login, logout - write utmp and wtmp entries

       For an explanation of the terms used in this section, see attributes(7).
       ┌───────────────────┬───────────────┬───────────────────────────────────────────────────────────────────┐
       │ 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.

       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 structutmp, ut, and writes it to both  the  utmp  and  the  wtmp
       file.

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

   GNUdetails
       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.

/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>

       System utilities library (libutil, -lutil)

       login, logout - write utmp and wtmp entries

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

getutent(3), utmp(5)

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

       BSD.

#include<utmp.h>voidlogin(conststructutmp*ut);intlogout(constchar*ut_line);

       The member ut_user of structutmp is called ut_name in BSD.  Therefore, ut_name is defined  as  an  alias
       for ut_user in <utmp.h>.