utmp(5) — Linux manual page


UTMP(5)                   Linux Programmer's 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

       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 int e_termination;      /* Process termination status */
               short int 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) cre‐
       ates 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.

       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 cre‐
       ate 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 asso‐
       ciated 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 ver‐
       sions 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


CONFORMING TO         top

       POSIX.1 does not specify a utmp structure, but rather one named
       utmpx, 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 fields.

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

   Comparison with historical systems
       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

       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 created.

       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

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-08-13                          UTMP(5)

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