LOGIN(3) Linux Programmer's Manual LOGIN(3)
login, logout - write utmp and wtmp entries
void login(const struct utmp *ut);
int logout(const char *ut_line);
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.
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 lead-
ing /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
user accounting log file, configured through _PATH_WTMP in
For an explanation of the terms used in this section, see
|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), 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
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>.
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