login(3) — Linux manual page


login(3)                Library Functions Manual                login(3)

NAME         top

       login, logout - write utmp and wtmp entries

LIBRARY         top

       System utilities library (libutil, -lutil)

SYNOPSIS         top

       #include <utmp.h>

       void login(const struct utmp *ut);
       int logout(const char *ut_line);

DESCRIPTION         top

       The utmp file records who is currently using the system.  The
       wtmp file records all logins and logouts.  See utmp(5).

       The function login() takes the supplied struct utmp, ut, and
       writes it to both the utmp and the wtmp file.

       The function logout() clears the entry in the utmp file again.

   GNU details
       More precisely, login() takes the argument ut struct, fills the
       field ut->ut_type (if there is such a field) with the value
       USER_PROCESS, and fills the field ut->ut_pid (if there is such a
       field) with the process ID of the calling process.  Then it tries
       to fill the field ut->ut_line.  It takes the first of stdin,
       stdout, stderr that is a terminal, and stores the corresponding
       pathname minus a possible leading /dev/ into this field, and then
       writes the struct to the utmp file.  On the other hand, if no
       terminal name was found, this field is filled with "???"  and the
       struct is not written to the utmp file.  After this, the struct
       is written to the wtmp file.

       The logout() function searches the utmp file for an entry
       matching the ut_line argument.  If a record is found, it is
       updated by zeroing out the ut_name and ut_host fields, updating
       the ut_tv timestamp field and setting ut_type (if there is such a
       field) to DEAD_PROCESS.

RETURN VALUE         top

       The logout() function returns 1 if the entry was successfully
       written to the database, or 0 if an error occurred.

FILES         top

              user accounting database, configured through _PATH_UTMP in

              user accounting log file, configured through _PATH_WTMP in

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface         Attribute     Value                     │
       │ login(), logout() │ Thread safety │ MT-Unsafe race:utent      │
       │                   │               │ sig:ALRM timer            │

       In the above table, utent in race:utent signifies that if any of
       the functions setutent(3), getutent(3), or endutent(3) are used
       in parallel in different threads of a program, then data races
       could occur.  login() and logout() calls those functions, so we
       use race:utent to remind users.

VERSIONS         top

       The member ut_user of struct utmp is called ut_name in BSD.
       Therefore, ut_name is defined as an alias for ut_user in

STANDARDS         top


SEE ALSO         top

       getutent(3), utmp(5)

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                       login(3)

Pages that refer to this page: utmp(5)