#define INIT_PROCESS 5

#define LOGIN_PROCESS 6

#define USER_PROCESS 7

#define DEAD_PROCESS 8



#define UT_LINESIZE 12

#define UT_NAMESIZE 8

#define UT_HOSTSIZE 16



struct utmp {

    short ut_type;               /* type of login */

    pid_t ut_pid;                /* pid of process */

    char ut_line[UT_LINESIZE];   /* device name of tty _ "/dev/" */

    char ut_id[2];               /* init id or abbrev. ttyname */

    time_t ut_time;              /* login time */

    char ut_user[UT_NAMESIZE];   /* user name */

    char ut_host[UT_HOSTSIZE];   /* host name for remote login */

    long ut_addr;                /* IP addr of remote host */

};

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 \0 if they are shorter than the size of the field.

The first entries ever created result from init(8) processing inittab(5). Before an entry is processed, though, init(8) cleans up utmp by setting ut_type to DEAD_PROCESS, clearing ut_user, ut_host and ut_time with null bytes for each record that 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 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.

getty(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(8), after a user has been authenticated, changes ut_type to USER_PROCESS, changes ut_time, and sets ut_host and ut_addr. Depending on getty(8) and login(8), records may be located by ut_line instead of the preferable ut_pid.

When init(8) 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 last two letters of /dev/ttyp%c or by using p%d for /dev/pts/%d.

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.

xdm(8) should not create an utmp record because there is no assigned terminal. Letting it create one will result in trouble such as finger: cannot stat /dev/machine.dom. It should create wtmp entries, though, just like ftpd(8) does.

telnetd(8) sets up a LOGIN_PROCESS entry and leaves the rest to login(8) 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) and init(1) and some variation of getty(1). Neither 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

Linux utmp entries conform neither to v7/BSD nor to SYSV: 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 that allocates slots to sessions. BSD does so because it lacks ut_id fields. In Linux (as in SYSV), the ut_id field of a record will never change once it is set, which reserves that slot without needing a configuration file. Clearing ut_id may result in race conditions leading to corrupted utmp entries and potential security holes. Clearing the previously mentioned fields by filling them with null bytes is not required by SYSV semantics, but it allows you to run many programs that assume BSD semantics and that do not modify utmp. Linux uses the BSD conventions for line contents. SYSV only uses the type field to mark them and logs informative messages such as new time in the line field. SYSV has one more field to log the exit status of dead processes. UT_UNKNOWN seems to be a Linux invention. There is no type ACCOUNTING in Linux. SYSV has no ut_host or ut_addr fields. 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.

RESTRICTIONS

The file format is machine dependent, so it is recommended that it be processed only on the machine architecture where it got created.

SEE ALSO

ac(1), date(1), last(1), login(1), who(1), getutent(3), init(8)

20 July 1996

uuencode

uuencode—Format of an encoded uuencode file.

DESCRIPTION

Files output by uuencode(1) consist of a header line, followed by a number of body lines, and a trailer line. The uudecode(1) command will ignore any lines preceding the header or following the trailer. Lines preceding a header must not, of course, look like a header.

The header line is distinguished by having the first six characters begin. The word begin is followed by a mode (in octal) and a string that names the remote file. A space separates the three items in the header line.

The body consists of a number of lines, each at most 62 characters long (including the trailing newline). These consist of a character count, followed by encoded characters, followed by a newline. The character count is a single printing character and represents an integer, the number of bytes the rest of the line represents. Such integers are always in the range from 0 to 63 and can be determined by subtracting the character space (octal 40) from the character.

Groups of three bytes are stored in four characters, six bits per character. All are offset by a space to make the characters print. The last line may be shorter than the normal 45 bytes. If the size is not a multiple of three, this fact can be determined by the value of the count on the last line. Extra garbage will be included to make the character count a multiple of four. The body is terminated by a line with a count of zero. This line consists of one ASCII space.

The trailer line consists of end on a line by itself.

SEE ALSO

uuencode(1), uudecode(1), uusend(1), uucp(1), mail(1)

HISTORY

The uuencode file format appeared in BSD 4.0.