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SD_JOURNAL_GET_FD(3)	       sd_journal_get_fd	  SD_JOURNAL_GET_FD(3)

NAME
       sd_journal_get_fd, sd_journal_get_events, sd_journal_get_timeout,
       sd_journal_process, sd_journal_wait, sd_journal_reliable_fd,
       SD_JOURNAL_NOP, SD_JOURNAL_APPEND, SD_JOURNAL_INVALIDATE - Journal
       change notification interface

SYNOPSIS
       #include <systemd/sd-journal.h>

       int sd_journal_get_fd(sd_journal* j);

       int sd_journal_get_events(sd_journal* j);

       int sd_journal_get_timeout(sd_journal* j, uint64_t* timeout_usec);

       int sd_journal_process(sd_journal* j);

       int sd_journal_wait(sd_journal* j, uint64_t timeout_usec);

       int sd_journal_reliable_fd(sd_journal* j);

DESCRIPTION
       sd_journal_get_fd() returns a file descriptor that may be
       asynchronously polled in an external event loop and is signaled as soon
       as the journal changes, because new entries or files were added,
       rotation took place, or files have been deleted, and similar. The file
       descriptor is suitable for usage in poll(2). Use
       sd_journal_get_events() for an events mask to watch for. The call takes
       one argument: the journal context object. Note that not all file
       systems are capable of generating the necessary events for wakeups from
       this file descriptor for changes to be noticed immediately. In
       particular network files systems do not generate suitable file change
       events in all cases. Cases like this can be detected with
       sd_journal_reliable_fd(), below.	 sd_journal_get_timeout() will ensure
       in these cases that wake-ups happen frequently enough for changes to be
       noticed, although with a certain latency.

       sd_journal_get_events() will return the poll() mask to wait for. This
       function will return a combination of POLLIN and POLLOUT and similar to
       fill into the ".events" field of struct pollfd.

       sd_journal_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_journal_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.

       After each poll() wake-up sd_journal_process() needs to be called to
       process events. This call will also indicate what kind of change has
       been detected (see below; note that spurious wake-ups are possible).

       A synchronous alternative for using sd_journal_get_fd(),
       sd_journal_get_events(), sd_journal_get_timeout() and
       sd_journal_process() is sd_journal_wait(). It will synchronously wait
       until the journal gets changed. The maximum time this call sleeps may
       be controlled with the timeout_usec parameter. Pass (uint64_t) -1 to
       wait indefinitely. Internally this call simply combines
       sd_journal_get_fd(), sd_journal_get_events(), sd_journal_get_timeout(),
       poll() and sd_journal_process() into one.

       sd_journal_reliable_fd() may be used to check whether the wakeup events
       from the file descriptor returned by sd_journal_get_fd() are known to
       be immediately triggered. On certain file systems where file change
       events from the OS are not available (such as NFS) changes need to be
       polled for repeatedly, and hence are detected only with a certain
       latency. This call will return a positive value if the journal changes
       are detected immediately and zero when they need to be polled for and
       hence might be noticed only with a certain latency. Note that there's
       usually no need to invoke this function directly as
       sd_journal_get_timeout() on these file systems will ask for timeouts
       explicitly anyway.

RETURN VALUE
       sd_journal_get_fd() returns a valid file descriptor on success or a
       negative errno-style error code.

       sd_journal_get_events() returns a combination of POLLIN, POLLOUT and
       suchlike on success or a negative errno-style error code.

       sd_journal_reliable_fd() returns a positive integer if the file
       descriptor returned by sd_journal_get_fd() will generate wake-ups
       immediately for all journal changes. Returns 0 if there might be a
       latency involved.

       sd_journal_process() and sd_journal_wait() return one of
       SD_JOURNAL_NOP, SD_JOURNAL_APPEND or SD_JOURNAL_INVALIDATE on success
       or a negative errno-style error code. If SD_JOURNAL_NOP is returned,
       the journal did not change since the last invocation. If
       SD_JOURNAL_APPEND is returned, new entries have been appended to the
       end of the journal. If SD_JOURNAL_INVALIDATE, journal files were added
       or removed (possibly due to rotation). In the latter event, live-view
       UIs should probably refresh their entire display, while in the case of
       SD_JOURNAL_APPEND, it is sufficient to simply continue reading at the
       previous end of the journal.

NOTES
       The sd_journal_get_fd(), sd_journal_get_events(),
       sd_journal_reliable_fd(), sd_journal_process() and sd_journal_wait()
       interfaces are available as a shared library, which can be compiled and
       linked to with the libsystemd-journal pkg-config(1) file.

EXAMPLES
       Iterating through the journal, in a live view tracking all changes:

	   #include <stdio.h>
	   #include <string.h>
	   #include <systemd/sd-journal.h>

	   int main(int argc, char *argv[]) {
		   int r;
		   sd_journal *j;
		   r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
		   if (r < 0) {
			   fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
			   return 1;
		   }
		   for (;;)  {
			   const void *d;
			   size_t l;
			   r = sd_journal_next(j);
			   if (r < 0) {
				   fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r));
				   break;
			   }
			   if (r == 0) {
				   /* Reached the end, let's wait for changes, and try again */
				   r = sd_journal_wait(j, (uint64_t) -1);
				   if (r < 0) {
					   fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r));
					   break;
				   }
				   continue;
			   }
			   r = sd_journal_get_data(j, "MESSAGE", &d, &l);
			   if (r < 0) {
				   fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
				   continue;
			   }
			   printf("%.*s\n", (int) l, (const char*) d);
		   }
		   sd_journal_close(j);
		   return 0;
	   }

       Waiting with poll() (this example lacks all error checking for the sake
       of simplicity):

	   #include <sys/poll.h>
	   #include <systemd/sd-journal.h>

	   int wait_for_changes(sd_journal *j) {
		   struct pollfd pollfd;
		   int msec;

		   sd_journal_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;
		   }

		   pollfd.fd = sd_journal_get_fd(j);
		   pollfd.events = sd_journal_get_events(j);
		   poll(&pollfd, 1, msec);
		   return sd_journal_process(j);
	   }

SEE ALSO
       systemd(1), sd-journal(3), sd_journal_open(3), sd_journal_next(3),
       poll(2), clock_gettime(2)

systemd 208						  SD_JOURNAL_GET_FD(3)
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