sd_login_monitor_new (3) - Linux Manuals

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_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_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 'us' 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_getttime(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.

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.