#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
                                                kernel version for run-level
                                                messages */
               struct  exit_status ut_exit;  /* Exit status of a process

                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  init-
       tab(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 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(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


       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);

       the following method of setting this field is recommended:

       This page is part of release 4.04 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                             2014-08-19                           UTMP(5)
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