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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(1)) */
           #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(1) */
           #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 e_termination;          /* Process termination status */
               short 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  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(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 cre‐
       ate 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 ter‐
       minal  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  re‐
       sult  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 read‐
       able.

       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 applica‐
       tions (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 5.10 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/.



Linux                                        2020-12-21                                      UTMP(5)