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
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:
if (t == (uint64_t) -1)
msec = -1;
struct timespec 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_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.
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
sd_journal_process() and sd_journal_wait() return a negative
errno-style error code, or one of SD_JOURNAL_NOP, SD_JOURNAL_APPEND
or SD_JOURNAL_INVALIDATE on success:
· 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. In this case it is sufficient to
simply continue reading at the previous end location of the
journal, to read the newly added entries.
· If SD_JOURNAL_INVALIDATE, journal files were added to or removed
from the set of journal files watched (e.g. due to rotation or
vacuuming), and thus entries might have appeared or disappeared
at arbitrary places in the log stream, possibly before or after
the previous end of the log stream. If SD_JOURNAL_INVALIDATE is
returned, live-view UIs that want to reflect on screen the
precise state of the log data on disk should probably refresh
their entire display (relative to the cursor of the log entry on
the top of the screen). Programs only interested in a strictly
sequential stream of log data may treat SD_JOURNAL_INVALIDATE the
same way as SD_JOURNAL_APPEND, thus ignoring any changes to the
log view earlier than the old end of the log stream.
In general, sd_journal_get_fd(), sd_journal_get_events(), and
sd_journal_get_timeout() are not "async signal safe" in the meaning
of signal-safety(7). Nevertheless, only the first call to any of
those three functions performs unsafe operations, so subsequent calls
sd_journal_process() and sd_journal_wait() are not safe.
sd_journal_reliable_fd() is safe.
All functions listed here are thread-agnostic and only a single
specific thread may operate on a given object during its entire
lifetime. It's safe to allocate multiple independent objects and use
each from a specific thread in parallel. However, it's not safe to
allocate such an object in one thread, and operate or free it from
any other, even if locking is used to ensure these threads don't
operate on it at the very same time.
These APIs are implemented as a shared library, which can be compiled
and linked to with the libsystemd pkg-config(1) file.
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2020-07-14. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-07-14.) If you discover any rendering problems in
this HTML version of the page, or you believe there is a better or
more up-to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not part
of the original manual page), send a mail to email@example.com
systemd 246 SD_JOURNAL_GET_FD(3)