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Linux 2.6 - man page for wtmp (linux section 5)

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UTMP(5) 			    Linux Programmer's Manual				  UTMP(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 log-
       ging.

       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(8) */
	       /* 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 termi-
       nated by a null byte ('\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(8) 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 string that suffix part of the terminal name (the  characters  fol-
       lowing  /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 main-
       tained by  login(1),  init(8),  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 con-
       figuration 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 Sys-
       tem 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;

       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.

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), utmpdump(1), who(1), getutent(3), getutmp(3), login(3),
       logout(3), logwtmp(3), updwtmp(3), init(8)

COLOPHON
       This page is part of release 3.55 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/.

Linux					    2013-02-11					  UTMP(5)
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