Fix version numbers in cross references.

[thirdparty/man-pages.git] / man5 / utmp.5

.\" Copyright (c) 1993 Michael Haardt (michael@cantor.informatik.rwth-aachen.de), Fri Apr  2 11:32:09 MET DST 1993
.\"
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.\" Modified 1993-07-25 by Rik Faith (faith@cs.unc.edu)
.\" Modified 1995-02-26 by Michael Haardt
.\" Modified 1996-07-20 by Michael Haardt
.\" Modified 1997-07-02 by Nicolás Lichtmaier <nick@debian.org>
.\" Modified 2004-10-31 by aeb, following Gwenole Beauchesne
.TH UTMP 5 2004-10-31 "Linux" "Linux Programmer's Manual"
.SH NAME
utmp, wtmp \- login records
.SH SYNOPSIS
#include <utmp.h>
.SH DESCRIPTION
The
.I 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.
.PP
.B Warning:
.I utmp
must not be writable, because many system programs (foolishly)
depend on its integrity.
You risk faked system logfiles and
modifications of system files if you leave
.I utmp
writable to any user.
.PP
The file is a sequence of entries with the following structure declared
in the include file (note that this is only one of several definitions
around; details depend on the version of libc):
.in +3
.nf
.sp
.ta 3i
#define UT_UNKNOWN      0
#define RUN_LVL         1
#define BOOT_TIME       2
#define NEW_TIME        3
#define OLD_TIME        4
#define INIT_PROCESS    5
#define LOGIN_PROCESS   6
#define USER_PROCESS    7
#define DEAD_PROCESS    8
#define ACCOUNTING      9

#define UT_LINESIZE     12
#define UT_NAMESIZE     32
#define UT_HOSTSIZE     256

struct exit_status {
    short int e_termination;    /* process termination status */
    short int e_exit;           /* process exit status */
};

struct utmp {
    short ut_type;              /* type of login */
    pid_t ut_pid;               /* PID of login process */
    char ut_line[UT_LINESIZE];  /* device name of tty \- "/dev/" */
    char ut_id[4];              /* init id or abbrev. ttyname */
    char ut_user[UT_NAMESIZE];  /* user name */
    char ut_host[UT_HOSTSIZE];  /* hostname for remote login */
    struct exit_status ut_exit; /* The exit status of a process
                                   marked as DEAD_PROCESS */

    /* 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, used for windowing */
    struct {
        int32_t tv_sec;         /* Seconds */
        int32_t tv_usec;        /* Microseconds */
    } ut_tv;                    /* Time entry was made */
#else
     long int ut_session;        /* Session ID, used for windowing */
     struct timeval ut_tv;       /* Time entry was made */
#endif

    int32_t ut_addr_v6[4];       /* IP address of remote host */
    char __unused[20];           /* Reserved for future use */
};

/* Backwards 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]
.sp
.fi
.in
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
.BR time (2).
String fields are terminated by \fB'\e0'\fP if they are shorter than the size
of the field.
.PP
The first entries ever created result from
.BR init (8)
processing
.BR inittab (5).
Before an entry is processed, though,
.BR init (8)
cleans up utmp by setting \fIut_type\fP to \fBDEAD_PROCESS\fP, clearing
\fIut_user\fP, \fIut_host\fP, and \fIut_time\fP with null bytes for each
record which \fIut_type\fP is not \fBDEAD_PROCESS\fP or \fBRUN_LVL\fP
and where no process with PID \fIut_pid\fP exists.
If no empty record
with the needed \fIut_id\fP can be found, init creates a new one.
It sets \fIut_id\fP from the inittab, \fIut_pid\fP and \fIut_time\fP to the
current values, and \fIut_type\fP to \fBINIT_PROCESS\fP.
.PP
.BR getty (8)
locates the entry by the PID, changes \fIut_type\fP to
\fBLOGIN_PROCESS\fP, changes \fIut_time\fP, sets \fIut_line\fP, and waits
for connection to be established.
.BR login (1),
after a user has been
authenticated, changes \fIut_type\fP to \fBUSER_PROCESS\fP, changes
\fIut_time\fP, and sets \fIut_host\fP and \fIut_addr\fP.
Depending on
.BR getty (8)
and
.BR login (1),
records may be located by
\fIut_line\fP instead of the preferable \fIut_pid\fP.
.PP
When
.BR init (8)
finds that a process has exited, it locates its utmp
entry by \fIut_pid\fP, sets \fIut_type\fP to \fBDEAD_PROCESS\fP, and
clears \fIut_user\fP, \fIut_host\fP and \fIut_time\fP with null bytes.
.PP
.BR xterm (1)
and other terminal emulators directly create a
\fBUSER_PROCESS\fP record and generate the \fIut_id\fP by using the last
two letters of \fI/dev/ttyp\fP\fI%c\fP or by using \fIp\fP\fI%d\fP for
\fI/dev/pts/\fP\fI%d\fP.
If they find a \fBDEAD_PROCESS\fP for this ID,
they recycle it, otherwise they create a new entry.
If they can, they
will mark it as \fBDEAD_PROCESS\fP on exiting and it is advised that
they null \fIut_line\fP, \fIut_time\fP, \fIut_user\fP, and \fIut_host\fP
as well.
.PP
.BR telnetd (8)
sets up a \fBLOGIN_PROCESS\fP entry and leaves the rest to
.BR login (1)
as usual.
After the telnet session ends,
.BR telnetd (8)
cleans up utmp in the described way.
.PP
The \fIwtmp\fP file records all logins and logouts.
Its format is exactly like \fIutmp\fP except that a null user name
indicates a logout
on the associated terminal.
Furthermore, the terminal name \fB~\fP
with user name \fBshutdown\fP or \fBreboot\fP indicates a system
shutdown or reboot and the pair of terminal names \fB|\fP/\fB}\fP
logs the old/new system time when
.BR date (1)
changes it.
\fIwtmp\fP is maintained by
.BR login (1),
.BR init (8),
and some versions of
.BR getty (8).
Neither of these programs creates the file, so if it is
removed, record-keeping is turned off.
.PP
Note that on \fIbiarch\fP platforms, that is, systems which can run both
32-bit and 64-bit applications (x86-64, ppc64, s390x, etc.),
\fIut_tv\fP is the same size in 32-bit mode as in 64-bit mode.
The same goes for \fIut_session\fP and \fIut_time\fP if they are present.
This allows data files and shared memory to be shared between
32-bit and 64-bit applications.
Since \fIut_tv\fP may not be the same as \fIstruct timeval\fP,
then instead of the call:
.RS
.nf
.sp
.ta 3i
gettimeofday((struct timeval *) &ut.ut_tv, NULL);
.fi
.RE

the following method of setting this field is recommended:
.RS
.nf
.sp
.ta 3i
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;
.fi
.RE
.SH FILES
/var/run/utmp
.br
/var/log/wtmp
.SH "CONFORMING TO"
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
\fIut_type\fP, 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 \fIut_id\fP fields.
In Linux (as in System V), the \fIut_id\fP field of a
record will never change once it has been set, which reserves that slot
without needing a configuration file.
Clearing \fIut_id\fP may result
in race conditions leading to corrupted utmp entries and potential
security holes.
Clearing the above mentioned fields by filling them
with null bytes is not required by System V semantics, but it allows 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.
.PP
System V only uses the type field to mark them and logs informative messages
such as \fB"new time"\fP in the line field.
\fBUT_UNKNOWN\fP seems
to be a Linux invention.
System V has no \fIut_host\fP or \fIut_addr_v6\fP fields.
.PP
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
.BR who (1)
then do not make utmp world readable.
.PP
Note that the \fIutmp\fP struct from libc5 has changed in libc6.
Because of this,
binaries using the old libc5 struct will corrupt
.IR /var/run/utmp " and/or " /var/log/wtmp .
.SH NOTES
The file format is machine dependent, so it is recommended that it be
processed only on the machine architecture where it was created.
.PP
Note that on platforms which can run both 32-bit and 64-bit applications
(x86-64, ppc64, s390x, etc.), the sizes of the fields of a \fIutmp\fP struct
must be the same in 32-bit mode as in 64-bit mode.
This is achieved by changing the type of
.I ut_session
to
.IR int32_t ,
and that of
.I ut_tv
to a struct with two
.I int32_t
fields
.I tv_sec
and
.IR tv_usec .
(Thus, in order to fill it, first get the
time into a real \fIstruct timeval\fP,
then copy the two fields to
.IR ut_tv .)
.SH BUGS
This man page is based on the libc5 one, things may work differently now.
.SH "SEE ALSO"
.BR ac (1),
.BR date (1),
.BR last (1),
.BR login (1),
.BR who (1),
.BR getutent (3),
.BR updwtmp (3),
.BR init (8)