utmp(5) — Linux manual page


utmp(5)                    File Formats Manual                   utmp(5)

NAME         top

       utmp, wtmp - login records

SYNOPSIS         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)) */
           #define BOOT_TIME     2 /* Time of system boot (in ut_tv) */
           #define NEW_TIME      3 /* Time after system clock change
                                      (in ut_tv) */
           #define OLD_TIME      4 /* Time before system clock change
                                      (in ut_tv) */
           #define INIT_PROCESS  5 /* Process spawned by init(1) */
           #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) */
               /* 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)),
                                                used for windowing */
               struct {
                   int32_t tv_sec;           /* Seconds */
                   int32_t tv_usec;          /* Microseconds */
               } ut_tv;                      /* Time entry was made */
                long   ut_session;           /* Session ID */
                struct timeval ut_tv;        /* Time entry was made */

               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
           #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 a null
       byte ('\0') if they are shorter than the size of the field.

       The first entries ever created result from init(1) processing
       inittab(5).  Before an entry is processed, though, init(1) 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(1) creates a new one.  It sets ut_id from the
       inittab, ut_pid and ut_time to the current values, and ut_type to

       mingetty(8) (or agetty(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(1), after a user
       has been authenticated, changes ut_type to USER_PROCESS, changes
       ut_time, and sets ut_host and ut_addr.  Depending on mingetty(8)
       (or agetty(8)) and login(1), records may be located by ut_line
       instead of the preferable ut_pid.

       When init(1) 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 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 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(1) 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) changes it.  wtmp is maintained by login(1),
       init(1), and some versions of getty(8) (e.g., mingetty(8) or
       agetty(8)).  None of these programs creates the file, so if it is
       removed, record-keeping is turned off.

FILES         top


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 ut_type, ut_pid, ut_line, ut_id, ut_user, and ut_tv.
       POSIX.1 does not specify the lengths of the ut_line and ut_user

       Linux defines the utmpx structure to be the same as the utmp

STANDARDS         top


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 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

       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 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 ut_host or ut_addr_v6 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), 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

       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.  This is achieved by changing the type
       of ut_session to int32_t, and that of ut_tv to a struct with two
       int32_t fields tv_sec and tv_usec.  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;

SEE ALSO         top

       ac(1), date(1), init(1), last(1), login(1), logname(1),
       lslogins(1), users(1), utmpdump(1), who(1), getutent(3),
       getutmp(3), login(3), logout(3), logwtmp(3), updwtmp(3)

COLOPHON         top

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       for this manual page, see
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
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       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
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Linux man-pages 6.9.1          2024-06-15                        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)