login, logout — write utmp and wtmp entries


#include <utmp.h>
void login( const struct utmp *ut);
int logout( const char *ut_line);
[Note] Note

Link with −lutil.


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.



user accounting database, configured through _PATH_UTMP in <paths.h>


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



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.


Not in POSIX.1. Present on the BSDs.


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>


getutent(3), utmp(5)


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