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, because many system programs (fool‐
17 ishly) depend on its integrity. You risk faked system logfiles and
18 modifications of system files if you leave utmp writable to any user.
19 The file is a sequence of entries with the following structure declared
21 in the include file (note that this is only one of several definitions
22 around; details depend on the version of libc):
23 #define UT_UNKNOWN 0
25 #define RUN_LVL 1
26 #define BOOT_TIME 2
27 #define NEW_TIME 3
28 #define OLD_TIME 4
29 #define INIT_PROCESS 5
30 #define LOGIN_PROCESS 6
31 #define USER_PROCESS 7
32 #define DEAD_PROCESS 8
33 #define ACCOUNTING 9
34 #define UT_LINESIZE 12
36 #define UT_NAMESIZE 32
37 #define UT_HOSTSIZE 256
38 struct exit_status {
40 short int e_termination; /* process termination status */
41 short int e_exit; /* process exit status */
42 };
43 struct utmp {
45 short ut_type; /* type of login */
46 pid_t ut_pid; /* PID of login process */
47 char ut_line[UT_LINESIZE]; /* device name of tty - "/dev/" */
48 char ut_id[4]; /* init id or abbrev. ttyname */
49 char ut_user[UT_NAMESIZE]; /* user name */
50 char ut_host[UT_HOSTSIZE]; /* hostname for remote login */
51 struct exit_status ut_exit; /* The exit status of a process
52 marked as DEAD_PROCESS */
53 /* The ut_session and ut_tv fields must be the same size when
55 compiled 32- and 64-bit. This allows data files and shared
56 memory to be shared between 32- and 64-bit applications */
57 #if __WORDSIZE == 64 && defined __WORDSIZE_COMPAT32
58 int32_t ut_session; /* Session ID, used for windowing */
59 struct {
60 int32_t tv_sec; /* Seconds */
61 int32_t tv_usec; /* Microseconds */
62 } ut_tv; /* Time entry was made */
63 #else
64 long int ut_session; /* Session ID, used for windowing */
65 struct timeval ut_tv; /* Time entry was made */
66 #endif
67 int32_t ut_addr_v6[4]; /* IP address of remote host */
69 char __unused[20]; /* Reserved for future use */
70 };
71 /* Backwards compatibility hacks. */
73 #define ut_name ut_user
74 #ifndef _NO_UT_TIME
75 #define ut_time ut_tv.tv_sec
76 #endif
77 #define ut_xtime ut_tv.tv_sec
78 #define ut_addr ut_addr_v6[0]
79 This structure gives the name of the special file associated with the
81 user's terminal, the user's login name, and the time of login in the
82 form of time(2). String fields are terminated by '\0' if they are
83 shorter than the size of the field.
84 The first entries ever created result from init(8) processing init‐
86 tab(5). Before an entry is processed, though, init(8) cleans up utmp
87 by setting ut_type to DEAD_PROCESS, clearing ut_user, ut_host, and
88 ut_time with null bytes for each record which ut_type is not
89 DEAD_PROCESS or RUN_LVL and where no process with PID ut_pid exists.
90 If no empty record with the needed ut_id can be found, init creates a
91 new one. It sets ut_id from the inittab, ut_pid and ut_time to the
92 current values, and ut_type to INIT_PROCESS.
93 getty(8) locates the entry by the PID, changes ut_type to
95 LOGIN_PROCESS, changes ut_time, sets ut_line, and waits for connection
96 to be established. login(8), after a user has been authenticated,
97 changes ut_type to USER_PROCESS, changes ut_time, and sets ut_host and
98 ut_addr. Depending on getty(8) and login(8), records may be located by
99 ut_line instead of the preferable ut_pid.
100 When init(8) finds that a process has exited, it locates its utmp entry
102 by ut_pid, sets ut_type to DEAD_PROCESS, and clears ut_user, ut_host
103 and ut_time with null bytes.
104 xterm(1) and other terminal emulators directly create a USER_PROCESS
106 record and generate the ut_id by using the last two letters of
107 /dev/ttyp%c or by using p%d for /dev/pts/%d. If they find a
108 DEAD_PROCESS for this ID, they recycle it, otherwise they create a new
109 entry. If they can, they will mark it as DEAD_PROCESS on exiting and
110 it is advised that they null ut_line, ut_time, ut_user, and ut_host as
111 well.
112 xdm(8) should not create a utmp record, because there is no assigned
114 terminal. Letting it create one will result in errors, such as 'fin‐
115 ger: cannot stat /dev/machine.dom'. It should create wtmp entries,
116 though, just like ftpd(8) does.
117 telnetd(8) sets up a LOGIN_PROCESS entry and leaves the rest to
119 login(8) as usual. After the telnet session ends, telnetd(8) cleans up
120 utmp in the described way.
121 The wtmp file records all logins and logouts. Its format is exactly
123 like utmp except that a null user name indicates a logout on the asso‐
124 ciated terminal. Furthermore, the terminal name ~ with user name shut‐
125 down or reboot indicates a system shutdown or reboot and the pair of
126 terminal names |/} logs the old/new system time when date(1) changes
127 it. wtmp is maintained by login(1), init(1), and some versions of
128 getty(8). Neither of these programs creates the file, so if it is
129 removed, record-keeping is turned off.
130 Note that on biarch platforms, i.e. systems which can run both 32-bit
132 and 64-bit applications (x86-64, ppc64, s390x, etc.), ut_tv is the same
133 size in 32-bit mode as in 64-bit mode. The same goes for ut_session
134 and ut_time if they are present. This allows data files and shared
135 memory to be shared between 32-bit and 64-bit applications. Since
136 ut_tv may not be the same as struct timeval, then instead of the call:
137 gettimeofday((struct timeval *) &ut.ut_tv, NULL);
139 the following method of setting this field is recommended:
142 struct utmp ut;
144 struct timeval tv;
145 gettimeofday(&tv, NULL);
147 ut.ut_tv.tv_sec = tv.tv_sec;
148 ut.ut_tv.tv_usec = tv.tv_usec;
149
152 /var/run/utmp
153 /var/log/wtmp
154
156 Linux utmp entries conform neither to v7/BSD nor to System V; they are
157 a mix of the two. v7/BSD has fewer fields; most importantly it lacks
158 ut_type, which causes native v7/BSD-like programs to display (for exam‐
159 ple) dead or login entries. Further, there is no configuration file
160 which allocates slots to sessions. BSD does so because it lacks ut_id
161 fields. In Linux (as in System V), the ut_id field of a record will
162 never change once it has been set, which reserves that slot without
163 needing a configuration file. Clearing ut_id may result in race condi‐
164 tions leading to corrupted utmp entries and potential security holes.
165 Clearing the above mentioned fields by filling them with null bytes is
166 not required by System V semantics, but it allows to run many programs
167 which assume BSD semantics and which do not modify utmp. Linux uses
168 the BSD conventions for line contents, as documented above.
169 System V only uses the type field to mark them and logs informative
171 messages such as e.g. "new time" in the line field. UT_UNKNOWN seems to
172 be a Linux invention. System V has no ut_host or ut_addr_v6 fields.
173 Unlike various other systems, where utmp logging can be disabled by
175 removing the file, utmp must always exist on Linux. If you want to
176 disable who(1) then do not make utmp world readable.
177 Note that the utmp struct from libc5 has changed in libc6. Because of
179 this, binaries using the old libc5 struct will corrupt /var/run/utmp
180 and/or /var/log/wtmp. Debian systems include a patched libc5 which
181 uses the new utmp format. The problem still exists with wtmp since
182 it's accessed directly in libc5.
183
185 The file format is machine dependent, so it is recommended that it be
186 processed only on the machine architecture where it was created.
187 Note that on platforms which can run both 32-bit and 64-bit applica‐
189 tions (x86-64, ppc64, s390x, etc.), the sizes of the fields of a struct
190 utmp must be the same in 32-bit mode as in 64-bit mode. This is
191 achieved by changing the type of ut_session to int32_t, and that of
192 ut_tv to a struct with two int32_t fields tv_sec and tv_usec. (Thus,
193 in order to fill it, first get the time into a real struct timeval,
194 then copy the two fields to ut_tv.)
195
197 This manpage is based on the libc5 one, things may work differently
198 now.
199
201 ac(1), date(1), last(1), login(1), who(1), getutent(3), updwtmp(3),
202 init(8)
203File formats 2004-10-31 UTMP(5)