# utmp

## 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(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 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_
    


## VERSIONS



           POSIX.1 does not specify a _utmp_ structure, but rather one named
           _utmpx_ (as part of the XSI extension), 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.
    


## STANDARDS



           Linux.
    


## 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;
    


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