit applications.
Since
ut_tv may not be the same as
struct timeval,
then instead of the call:
gettimeofday((struct timeval *) &ut.ut_tv, NULL);
the following method of setting this field is recommended:
struct utmp ut;
struct timeval tv;
gettimeofday(&tv, NULL);
ut.ut_tv.tv_sec = tv.tv_sec;
ut.ut_tv.tv_usec = tv.tv_usec;
FILES
/var/run/utmp
/var/log/wtmp
CONFORMING TO
Linux utmp entries conform neither to v7/BSD nor to System V; they are a
mix of the two. v7/BSD has fewer fields; most importantly it lacks
ut_type, which causes native v7/BSD-like programs to display (for
example) dead or login entries.
Further, there is no configuration file
which allocates slots to sessions.
BSD does so because it lacks ut_id fields.
In Linux (as in System V), the ut_id field of a
record will never change once it has been set, which reserves that slot
without needing a configuration file.
Clearing ut_id may result
in race conditions leading to corrupted utmp entries and potential
security holes.
Clearing the above mentioned fields by filling them
with null bytes is not required by System V semantics, but it allows to run
many programs which assume BSD semantics and which do not modify utmp.
Linux uses the BSD conventions for line contents, as documented above.
System V only uses the type field to mark them and logs informative messages
such as "new time" in the line field.
UT_UNKNOWN seems
to be a Linux invention.
System V has no ut_host or ut_addr_v6 fields.
Unlike various other
systems, where utmp logging can be disabled by removing the file, utmp
must always exist on Linux.
If you want to disable
who(1)
then do not make utmp world readable.
Note that the utmp struct from libc5 has changed in libc6.
Because of this,
binaries using the old libc5 struct will corrupt
R /var/run/utmp and/or /var/log/wtmp .
NOTES
The file format is machine dependent, so it is recommended that it be
processed only on the machine architecture where it was created.
Note that on platforms which can run both 32-bit and 64-bit applications
(x86-64, ppc64, s390x, etc.), the sizes of the fields of a utmp struct
must be the same in 32-bit mode as in 64-bit mode.
This is achieved by changing the type of
ut_session
to
R int32_t ,
and that of
ut_tv
to a struct with two
int32_t
fields
tv_sec
and
R tv_usec .
(Thus, in order to fill it, first get the
time into a real struct timeval,
then copy the two fields to
R ut_tv .)
BUGS
This man page is based on the libc5 one, things may work differently now.
SEE ALSO