# phpman > man > utmp(5) [UTMP(5)](https://www.chedong.com/phpMan.php/man/UTMP/5/markdown) Linux Programmer's Manual [UTMP(5)](https://www.chedong.com/phpMan.php/man/UTMP/5/markdown) ## NAME utmp, wtmp - login records ## SYNOPSIS ### #include ## 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 __ (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)](https://www.chedong.com/phpMan.php/man/init/1/markdown)) */ #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)](https://www.chedong.com/phpMan.php/man/init/1/markdown) */ #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)](https://www.chedong.com/phpMan.php/man/inittab/5/markdown) 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)](https://www.chedong.com/phpMan.php/man/init/1/markdown) */ /* 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)](https://www.chedong.com/phpMan.php/man/getsid/2/markdown)), 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)](https://www.chedong.com/phpMan.php/man/time/2/markdown). 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)](https://www.chedong.com/phpMan.php/man/init/1/markdown) processing [**inittab**(5)](https://www.chedong.com/phpMan.php/man/inittab/5/markdown). Before an entry is processed, though, [**init**(1)](https://www.chedong.com/phpMan.php/man/init/1/markdown) 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)](https://www.chedong.com/phpMan.php/man/init/1/markdown) 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)](https://www.chedong.com/phpMan.php/man/mingetty/8/markdown) (or [**agetty**(8)](https://www.chedong.com/phpMan.php/man/agetty/8/markdown)) 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)](https://www.chedong.com/phpMan.php/man/login/1/markdown), 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)](https://www.chedong.com/phpMan.php/man/mingetty/8/markdown) (or [**agetty**(8)](https://www.chedong.com/phpMan.php/man/agetty/8/markdown)) and [**login**(1)](https://www.chedong.com/phpMan.php/man/login/1/markdown), records may be located by _ut_line_ instead of the preferable _ut_pid_. When [**init**(1)](https://www.chedong.com/phpMan.php/man/init/1/markdown) 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)](https://www.chedong.com/phpMan.php/man/xterm/1/markdown) 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)](https://www.chedong.com/phpMan.php/man/telnetd/8/markdown) sets up a **LOGIN**___**PROCESS** entry and leaves the rest to [**login**(1)](https://www.chedong.com/phpMan.php/man/login/1/markdown) as usual. After the telnet session ends, [**telnetd**(8)](https://www.chedong.com/phpMan.php/man/telnetd/8/markdown) 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)](https://www.chedong.com/phpMan.php/man/date/1/markdown) changes it. _wtmp_ is maintained by [**login**(1)](https://www.chedong.com/phpMan.php/man/login/1/markdown), [**init**(1)](https://www.chedong.com/phpMan.php/man/init/1/markdown), and some versions of [**getty**(8)](https://www.chedong.com/phpMan.php/man/getty/8/markdown) (e.g., [**mingetty**(8)](https://www.chedong.com/phpMan.php/man/mingetty/8/markdown) or [**agetty**(8)](https://www.chedong.com/phpMan.php/man/agetty/8/markdown)). 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)](https://www.chedong.com/phpMan.php/man/who/1/markdown), 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)](https://www.chedong.com/phpMan.php/man/ac/1/markdown), [**date**(1)](https://www.chedong.com/phpMan.php/man/date/1/markdown), [**init**(1)](https://www.chedong.com/phpMan.php/man/init/1/markdown), [**last**(1)](https://www.chedong.com/phpMan.php/man/last/1/markdown), [**login**(1)](https://www.chedong.com/phpMan.php/man/login/1/markdown), [**logname**(1)](https://www.chedong.com/phpMan.php/man/logname/1/markdown), [**lslogins**(1)](https://www.chedong.com/phpMan.php/man/lslogins/1/markdown), [**users**(1)](https://www.chedong.com/phpMan.php/man/users/1/markdown), [**utmpdump**(1)](https://www.chedong.com/phpMan.php/man/utmpdump/1/markdown), [**who**(1)](https://www.chedong.com/phpMan.php/man/who/1/markdown), [**getutent**(3)](https://www.chedong.com/phpMan.php/man/getutent/3/markdown), [**getutmp**(3)](https://www.chedong.com/phpMan.php/man/getutmp/3/markdown), [**login**(3)](https://www.chedong.com/phpMan.php/man/login/3/markdown), [**logout**(3)](https://www.chedong.com/phpMan.php/man/logout/3/markdown), [**logwtmp**(3)](https://www.chedong.com/phpMan.php/man/logwtmp/3/markdown), [**updwtmp**(3)](https://www.chedong.com/phpMan.php/man/updwtmp/3/markdown) ## 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 . Linux 2020-12-21 [UTMP(5)](https://www.chedong.com/phpMan.php/man/UTMP/5/markdown)