LOGIN(3)                                  Linux Programmer's Manual                                  LOGIN(3)

NAME
       login, logout - write utmp and wtmp entries

SYNOPSIS
       #include <utmp.h>

       void login(const struct utmp *ut);

       int logout(const char *ut_line);

       Link with -lutil.

DESCRIPTION
       The  utmp  file  records  who is currently using the system.  The wtmp file records all logins and lo‐
       gouts.  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
       The  logout() function returns 1 if the entry was successfully written to the database, or 0 if an er‐
       ror occurred.

FILES
       /var/run/utmp
              user accounting database, configured through _PATH_UTMP in <paths.h>

       /var/log/wtmp
              user accounting log file, configured through _PATH_WTMP in <paths.h>

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────┬───────────────┬──────────────────────┐
       │Interface │ Attribute     │ Value                │
       ├──────────┼───────────────┼──────────────────────┤
       │login(),  │ Thread safety │ MT-Unsafe race:utent │
       │logout()  │               │ sig:ALRM timer       │
       └──────────┴───────────────┴──────────────────────┘
       In the above table, utent in race:utent signifies that if any  of  the  functions  setutent(3),  getu‐
       tent(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.

CONFORMING TO
       Not in POSIX.1.  Present on the BSDs.

NOTES
       Note that 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 <utmp.h>.

SEE ALSO
       getutent(3), utmp(5)

COLOPHON
       This  page  is part of release 4.16 of the Linux man-pages project.  A description of the project, in‐
       formation  about  reporting  bugs,  and  the  latest  version  of  this  page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.

GNU                                               2017-09-15                                         LOGIN(3)

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