wtmp(5)

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 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
   file.

   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
                                            marked as DEAD_PROCESS; not
                                            used by Linux init (1 */
           /* 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 (getsid(2)),
                                            used for windowing */
           struct {
               int32_t tv_sec;           /* Seconds */
               int32_t tv_usec;          /* Microseconds */
           } ut_tv;                      /* Time entry was made */
       #else
            long   ut_session;           /* Session ID */
            struct timeval ut_tv;        /* Time entry was made */
       #endif

           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
       #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 a null byte ('\0') if
   they are shorter than the size of the field.

   The  first  entries  ever  created  result  from   init(1)   processing
   inittab(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
   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(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
   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  username  indicates  a  logout  on  the
   associated  terminal.   Furthermore,  the terminal name ~ with username
   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(8)  (e.g.,  mingetty(8)  or  agetty(8)).   None of these programs
   creates the file, so if it is removed, record-keeping is turned off.

FILES

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

CONFORMING 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
   leading  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
   programs  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.

NOTES

   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
   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;

SEE ALSO

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

COLOPHON

   This  page  is  part of release 4.09 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
   https://www.kernel.org/doc/man-pages/.

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