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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | FILES | ATTRIBUTES | VERSIONS | STANDARDS | SEE ALSO | COLOPHON |
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login(3) Library Functions Manual login(3)
login, logout - write utmp and wtmp entries
System utilities library (libutil, -lutil)
#include <utmp.h>
void login(const struct utmp *ut);
int logout(const char *ut_line);
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.
The logout() function returns 1 if the entry was successfully
written to the database, or 0 if an error occurred.
/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>
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 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>.
BSD.
getutent(3), utmp(5)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 login(3)
Pages that refer to this page: utmp(5)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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