journal: add public API call sd_journal_get_events()

This function should be used when filling in "struct pollfd"'s .events
field for watching the journal. It will always return POLLIN for now,
but we should keep our options open to change this later on.

This mimics libsystemd-bus' sd_bus_get_events() call with the same
purpose.

1 files changed, 45 insertions, 24 deletions

diff --git a/man/sd_journal_get_fd.xml b/man/sd_journal_get_fd.xml
index 3fc9c5f5b5..e57431ea9e 100644
--- a/man/sd_journal_get_fd.xml
+++ b/man/sd_journal_get_fd.xml
@@ -44,6 +44,7 @@
 
         <refnamediv>
                 <refname>sd_journal_get_fd</refname>
+                <refname>sd_journal_get_events</refname>
                 <refname>sd_journal_reliable_fd</refname>
                 <refname>sd_journal_process</refname>
                 <refname>sd_journal_wait</refname>
@@ -64,6 +65,11 @@
                         </funcprototype>
 
                         <funcprototype>
+                                <funcdef>int <function>sd_journal_get_events</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
                                 <funcdef>int <function>sd_journal_reliable_fd</function></funcdef>
                                 <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                         </funcprototype>
@@ -87,26 +93,35 @@
 
                 <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, 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
-                <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>
-                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
+                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
+                <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>. Use
+                <function>sd_journal_get_events()</function> 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 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
                 <function>sd_journal_reliable_fd()</function>,
                 below.</para>
 
+                <para><function>sd_journal_get_events()</function>
+                will return the
+                <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+                mask to wait for. This function will return a
+                combination of POLLIN and POLLOUT and similar to fill
+                into the <literal>.events</literal> field of
+                <literal>struct pollfd</literal>.</para>
+
                 <para><function>sd_journal_reliable_fd()</function>
                 may be used to check whether the wakeup events from
                 the file descriptor returned by
@@ -119,15 +134,15 @@
                 <function>sd_journal_get_fd()</function> are
                 sufficient as only source for wake-ups.</para>
 
-                <para>After each POLLIN wake-up
+                <para>After each <literal>poll()</literal> wake-up
                 <function>sd_journal_process()</function> needs to be
-                called to process events and reset the readable state
-                of the file descriptor. This call will also indicate
+                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).</para>
 
                 <para>A synchronous alternative for using
                 <function>sd_journal_get_fd()</function>,
+                <function>sd_journal_get_events()</function>,
                 <function>sd_journal_reliable_fd()</function> and
                 <function>sd_journal_process()</function> is
                 <function>sd_journal_wait()</function>. It will
@@ -139,17 +154,23 @@
                 <literal>(uint64_t) -1</literal> to wait
                 indefinitely. Internally this call simply combines
                 <function>sd_journal_get_fd()</function>,
+                <function>sd_journal_get_events()</function>,
                 <function>sd_journal_reliable_fd()</function>,
                 <function>poll()</function> and
                 <function>sd_journal_process()</function> into
                 one.</para>
-
         </refsect1>
 
         <refsect1>
                 <title>Return Value</title>
 
-                <para><function>sd_journal_get_fd()</function> returns a valid file descriptor on success or a negative errno-style error
+                <para><function>sd_journal_get_fd()</function> returns
+                a valid file descriptor on success or a negative
+                errno-style error code.</para>
+
+                <para><function>sd_journal_get_events()</function>
+                returns a combination of POLLIN, POLLOUT and suchlike
+                on success or a negative errno-style error
                 code.</para>
 
                 <para><function>sd_journal_reliable_fd()</function>
@@ -184,6 +205,7 @@
                 <title>Notes</title>
 
                 <para>The <function>sd_journal_get_fd()</function>,
+                <function>sd_journal_get_events()</function>,
                 <function>sd_journal_reliable_fd()</function>,
                 <function>sd_journal_process()</function> and
                 <function>sd_journal_wait()</function> interfaces are
@@ -249,14 +271,13 @@ int main(int argc, char *argv[]) {
 int wait_for_changes(sd_journal *j) {
         struct pollfd pollfd;
         pollfd.fd = sd_journal_get_fd(j);
-        pollfd.events = POLLIN;
+        pollfd.events = sd_journal_get_events(j);
         poll(&amp;pollfd, 1, sd_journal_reliable_fd(j) &gt; 0 ? -1 : 2000);
         return sd_journal_process(j);
 }
                 </programlisting>
         </refsect1>
 
-
         <refsect1>
                 <title>See Also</title>