UTMP(5)                   (2020-12-21)                    UTMP(5)

     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 cur-
          rently 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 modifica-
          tions 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 fol-
          lows in <utmp.h> (note that this is only one of several def-
          initions 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 */

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                  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 */
              #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).  String fields are
          terminated by a null byte (aq\0aq) if they are shorter than
          the size of the field.

          The first entries ever created result from init(1) process-
          ing 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

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          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 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 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 termi-
          nal name ti with username shutdown or reboot indicates a sys-
          tem 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
          /var/run/utmp
          /var/log/wtmp

     CONFORMING TO
          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

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          structure.

        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 con-
          figuration 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 abovementioned
          fields by filling them with null bytes is not required by
          System V semantics, but makes it possible to run many pro-
          grams 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
          Unlike various other systems, where utmp logging can be dis-
          abled 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;

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              gettimeofday(&tv, NULL);
              ut.ut_tv.tv_sec = tv.tv_sec;
              ut.ut_tv.tv_usec = tv.tv_usec;

     SEE ALSO
          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
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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