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       utmp, wtmp - login records


       #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,  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

       The file is a sequence of entries with the following structure declared
       in  the include file (note that this is only one of several definitions
       around; details depend on the version of libc):

          #define UT_UNKNOWN      0
          #define RUN_LVL         1
          #define BOOT_TIME       2
          #define NEW_TIME        3
          #define OLD_TIME        4
          #define INIT_PROCESS    5
          #define LOGIN_PROCESS   6
          #define USER_PROCESS    7
          #define DEAD_PROCESS    8
          #define ACCOUNTING      9

          #define UT_LINESIZE     12
          #define UT_NAMESIZE     32
          #define UT_HOSTSIZE     256

          struct exit_status {
            short int e_termination;    /* process termination status */
            short int e_exit;           /* process exit status */

          struct utmp {
            short ut_type;              /* type of login */
            pid_t ut_pid;               /* PID of login process */
            char ut_line[UT_LINESIZE];  /* device name of tty - "/dev/" */
            char ut_id[4];              /* init id or abbrev. ttyname */
            char ut_user[UT_NAMESIZE];  /* user name */
            char ut_host[UT_HOSTSIZE];  /* hostname for remote login */
            struct exit_status ut_exit; /* The exit status of a process
                                           marked as DEAD_PROCESS */

            /* 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, used for windowing */
            struct {
              int32_t tv_sec;           /* Seconds */
              int32_t tv_usec;          /* Microseconds */
            } ut_tv;                    /* Time entry was made */
             long int ut_session;        /* Session ID, used for windowing */
             struct timeval ut_tv;       /* Time entry was made */

            int32_t ut_addr_v6[4];       /* IP address of remote host */
            char __unused[20];           /* Reserved for future use */

          /* Backwards 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  ’\0’  if  they  are
       shorter than the size of the field.

       The   first   entries  ever  created  result  from  init(8)  processing
       inittab(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_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  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.

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

       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  last  two  letters  of
       /dev/ttyp%c   or  by  using  p%d  for  /dev/pts/%d.   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

       xdm(8)  should  not  create a utmp record, because there is no assigned
       terminal.  Letting it  create  one  will  result  in  errors,  such  as
       ’finger: cannot stat /dev/machine.dom’.  It should create wtmp entries,
       though, just like ftpd(8) does.

       telnetd(8) sets up  a  LOGIN_PROCESS  entry  and  leaves  the  rest  to
       login(8) 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  user  name indicates a logout on the
       associated terminal.  Furthermore, the terminal name ~ with  user  name
       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(1).  Neither of these programs creates the  file,  so  if  it  is
       removed, record-keeping is turned off.

       Note  that  on biarch platforms, i.e. 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.  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;




       Linux utmp entries conform neither to v7/BSD nor to SYSV;  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 SYSV), 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 above mentioned fields by filling them with null
       bytes is not required by SYSV semantics, but  it  allows  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.

       SYSV  only  uses  the  type  field  to  mark  them and logs informative
       messages such as e.g. "new time" in the line field. UT_UNKNOWN seems to
       be a Linux invention.  SYSV 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.

       Note  that  the utmp struct from libc5 has changed in libc6. Because of
       this, binaries using the old libc5 struct  will  corrupt  /var/run/utmp
       and/or  /var/log/wtmp.   Debian  systems  include a patched libc5 which
       uses the new utmp format.  The problem still  exists  with  wtmp  since
       it’s accessed directly in libc5.


       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  platforms  which  can  run  both  32-bit   and   64-bit
       applications (x86-64, ppc64, s390x, etc.), the sizes of the fields of a
       struct utmp must be the same in 32-bit mode as in 64-bit mode.  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.   (Thus,
       in  order  to  fill  it, first get the time into a real struct timeval,
       then copy the two fields to ut_tv.)


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


       ac(1),  date(1),  last(1),  login(1),  who(1), getutent(3), updwtmp(3),