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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | HISTORY | SEE ALSO | NOTES | COLOPHON |
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SD_LOGIN_MONITOR_NEW(3) sd_login_monitor_new SD_LOGIN_MONITOR_NEW(3)
sd_login_monitor_new, sd_login_monitor_unref,
sd_login_monitor_unrefp, sd_login_monitor_flush,
sd_login_monitor_get_fd, sd_login_monitor_get_events,
sd_login_monitor_get_timeout, sd_login_monitor - Monitor login
sessions, seats, users and virtual machines/containers
#include <systemd/sd-login.h>
int sd_login_monitor_new(const char *category,
sd_login_monitor **ret);
sd_login_monitor *sd_login_monitor_unref(sd_login_monitor *m);
void sd_login_monitor_unrefp(sd_login_monitor **m);
int sd_login_monitor_flush(sd_login_monitor *m);
int sd_login_monitor_get_fd(sd_login_monitor *m);
int sd_login_monitor_get_events(sd_login_monitor *m);
int sd_login_monitor_get_timeout(sd_login_monitor *m,
uint64_t *timeout_usec);
sd_login_monitor_new() may be used to monitor login sessions,
users, seats, and virtual machines/containers. Via a monitor
object a file descriptor can be integrated into an application
defined event loop which is woken up each time a user logs in,
logs out or a seat is added or removed, or a session, user, seat
or virtual machine/container changes state otherwise. The first
parameter takes a string which can be "seat" (to get only
notifications about seats being added, removed or changed),
"session" (to get only notifications about sessions being created
or removed or changed), "uid" (to get only notifications when a
user changes state in respect to logins) or "machine" (to get only
notifications when a virtual machine or container is started or
stopped). If notifications shall be generated in all these
conditions, NULL may be passed. Note that in the future additional
categories may be defined. The second parameter returns a monitor
object and needs to be freed with the sd_login_monitor_unref()
call after use.
sd_login_monitor_unref() may be used to destroy a monitor object.
Note that this will invalidate any file descriptor returned by
sd_login_monitor_get_fd().
sd_login_monitor_unrefp() is similar to sd_login_monitor_unref()
but takes a pointer to a pointer to an sd_login_monitor object.
This call is useful in conjunction with GCC's and LLVM's Clean-up
Variable Attribute[1]. Note that this function is defined as
inline function. Use a declaration like the following, in order to
allocate a login monitor object that is freed automatically as the
code block is left:
{
__attribute__((cleanup(sd_login_monitor_unrefp))) sd_login_monitor *m = NULL;
int r;
...
r = sd_login_monitor_new(NULL, &m);
if (r < 0) {
errno = -r;
fprintf(stderr, "Failed to allocate login monitor object: %m\n");
}
...
}
sd_login_monitor_flush() may be used to reset the wakeup state of
the monitor object. Whenever an event causes the monitor to wake
up the event loop via the file descriptor this function needs to
be called to reset the wake-up state. If this call is not invoked,
the file descriptor will immediately wake up the event loop again.
sd_login_monitor_unref() and sd_login_monitor_unrefp() execute no
operation if the passed in monitor object is NULL.
sd_login_monitor_get_fd() may be used to retrieve the file
descriptor of the monitor object that may be integrated in an
application defined event loop, based around poll(2) or a similar
interface. The application should include the returned file
descriptor as wake-up source for the events mask returned by
sd_login_monitor_get_events(). It should pass a timeout value as
returned by sd_login_monitor_get_timeout(). Whenever a wake-up is
triggered the file descriptor needs to be reset via
sd_login_monitor_flush(). An application needs to reread the login
state with a function like sd_get_seats(3) or similar to determine
what changed.
sd_login_monitor_get_events() will return the poll() mask to wait
for. This function will return a combination of POLLIN, POLLOUT
and similar to fill into the ".events" field of struct pollfd.
sd_login_monitor_get_timeout() will return a timeout value for
usage in poll(). This returns a value in microseconds since the
epoch of CLOCK_MONOTONIC for timing out poll() in timeout_usec.
See clock_gettime(2) for details about CLOCK_MONOTONIC. If there
is no timeout to wait for this will fill in (uint64_t) -1 instead.
Note that poll() takes a relative timeout in milliseconds rather
than an absolute timeout in microseconds. To convert the absolute
'μs' timeout into relative 'ms', use code like the following:
uint64_t t;
int msec;
sd_login_monitor_get_timeout(m, &t);
if (t == (uint64_t) -1)
msec = -1;
else {
struct timespec ts;
uint64_t n;
clock_gettime(CLOCK_MONOTONIC, &ts);
n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
}
The code above does not do any error checking for brevity's sake.
The calculated msec integer can be passed directly as poll()'s
timeout parameter.
On success, sd_login_monitor_new(), sd_login_monitor_flush() and
sd_login_monitor_get_timeout() return 0 or a positive integer. On
success, sd_login_monitor_get_fd() returns a Unix file descriptor.
On success, sd_login_monitor_get_events() returns a combination of
POLLIN, POLLOUT and suchlike. On failure, these calls return a
negative errno-style error code.
sd_login_monitor_unref() always returns NULL.
Errors
Returned errors may indicate the following problems:
-EINVAL
An input parameter was invalid (out of range, or NULL, where
that is not accepted). The specified category to watch is not
known.
-ENOMEM
Memory allocation failed.
Functions described here are available as a shared library, which
can be compiled against and linked to with the
libsystemd pkg-config(1) file.
The code described here uses getenv(3), which is declared to be
not multi-thread-safe. This means that the code calling the
functions described here must not call setenv(3) from a parallel
thread. It is recommended to only do calls to setenv() from an
early phase of the program when no other threads have been
started.
sd_login_monitor_get_events() and sd_login_monitor_get_timeout()
were added in version 201.
sd_login_monitor_unrefp() was added in version 229.
systemd(1), sd-login(3), sd_get_seats(3), poll(2),
clock_gettime(2)
1. Clean-up Variable Attribute
https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 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-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 258~rc2 SD_LOGIN_MONITOR_NEW(3)
Pages that refer to this page: sd-login(3), systemd.directives(7), systemd.index(7)
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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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