Commit 85210bff authored by Lennart Poettering's avatar Lennart Poettering
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journal: provide an API that allows client to figure out whether they need to...

journal: provide an API that allows client to figure out whether they need to recheck the journal manually for changes in regular intervals

Network file systems generally do not offer inotify() that would work
across the network. We hence cannot rely on inotify() exclusiely in
those case. Provide an API to determine these cases, and suggest doing
manual regular rechecks.

Note that this is not complete yet, as we need to rescan journal dirs on
network file systems explicitly to find new/removed files
parent e9f600f2
......@@ -576,6 +576,7 @@ MANPAGES_ALIAS = \
man/sd_journal_get_monotonic_usec.3 \
man/sd_journal_get_cutoff_monotonic_usec.3 \
man/sd_journal_reliable_fd.3 \
man/sd_journal_process.3 \
man/sd_journal_wait.3 \
......@@ -649,6 +650,7 @@ man/sd_journal_restart_data.3: man/sd_journal_get_data.3
man/SD_JOURNAL_FOREACH_DATA.3: man/sd_journal_get_data.3
man/sd_journal_get_monotonic_usec.3: man/sd_journal_get_realtime_usec.3
man/sd_journal_get_cutoff_monotonic_usec.3: man/sd_journal_get_cutoff_realtime_usec.3
man/sd_journal_reliable_fd.3: man/sd_journal_get_fd.3
man/sd_journal_process.3: man/sd_journal_get_fd.3
man/sd_journal_wait.3: man/sd_journal_get_fd.3
man/SD_JOURNAL_NOP.3: man/sd_journal_get_fd.3
......@@ -44,6 +44,7 @@
......@@ -62,6 +63,11 @@
<paramdef>sd_journal* <parameter>j</parameter></paramdef>
<funcdef>int <function>sd_journal_reliable_fd</function></funcdef>
<paramdef>sd_journal* <parameter>j</parameter></paramdef>
<funcdef>int <function>sd_journal_process</function></funcdef>
<paramdef>sd_journal* <parameter>j</parameter></paramdef>
......@@ -82,12 +88,36 @@
<para><function>sd_journal_get_fd()</function> returns
a file descriptor that may be asynchronously polled in
an external event loop and is signaled readable as
soon as the journal changes, for example because new
entries were added. The file descriptor is suitable
for usage in
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
where it will yield POLLIN on all changes. The call
takes one argument: the journal context object.</para>
where it will yield POLLIN on changes. 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
to be enirely reliable. In particular network files
systems do not generate suitable file change events in
all cases. In such a case an application should not
rely alone on wake-ups from this file descriptor but
wake up and recheck the journal in regular time
intervals, for example every 2s. To detect
cases where this is necessary, use
may be used to check whether the wakeup events from
the file descriptor returned by
<function>sd_journal_get_fd</function> are sufficient
to track changes to the journal. If this call returns
0, it is necessary to regularly recheck for journal
changes (suggestion: every 2s). If this call returns a
positive integer this is not necessary, and wakeups
from the file descriptor returned by
<function>sd_journal_get_fd()</function> are
sufficient as only source for wake-ups.</para>
<para>After each POLLIN wake-up
<function>sd_journal_process()</function> needs to be
......@@ -97,14 +127,19 @@
that spurious wake-ups are possible).</para>
<para>A synchronous alternative for using
<function>sd_journal_get_fd()</function> and
<function>sd_journal_reliable_fd()</function> and
<function>sd_journal_process()</function> is
<function>sd_journal_wait()</function>. It will
synchronously wait until the journal gets changed up
to a certain time-out as specified in its second
argument. Pass <literal>(uint64_t) -1</literal> to
wait indefinitely. Internally this call simply
combines <function>sd_journal_get_fd()</function>,
synchronously wait until the journal gets changed,
possibly using a 2s time-out if this is necessary (see
above). In either way the maximum time this call
sleeps may be controlled with the
<parameter>timeout_usec</parameter> parameter. Pass
<literal>(uint64_t) -1</literal> to wait
indefinitely. Internally this call simply combines
<function>poll()</function> and
<function>sd_journal_process()</function> into
......@@ -117,6 +152,15 @@
<para><function>sd_journal_get_fd()</function> returns a valid file descriptor on success or a negative errno-style error
returns a positive integer if the file descriptor
returned by <function>sd_journal_get_fd()</function>
is sufficient as sole wake-up source for journal
change events. Returns 0 if it is not sufficient and
the journal needs to be checked manually in regular
time intervals for changes. Returns a negative
errno-style error code on failure.</para>
<para><function>sd_journal_process()</function> and
<function>sd_journal_wait()</function> return one of
......@@ -140,6 +184,7 @@
<para>The <function>sd_journal_get_fd()</function>,
<function>sd_journal_process()</function> and
<function>sd_journal_wait()</function> interfaces are
available as shared library, which can be compiled and
......@@ -149,6 +194,69 @@
<para>Iterating through the journal, in a live view tracking all changes:</para>
<programlisting>#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;systemd/sd-journal.h&gt;
int main(int argc, char *argv[]) {
int r;
sd_journal *j;
r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
if (r &lt; 0) {
fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
return 1;
for (;;) {
const char *d;
size_t l;
r = sd_journal_next(j);
if (r &lt; 0) {
fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r));
if (r == 0) {
/* Reached the end, let's wait for changes, and try again */
r = sd_journal_wait(j, (uint64_t) -1);
if (r &lt; 0) {
fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r));
r = sd_journal_get_data(j, "MESSAGE", &amp;d, &amp;l);
if (r &lt; 0) {
fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
printf("%.*s\n", (int) l, d);
return 0;
<para>Waiting with <function>poll()</function> (this
example lacks all error checking for the sake of
<programlisting>#include &lt;sys/poll.h&gt;
#include &lt;systemd/sd-journal.h&gt;
int wait_for_changes(sd_journal *j) {
struct pollfd pollfd;
pollfd.fd = sd_journal_get_fd(); = POLLIN;
poll(&amp;pollfd, 1, sd_journal_reliable_fd() &gt; 0 ? -1 : 2000);
return sd_journal_process(j);
<title>See Also</title>
......@@ -156,7 +264,8 @@
......@@ -119,6 +119,8 @@ struct sd_journal {
char *unique_field;
JournalFile *unique_file;
uint64_t unique_offset;
bool on_network;
char *journal_make_match_string(sd_journal *j);
......@@ -80,3 +80,8 @@ global:
......@@ -25,6 +25,8 @@
#include <unistd.h>
#include <sys/inotify.h>
#include <sys/poll.h>
#include <sys/vfs.h>
#include <linux/magic.h>
#include "sd-journal.h"
#include "journal-def.h"
......@@ -35,9 +37,12 @@
#include "lookup3.h"
#include "compress.h"
#include "journal-internal.h"
#include "missing.h"
#define JOURNAL_FILES_MAX 1024
static void detach_location(sd_journal *j) {
Iterator i;
JournalFile *f;
......@@ -1184,6 +1189,25 @@ _public_ int sd_journal_seek_tail(sd_journal *j) {
return 0;
static void check_network(sd_journal *j, int fd) {
struct statfs sfs;
if (j->on_network)
if (fstatfs(fd, &sfs) < 0)
j->on_network =
sfs.f_type == CIFS_MAGIC_NUMBER ||
sfs.f_type == CODA_SUPER_MAGIC ||
sfs.f_type == NCP_SUPER_MAGIC ||
sfs.f_type == NFS_SUPER_MAGIC ||
sfs.f_type == SMB_SUPER_MAGIC;
static int add_file(sd_journal *j, const char *prefix, const char *filename) {
char *path;
int r;
......@@ -1233,6 +1257,8 @@ static int add_file(sd_journal *j, const char *prefix, const char *filename) {
return r;
check_network(j, f->fd);
j->current_invalidate_counter ++;
log_debug("File %s got added.", f->path);
......@@ -1366,6 +1392,8 @@ static int add_directory(sd_journal *j, const char *prefix, const char *dirname)
check_network(j, dirfd(d));
return 0;
......@@ -1453,6 +1481,8 @@ static int add_root_directory(sd_journal *j, const char *p) {
check_network(j, dirfd(d));
return 0;
......@@ -2079,6 +2109,14 @@ _public_ int sd_journal_wait(sd_journal *j, uint64_t timeout_usec) {
return determine_change(j);
if (j->on_network) {
/* If we are on the network we need to regularly check
* for changes manually */
if (timeout_usec == (uint64_t) -1 || timeout_usec > JOURNAL_FILES_RECHECK_USEC)
do {
r = fd_wait_for_event(j->inotify_fd, POLLIN, timeout_usec);
} while (r == -EINTR);
......@@ -2344,3 +2382,10 @@ _public_ void sd_journal_restart_unique(sd_journal *j) {
j->unique_file = NULL;
j->unique_offset = 0;
_public_ int sd_journal_reliable_fd(sd_journal *j) {
if (!j)
return -EINVAL;
return !j->on_network;
......@@ -249,3 +249,7 @@ static inline int name_to_handle_at(int fd, const char *name, struct file_handle
# error neither secure_getenv nor __secure_getenv are available
# endif
#define CIFS_MAGIC_NUMBER 0xFF534D42
......@@ -124,6 +124,7 @@ int sd_journal_enumerate_unique(sd_journal *j, const void **data, size_t *l);
void sd_journal_restart_unique(sd_journal *j);
int sd_journal_get_fd(sd_journal *j);
int sd_journal_reliable_fd(sd_journal *j);
int sd_journal_process(sd_journal *j);
int sd_journal_wait(sd_journal *j, uint64_t timeout_usec);
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