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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | SIGNAL SAFETY | NOTES | EXAMPLES | HISTORY | SEE ALSO | COLOPHON |
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SD_JOURNAL_GET_FD(3) sd_journal_get_fd SD_JOURNAL_GET_FD(3)
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
#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);
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 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 wake-up
events from the file descriptor returned by sd_journal_get_fd()
are known to be quickly 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 considerable latency. This call will return a positive
value if the journal changes are detected quickly and zero when
they need to be polled for. Note that there is usually no need to
invoke this function directly as sd_journal_get_timeout() will
request appropriate timeouts anyway.
Note that all of the above change notification interfaces do not
report changes instantly. Latencies are introduced for multiple
reasons: as mentioned certain storage backends require time-based
polling, in other cases wake-ups are optimized by coalescing
events, and the OS introduces additional IO/CPU scheduling
latencies.
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 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 are safe.
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 is safe to allocate multiple independent objects and
use each from a specific thread in parallel. However, it is 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
do not operate on it at the very same time.
Functions described here are available as a shared library, which
can be compiled against and linked to with the
libsystemd pkg-config(1) file.
Iterating through the journal, in a live view tracking all
changes:
/* SPDX-License-Identifier: MIT-0 */
#include <errno.h>
#include <stdio.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):
/* SPDX-License-Identifier: MIT-0 */
#define _GNU_SOURCE 1
#include <poll.h>
#include <time.h>
#include <systemd/sd-journal.h>
int wait_for_changes(sd_journal *j) {
uint64_t t;
int msec;
struct pollfd pollfd;
sd_journal_get_timeout(j, &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);
pollfd.events = sd_journal_get_events(j);
poll(&pollfd, 1, msec);
return sd_journal_process(j);
}
sd_journal_get_fd(), sd_journal_process(), and sd_journal_wait()
were added in version 187.
sd_journal_reliable_fd() was added in version 196.
sd_journal_get_events() and sd_journal_get_timeout() were added in
version 201.
systemd(1), sd-journal(3), sd_journal_open(3), sd_journal_next(3),
poll(2), clock_gettime(2)
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
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) 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
man-pages@man7.org
systemd 258~rc2 SD_JOURNAL_GET_FD(3)
Pages that refer to this page: sd-journal(3), systemd.directives(7), systemd.index(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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