Provided by: systemd_204-5ubuntu20.31_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's 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 didn't 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 shared library,
which can be compiled and linked to with the libsystemd-journalpkg-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)