utmp(5) - Linux manual page (original) (raw)
utmp(5) File Formats Manual utmp(5)
NAME top
utmp, wtmp - login recordsSYNOPSIS top
**#include <utmp.h>**DESCRIPTION top
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 by the user class "other",
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 other than the owner and
group owner of the file.
The file is a sequence of _utmp_ structures, declared as follows in
_<utmp.h>_ (note that this is only one of several definitions
around; details depend on the version of libc):
/* Values for ut_type field, below */
#define EMPTY 0 /* Record does not contain valid info
(formerly known as UT_UNKNOWN on Linux) */
#define RUN_LVL 1 /* Change in system run-level (see
[init(1)](../man1/init.1.html)) */
#define BOOT_TIME 2 /* Time of system boot (in _uttv_) */
#define NEW_TIME 3 /* Time after system clock change
(in _uttv_) */
#define OLD_TIME 4 /* Time before system clock change
(in _uttv_) */
#define INIT_PROCESS 5 /* Process spawned by [init(1)](../man1/init.1.html) */
#define LOGIN_PROCESS 6 /* Session leader process for user login */
#define USER_PROCESS 7 /* Normal process */
#define DEAD_PROCESS 8 /* Terminated process */
#define ACCOUNTING 9 /* Not implemented */
#define UT_LINESIZE 32
#define UT_NAMESIZE 32
#define UT_HOSTSIZE 256
struct exit_status { /* Type for ut_exit, below */
short e_termination; /* Process termination status */
short e_exit; /* Process exit status */
};
struct utmp {
short ut_type; /* Type of record */
pid_t ut_pid; /* PID of login process */
char ut_line[UT_LINESIZE]; /* Device name of tty - "/dev/" */
char ut_id[4]; /* Terminal name suffix,
or inittab(5) ID */
char ut_user[UT_NAMESIZE]; /* Username */
char ut_host[UT_HOSTSIZE]; /* Hostname for remote login, or
kernel version for run-level
messages */
struct exit_status ut_exit; /* Exit status of a process
marked as DEAD_PROCESS; not
used by Linux [init(1)](../man1/init.1.html) */
/* 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 ([getsid(2)](../man2/getsid.2.html)),
used for windowing */
struct {
int32_t tv_sec; /* Seconds */
int32_t tv_usec; /* Microseconds */
} ut_tv; /* Time entry was made */
#else
long ut_session; /* Session ID */
struct timeval ut_tv; /* Time entry was made */
#endif
int32_t ut_addr_v6[4]; /* Internet address of remote
host; IPv4 address uses
just ut_addr_v6[0] */
char __unused[20]; /* Reserved for future use */
};
/* Backward 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)](../man2/time.2.html). String fields are terminated by a null
byte ('\0') if they are shorter than the size of the field.
The first entries ever created result from [init(1)](../man1/init.1.html) processing
**inittab**(5). Before an entry is processed, though, [init(1)](../man1/init.1.html) cleans
up utmp by setting _uttype_ to **DEAD_PROCESS**, clearing _utuser_,
_uthost_, and _uttime_ with null bytes for each record which _uttype_
is not **DEAD_PROCESS** or **RUN_LVL** and where no process with PID
_utpid_ exists. If no empty record with the needed _utid_ can be
found, [init(1)](../man1/init.1.html) creates a new one. It sets _utid_ from the inittab,
_utpid_ and _uttime_ to the current values, and _uttype_ to
**INIT_PROCESS**.
**mingetty**(8) (or [agetty(8)](../man8/agetty.8.html)) locates the entry by the PID, changes
_uttype_ to **LOGIN_PROCESS**, changes _uttime_, sets _utline_, and waits
for connection to be established. [login(1)](../man1/login.1.html), after a user has been
authenticated, changes _uttype_ to **USER_PROCESS**, changes _uttime_,
and sets _uthost_ and _utaddr_. Depending on **mingetty**(8) (or
[agetty(8)](../man8/agetty.8.html)) and [login(1)](../man1/login.1.html), records may be located by _utline_ instead
of the preferable _utpid_.
When [init(1)](../man1/init.1.html) finds that a process has exited, it locates its utmp
entry by _utpid_, sets _uttype_ to **DEAD_PROCESS**, and clears _utuser_,
_uthost_, and _uttime_ with null bytes.
**xterm**(1) and other terminal emulators directly create a
**USER_PROCESS** record and generate the _utid_ by using the string
that suffix part of the terminal name (the characters following
_/dev/_[pt]_ty_). 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 _utline_, _uttime_, _utuser_, and _uthost_ as well.
**telnetd**(8) sets up a **LOGIN_PROCESS** entry and leaves the rest to
[login(1)](../man1/login.1.html) 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 username indicates a logout
on the associated terminal. Furthermore, the terminal name **~** with
username **shutdown** or **reboot** indicates a system shutdown or reboot
and the pair of terminal names **|**/**}** logs the old/new system time
when [date(1)](../man1/date.1.html) changes it. _wtmp_ is maintained by [login(1)](../man1/login.1.html), [init(1)](../man1/init.1.html),
and some versions of **getty**(8) (e.g., **mingetty**(8) or [agetty(8)](../man8/agetty.8.html)).
None of these programs creates the file, so if it is removed,
record-keeping is turned off.FILES top
_/var/run/utmp_
_/var/log/wtmp_VERSIONS top
POSIX.1 does not specify a _utmp_ structure, but rather one named
_utmpx_ (as part of the XSI extension), with specifications for the
fields _uttype_, _utpid_, _utline_, _utid_, _utuser_, and _uttv_.
POSIX.1 does not specify the lengths of the _utline_ and _utuser_
fields.
Linux defines the _utmpx_ structure to be the same as the _utmp_
structure.STANDARDS top
Linux.HISTORY top
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 _uttype_, 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 _utid_
fields.
In Linux (as in System V), the _utid_ field of a record will never
change once it has been set, which reserves that slot without
needing a configuration file. Clearing _utid_ may result in race
conditions leading to corrupted utmp entries and potential
security holes. Clearing the abovementioned fields by filling
them with null bytes is not required by System V semantics, but
makes it possible 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 has no _uthost_ or _utaddrv6_ fields.NOTES top
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)](../man1/who.1.html), then do not make utmp world readable.
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 _biarch_ platforms, that is, systems which can run both
32-bit and 64-bit applications (x86-64, ppc64, s390x, etc.), _uttv_
is the same size in 32-bit mode as in 64-bit mode. The same goes
for _utsession_ and _uttime_ if they are present. This allows data
files and shared memory to be shared between 32-bit and 64-bit
applications. This is achieved by changing the type of _utsession_
to _int32t_, and that of _uttv_ to a struct with two _int32t_ fields
_tvsec_ and _tvusec_. Since _uttv_ 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;SEE ALSO top
[ac(1)](../man1/ac.1.html), [date(1)](../man1/date.1.html), [init(1)](../man1/init.1.html), [last(1)](../man1/last.1.html), [login(1)](../man1/login.1.html), [logname(1)](../man1/logname.1.html),
[lslogins(1)](../man1/lslogins.1.html), [users(1)](../man1/users.1.html), [utmpdump(1)](../man1/utmpdump.1.html), [who(1)](../man1/who.1.html), [getutent(3)](../man3/getutent.3.html),
[getutmp(3)](../man3/getutmp.3.html), [login(3)](../man3/login.3.html), [logout(3)](../man3/logout.3.html), [logwtmp(3)](../man3/logwtmp.3.html), [updwtmp(3)](../man3/updwtmp.3.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-09-01 utmp(5)
Pages that refer to this page:ac(1), last(1), last(1@@util-linux), login(1), lslogins(1), pcp-dstat(1), screen(1), uptime(1), utmpdump(1), w(1), wall(1), getlogin(3), getutent(3), getutmp(3), login(3), ttyslot(3), updwtmp(3), org.freedesktop.login1(5), systemd.exec(5), string_copying(7), agetty(8), dump-utmp(8), systemd-update-utmp.service(8)