Provided by: libsystemd-dev_256.5-2ubuntu3.1_amd64 
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_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.
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 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.
SIGNAL SAFETY
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.
NOTES
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.
Functions described here are available as a shared library, which can be compiled against and linked to
with the libsystemd pkg-config(1) file.
EXAMPLES
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);
}
HISTORY
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.
SEE ALSO
systemd(1), sd-journal(3), sd_journal_open(3), sd_journal_next(3), poll(2), clock_gettime(2)