man3/login.3

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.TH LOGIN 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
login, logout \- write utmp and wtmp entries
.SH SYNOPSIS
.nf
.B #include <utmp.h>
.PP
.BI "void login(const struct utmp *" ut );
.BI "int logout(const char *" ut_line );
.fi
.PP
Link with \fI\-lutil\fP.
.SH DESCRIPTION
The utmp file records who is currently using the system.
The wtmp file records all logins and logouts.
See
.BR utmp (5).
.PP
The function
.BR login ()
takes the supplied
.IR "struct utmp" ,
.IR ut ,
and writes it to both the utmp and the wtmp file.
.PP
The function
.BR logout ()
clears the entry in the utmp file again.
.SS GNU details
More precisely,
.BR login ()
takes the argument
.I ut
struct, fills the field
.I ut\->ut_type
(if there is such a field) with the value
.BR USER_PROCESS ,
and fills the field
.I ut\->ut_pid
(if there is such a field) with the process ID of the calling process.
Then it tries to fill the field
.IR ut\->ut_line .
It takes the first of
.IR stdin ,
.IR stdout ,
.I stderr
that is a terminal, and
stores the corresponding pathname minus a possible leading
.I /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.
.PP
The
.BR logout ()
function searches the utmp file for an entry matching the
.I ut_line
argument.
If a record is found, it is updated by zeroing out the
.I ut_name
and
.I ut_host
fields, updating the
.I ut_tv
timestamp field and setting
.I ut_type
(if there is such a field) to
.BR DEAD_PROCESS .
.SH RETURN VALUE
The
.BR logout ()
function returns 1 if the entry was successfully written to the
database, or 0 if an error occurred.
.SH FILES
.TP
.I /var/run/utmp
user accounting database, configured through
.B _PATH_UTMP
in
.I <paths.h>
.TP
.I /var/log/wtmp
user accounting log file, configured through
.B _PATH_WTMP
in
.I <paths.h>
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.BR login (),
.BR logout ()
T}	Thread safety	T{
MT-Unsafe race:utent
sig:ALRM timer
T}
.TE
.hy
.ad
.sp 1
In the above table,
.I utent
in
.I race:utent
signifies that if any of the functions
.BR setutent (3),
.BR getutent (3),
or
.BR endutent (3)
are used in parallel in different threads of a program,
then data races could occur.
.BR login ()
and
.BR logout ()
calls those functions,
so we use race:utent to remind users.
.SH CONFORMING TO
Not in POSIX.1.
Present on the BSDs.
.SH NOTES
Note that the
member
.I ut_user
of
.I struct utmp
is called
.I ut_name
in BSD.
Therefore,
.I ut_name
is defined as an alias for
.I ut_user
in
.IR <utmp.h> .
.SH SEE ALSO
.BR getutent (3),
.BR utmp (5)