SD_JOURNAL_GET_FD(3) sd_journal_get_fd SD_JOURNAL_GET_FD(3)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);
DESCRIPTIONsd_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_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 VALUEsd_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.
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-journal 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 <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 ALSOsystemd(1), sd-journal(3), sd_journal_open(3), sd_journal_next(3),
poll(2), clock_gettime(2)systemd 208SD_JOURNAL_GET_FD(3)
