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UTMP(5) Linux Programmer's Manual UTMP(5)


     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 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
     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 init-
     tab(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  authen-
     ticated,  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
     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 associ-
     ated terminal.  Furthermore, the terminal name ~ with username shutdown
     or  reboot indicates a system shutdown or reboot and the pair of termi-
     nal 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.




     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 lead-
     ing 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  pro-
     grams  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.


     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
         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;


     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)


     This page is part of release 4.16 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

Linux 2017-09-15 UTMP(5)

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