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 pro-
     grams  (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  struc-
     ture  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. */
            long 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 pad[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

     The first entries ever created result from init(8)  process-
     ing  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 well.

     xdm(8) should not create an utmp record, because there is no
     assigned  terminal.   Letting  it  create one will result in
     trouble like:  finger: can not  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, tel-
     netd(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  termi-
     nal 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), and  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.




     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 allo-
     cates  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 cor-
     rupted  utmp  entries  and  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 informa-
     tive  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 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

     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 got created.


     This manpage is based on the libc5 one, things may work dif-
     ferently now.


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