NAME

       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

       #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

       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_default(&m);
                   if (r < 0)
                           fprintf(stderr, "Failed to allocate login monitor object: %s\n", strerror(-r));
                   ...
           }

       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

       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.

NOTES

       The sd_login_monitor_new(), sd_login_monitor_unref(), sd_login_monitor_flush(),
       sd_login_monitor_get_fd(), sd_login_monitor_get_events() and
       sd_login_monitor_get_timeout() interfaces are available as a shared library, which can be
       compiled and linked to with the libsystemd pkg-config(1) file.

SEE ALSO

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

NOTES

        1. Clean-up Variable Attribute
           https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html