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           #include <utmp.h>


           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(8)) */
               #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(8) */
               #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
                   } 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(8)  processing  init-
           tab(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_PRO-
           CESS 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(8) 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 authen-
           ticated, 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
           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 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




           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 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 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 lead-
           ing  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.


           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
               gettimeofday((struct timeval *) &ut.ut_tv, NULL);


           This man page is based on the libc5 one, things  may  work  differently


           ac(1),  date(1),  last(1),  login(1), utmpdump(1), who(1), getutent(3),
           getutmp(3), login(3), logout(3), logwtmp(3), updwtmp(3), init(8)

    Linux 2013-02-11 UTMP(5)


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