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utmp(5) - utmp, wtmp, utmp, wtmp - login records - man 5 utmp

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

       utmp, wtmp - login(1,3,5) records

       #include <utmp.h>

       The utmp file(1,n) allows one to discover(1,3,5) information about who is currently
       using the system.  There may be more users(1,5) currently using the  system,
       because not all programs use utmp logging.

       Warning: utmp must not be writable, because many system programs (fool-
       ishly) depend on its integrity.  You risk  faked  system  logfiles  and
       modifications of system files if(3,n) you leave utmp writable to any user.

       The file(1,n) is a sequence of entries with the following structure declared
       in(1,8) the include file(1,n) (note that this is only one of several  definitions
       around; details depend on the version(1,3,5) 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(3,n,1 builtins) status.  */

          struct utmp {
            short ut_type;              /* type of login(1,3,5) */
            pid_t ut_pid;               /* pid of login(1,3,5) process */
            char ut_line[UT_LINESIZE];  /* device name of tty(1,4) - "/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(1,3,5) */
            struct exit_status ut_exit; /* The exit(3,n,1 builtins) status of a process
                                           marked as DEAD_PROCESS. */
            long ut_session;            /* session ID, used for windowing*/
            struct timeval ut_tv;       /* time(1,2,n) entry was made.  */
            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
          #define ut_xtime ut_tv.tv_sec
          #define ut_addr ut_addr_v6[0]

       This  structure  gives the name of the special file(1,n) associated with the
       user's terminal, the user's login(1,3,5) name, and the time(1,2,n) of  login(1,3,5)  in(1,8)  the
       form  of  time(1,2,n)(2).   String  fields  are terminated by '\0' if(3,n) they are
       shorter than the size of the field.

       The first entries ever created result  from  init(8)  processing  init-
       tab(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(1,3,5)(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(1,3,5)(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

       xdm(8) should not create a utmp record, because there  is  no  assigned
       terminal.   Letting  it create one will result in(1,8) errors, such as 'fin-
       ger: cannot stat(1,2) /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(1,3,5)(8) as usual.  After the telnet session ends, telnetd(8) cleans up
       utmp in(1,8) the described way.

       The  wtmp  file(1,n)  records all logins and logouts.  Its format is exactly
       like utmp except that a null user name indicates a logout on the  asso-
       ciated terminal.  Furthermore, the terminal name ~ with user name shut-
       down or reboot indicates a system shutdown(2,8) or reboot and  the  pair  of
       terminal  names  |/}  logs the old/new system time(1,2,n) when date(1) changes
       it.  wtmp is maintained by login(1,3,5)(1),  init(1),  and  some  versions  of
       getty(1).   Neither  of  these  programs  creates the file(1,n), so if(3,n) 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 exam-
       ple)  dead  or  login(1,3,5) entries.  Further, there is no configuration file(1,n)
       which allocates slots to sessions.  BSD does so because it lacks  ut_id
       fields.   In Linux (as in(1,8) SYSV), the ut_id field of a record will never
       change once it has been set(7,n,1 builtins), which reserves that slot without needing a
       configuration file.  Clearing ut_id may result in(1,8) race conditions lead-
       ing to corrupted utmp entries and and potential security holes.  Clear-
       ing  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  informative  mes-
       sages such as e.g. "new time(1,2,n)" in(1,8) 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  disabled  by
       removing  the  file(1,n),  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(1,8) 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(1,8) libc5.

       The file(1,n) 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 applica-
       tions (x86-64, ppc64, s390x, etc.), the sizes of the fields of a struct
       utmp  must  be  the  same  in(1,8)  32-bit  mode as in(1,8) 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(1,8) order to fill it, first get the time(1,2,n) into  a  real  struct  timeval,
       then copy the two fields to ut_tv.)

       This  manpage  is  based  on the libc5 one, things may work differently

       ac(1), date(1), last(1),  login(1,3,5)(1),  who(1),  getutent(3),  updwtmp(3),

File formats                      2004-10-31                           UTMP(5)

References for this manual (incoming links)