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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 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
       user.

       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 */
           #else
                long int ut_session;       /* Session ID, used for windowing */
                struct timeval ut_tv;      /* Time entry was made */
           #endif

               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
           #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  ’\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.

       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(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
       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  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(8), and some versions of
       mingetty(8) (or agetty(8)).  Neither  of  these  programs  creates  the
       file, so if it is removed, record-keeping is turned off.

       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.
       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;

FILES

       /var/run/utmp
       /var/log/wtmp

CONFORMING TO

       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  leading  to  corrupted  utmp  entries  and  potential
       security holes.  Clearing the above mentioned fields  by  filling  them
       with null bytes is not required by System V 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.

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

       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.

NOTES

       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
       utmp struct 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.)

BUGS

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

SEE ALSO

       ac(1), date(1), last(1),  login(1),  who(1),  getutent(3),  updwtmp(3),
       init(8)

COLOPHON

       This  page  is  part of release 2.77 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.