1UTMP(5) Linux Programmer's Manual UTMP(5)
2
6 utmp, wtmp - login records
7
9 #include <utmp.h>
10
12 The utmp file allows one to discover information about who is currently
13 using the system. There may be more users currently using the system,
14 because not all programs use utmp logging.
15 Warning: utmp must not be writable by the user class "other", because
17 many system programs (foolishly) depend on its integrity. You risk
18 faked system logfiles and modifications of system files if you leave
19 utmp writable to any user other than the owner and group owner of the
20 file.
21 The file is a sequence of utmp structures, declared as follows in
23 <utmp.h> (note that this is only one of several definitions around; de‐
24 tails depend on the version of libc):
25 /* Values for ut_type field, below */
27 #define EMPTY 0 /* Record does not contain valid info
29 (formerly known as UT_UNKNOWN on Linux) */
30 #define RUN_LVL 1 /* Change in system run-level (see
31 init(1)) */
32 #define BOOT_TIME 2 /* Time of system boot (in ut_tv) */
33 #define NEW_TIME 3 /* Time after system clock change
34 (in ut_tv) */
35 #define OLD_TIME 4 /* Time before system clock change
36 (in ut_tv) */
37 #define INIT_PROCESS 5 /* Process spawned by init(1) */
38 #define LOGIN_PROCESS 6 /* Session leader process for user login */
39 #define USER_PROCESS 7 /* Normal process */
40 #define DEAD_PROCESS 8 /* Terminated process */
41 #define ACCOUNTING 9 /* Not implemented */
42 #define UT_LINESIZE 32
44 #define UT_NAMESIZE 32
45 #define UT_HOSTSIZE 256
46 struct exit_status { /* Type for ut_exit, below */
48 short e_termination; /* Process termination status */
49 short e_exit; /* Process exit status */
50 };
51 struct utmp {
53 short ut_type; /* Type of record */
54 pid_t ut_pid; /* PID of login process */
55 char ut_line[UT_LINESIZE]; /* Device name of tty - "/dev/" */
56 char ut_id[4]; /* Terminal name suffix,
57 or inittab(5) ID */
58 char ut_user[UT_NAMESIZE]; /* Username */
59 char ut_host[UT_HOSTSIZE]; /* Hostname for remote login, or
60 kernel version for run-level
61 messages */
62 struct exit_status ut_exit; /* Exit status of a process
63 marked as DEAD_PROCESS; not
64 used by Linux init(1) */
65 /* The ut_session and ut_tv fields must be the same size when
66 compiled 32- and 64-bit. This allows data files and shared
67 memory to be shared between 32- and 64-bit applications. */
68 #if __WORDSIZE == 64 && defined __WORDSIZE_COMPAT32
69 int32_t ut_session; /* Session ID (getsid(2)),
70 used for windowing */
71 struct {
72 int32_t tv_sec; /* Seconds */
73 int32_t tv_usec; /* Microseconds */
74 } ut_tv; /* Time entry was made */
75 #else
76 long ut_session; /* Session ID */
77 struct timeval ut_tv; /* Time entry was made */
78 #endif
79 int32_t ut_addr_v6[4]; /* Internet address of remote
81 host; IPv4 address uses
82 just ut_addr_v6[0] */
83 char __unused[20]; /* Reserved for future use */
84 };
85 /* Backward compatibility hacks */
87 #define ut_name ut_user
88 #ifndef _NO_UT_TIME
89 #define ut_time ut_tv.tv_sec
90 #endif
91 #define ut_xtime ut_tv.tv_sec
92 #define ut_addr ut_addr_v6[0]
93 This structure gives the name of the special file associated with the
95 user's terminal, the user's login name, and the time of login in the
96 form of time(2). String fields are terminated by a null byte ('\0') if
97 they are shorter than the size of the field.
98 The first entries ever created result from init(1) processing init‐
100 tab(5). Before an entry is processed, though, init(1) cleans up utmp
101 by setting ut_type to DEAD_PROCESS, clearing ut_user, ut_host, and
102 ut_time with null bytes for each record which ut_type is not
103 DEAD_PROCESS or RUN_LVL and where no process with PID ut_pid exists.
104 If no empty record with the needed ut_id can be found, init(1) creates
105 a new one. It sets ut_id from the inittab, ut_pid and ut_time to the
106 current values, and ut_type to INIT_PROCESS.
107 mingetty(8) (or agetty(8)) locates the entry by the PID, changes
109 ut_type to LOGIN_PROCESS, changes ut_time, sets ut_line, and waits for
110 connection to be established. login(1), after a user has been authen‐
111 ticated, changes ut_type to USER_PROCESS, changes ut_time, and sets
112 ut_host and ut_addr. Depending on mingetty(8) (or agetty(8)) and lo‐
113 gin(1), records may be located by ut_line instead of the preferable
114 ut_pid.
115 When init(1) finds that a process has exited, it locates its utmp entry
117 by ut_pid, sets ut_type to DEAD_PROCESS, and clears ut_user, ut_host,
118 and ut_time with null bytes.
119 xterm(1) and other terminal emulators directly create a USER_PROCESS
121 record and generate the ut_id by using the string that suffix part of
122 the terminal name (the characters following /dev/[pt]ty). If they find
123 a DEAD_PROCESS for this ID, they recycle it, otherwise they create a
124 new entry. If they can, they will mark it as DEAD_PROCESS on exiting
125 and it is advised that they null ut_line, ut_time, ut_user, and ut_host
126 as well.
127 telnetd(8) sets up a LOGIN_PROCESS entry and leaves the rest to lo‐
129 gin(1) as usual. After the telnet session ends, telnetd(8) cleans up
130 utmp in the described way.
131 The wtmp file records all logins and logouts. Its format is exactly
133 like utmp except that a null username indicates a logout on the associ‐
134 ated terminal. Furthermore, the terminal name ~ with username shutdown
135 or reboot indicates a system shutdown or reboot and the pair of termi‐
136 nal names |/} logs the old/new system time when date(1) changes it.
137 wtmp is maintained by login(1), init(1), and some versions of getty(8)
138 (e.g., mingetty(8) or agetty(8)). None of these programs creates the
139 file, so if it is removed, record-keeping is turned off.
140
142 /var/run/utmp
143 /var/log/wtmp
144
146 POSIX.1 does not specify a utmp structure, but rather one named utmpx,
147 with specifications for the fields ut_type, ut_pid, ut_line, ut_id,
148 ut_user, and ut_tv. POSIX.1 does not specify the lengths of the
149 ut_line and ut_user fields.
150 Linux defines the utmpx structure to be the same as the utmp structure.
152 Comparison with historical systems
154 Linux utmp entries conform neither to v7/BSD nor to System V; they are
155 a mix of the two.
156 v7/BSD has fewer fields; most importantly it lacks ut_type, which
158 causes native v7/BSD-like programs to display (for example) dead or lo‐
159 gin entries. Further, there is no configuration file which allocates
160 slots to sessions. BSD does so because it lacks ut_id fields.
161 In Linux (as in System V), the ut_id field of a record will never
163 change once it has been set, which reserves that slot without needing a
164 configuration file. Clearing ut_id may result in race conditions lead‐
165 ing to corrupted utmp entries and potential security holes. Clearing
166 the abovementioned fields by filling them with null bytes is not re‐
167 quired by System V semantics, but makes it possible to run many pro‐
168 grams which assume BSD semantics and which do not modify utmp. Linux
169 uses the BSD conventions for line contents, as documented above.
170 System V has no ut_host or ut_addr_v6 fields.
172
174 Unlike various other systems, where utmp logging can be disabled by re‐
175 moving the file, utmp must always exist on Linux. If you want to dis‐
176 able who(1), then do not make utmp world readable.
177 The file format is machine-dependent, so it is recommended that it be
179 processed only on the machine architecture where it was created.
180 Note that on biarch platforms, that is, systems which can run both
182 32-bit and 64-bit applications (x86-64, ppc64, s390x, etc.), ut_tv is
183 the same size in 32-bit mode as in 64-bit mode. The same goes for
184 ut_session and ut_time if they are present. This allows data files and
185 shared memory to be shared between 32-bit and 64-bit applications.
186 This is achieved by changing the type of ut_session to int32_t, and
187 that of ut_tv to a struct with two int32_t fields tv_sec and tv_usec.
188 Since ut_tv may not be the same as struct timeval, then instead of the
189 call:
190 gettimeofday((struct timeval *) &ut.ut_tv, NULL);
192 the following method of setting this field is recommended:
194 struct utmp ut;
196 struct timeval tv;
197 gettimeofday(&tv, NULL);
199 ut.ut_tv.tv_sec = tv.tv_sec;
200 ut.ut_tv.tv_usec = tv.tv_usec;
201
203 ac(1), date(1), init(1), last(1), login(1), logname(1), lslogins(1),
204 users(1), utmpdump(1), who(1), getutent(3), getutmp(3), login(3), lo‐
205 gout(3), logwtmp(3), updwtmp(3)
206
208 This page is part of release 5.12 of the Linux man-pages project. A
209 description of the project, information about reporting bugs, and the
210 latest version of this page, can be found at
211 https://www.kernel.org/doc/man-pages/.
212Linux 2021-03-22 UTMP(5)