Provided by: libsystemd-dev_237-3ubuntu10.57_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 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
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 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.
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
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
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 <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);
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)