SD_JOURNAL_GET_FD(3)          sd_journal_get_fd         SD_JOURNAL_GET_FD(3)

NAME         top

       sd_journal_get_fd, sd_journal_get_events, sd_journal_get_timeout,
       sd_journal_process, sd_journal_wait, sd_journal_reliable_fd,
       change notification interface

SYNOPSIS         top

       #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         top

       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_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

       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 is 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         top

       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
       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         top

       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 pkg-config(1) file.

EXAMPLES         top

       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));
               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));
               r = sd_journal_get_data(j, "MESSAGE", &d, &l);
               if (r < 0) {
                 fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
               printf("%.*s\n", (int) l, (const char*) d);
             return 0;

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

           #include <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_gettime(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);
    = sd_journal_get_events(j);
             poll(&pollfd, 1, msec);
             return sd_journal_process(j);

SEE ALSO         top

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

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
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       report for this manual page, see
       ⟨⟩.  This
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systemd 234                                             SD_JOURNAL_GET_FD(3)

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