sd_login_monitor_new(3) — Linux manual page


SD_LOGIN_MONITOR_NEW(3)   sd_login_monitor_new   SD_LOGIN_MONITOR_NEW(3)

NAME         top

       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

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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_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.

RETURN VALUE         top

       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.

       Returned errors may indicate the following problems:

           An input parameter was invalid (out of range, or NULL, where
           that is not accepted). The specified category to watch is not

           Memory allocation failed.

NOTES         top

       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

HISTORY         top

       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.

SEE ALSO         top

       systemd(1), sd-login(3), sd_get_seats(3), poll(2),

NOTES         top

        1. Clean-up Variable Attribute

COLOPHON         top

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systemd 257~devel                                SD_LOGIN_MONITOR_NEW(3)

Pages that refer to this page: sd-login(3)systemd.directives(7)systemd.index(7)