diff options
Diffstat (limited to 'man')
| -rw-r--r-- | man/sd_journal_get_fd.xml | 130 |
1 files changed, 91 insertions, 39 deletions
diff --git a/man/sd_journal_get_fd.xml b/man/sd_journal_get_fd.xml index f29aeaab23..33d2980b3b 100644 --- a/man/sd_journal_get_fd.xml +++ b/man/sd_journal_get_fd.xml @@ -45,9 +45,10 @@ <refnamediv> <refname>sd_journal_get_fd</refname> <refname>sd_journal_get_events</refname> - <refname>sd_journal_reliable_fd</refname> + <refname>sd_journal_get_timeout</refname> <refname>sd_journal_process</refname> <refname>sd_journal_wait</refname> + <refname>sd_journal_reliable_fd</refname> <refname>SD_JOURNAL_NOP</refname> <refname>SD_JOURNAL_APPEND</refname> <refname>SD_JOURNAL_INVALIDATE</refname> @@ -70,8 +71,9 @@ </funcprototype> <funcprototype> - <funcdef>int <function>sd_journal_reliable_fd</function></funcdef> + <funcdef>int <function>sd_journal_get_timeout</function></funcdef> <paramdef>sd_journal* <parameter>j</parameter></paramdef> + <paramdef>uint64_t* <parameter>timeout_usec</parameter></paramdef> </funcprototype> <funcprototype> @@ -85,6 +87,11 @@ <paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef> </funcprototype> + <funcprototype> + <funcdef>int <function>sd_journal_reliable_fd</function></funcdef> + <paramdef>sd_journal* <parameter>j</parameter></paramdef> + </funcprototype> + </funcsynopsis> </refsynopsisdiv> @@ -103,16 +110,15 @@ 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 + 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 <function>sd_journal_reliable_fd()</function>, - below.</para> + below. <function>sd_journal_get_timeout()</function> + will ensure in these cases that wake-ups happen + frequently enough for changes to be noticed, although + with a certain latency.</para> <para><function>sd_journal_get_events()</function> will return the <function>poll()</function> mask to @@ -122,17 +128,36 @@ 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 - <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><function>sd_journal_get_timeout()</function> + will return a timeout value for usage in <function>poll()</function>. This returns a value in microseconds since the epoch of CLOCK_MONOTONIC for timing out <function>poll()</function> in <literal>timeout_usec</literal>. See + <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry> + for details about + <literal>CLOCK_MONOTONIC</literal>. If there's no + timeout to wait for this will fill in + <literal>(uint64_t) -1</literal> instead. Note that + <function>poll()</function> 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:</para> + + <programlisting>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; +}</programlisting> + + <para>The code above does not do any error checking + for brevity's sake. The calculated <literal>msec</literal> + integer can be passed directly as + <function>poll()</function>'s timeout + parameter.</para> <para>After each <function>poll()</function> wake-up <function>sd_journal_process()</function> needs to be @@ -143,22 +168,37 @@ <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_get_timeout()</function> and <function>sd_journal_process()</function> is <function>sd_journal_wait()</function>. It will - 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>sd_journal_get_fd()</function>, + synchronously wait until the journal gets changed. 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>sd_journal_get_fd()</function>, <function>sd_journal_get_events()</function>, - <function>sd_journal_reliable_fd()</function>, + <function>sd_journal_get_timeout()</function>, <function>poll()</function> and <function>sd_journal_process()</function> into one.</para> + + <para><function>sd_journal_reliable_fd()</function> + may be used to check whether the wakeup events from + the file descriptor returned by + <function>sd_journal_get_fd()</function> 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 + <function>sd_journal_get_timeout()</function> on these + file systems will ask for timeouts explicitly + anyway.</para> </refsect1> <refsect1> @@ -176,11 +216,9 @@ <para><function>sd_journal_reliable_fd()</function> 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> + will generate wake-ups immediately for all journal + changes. Returns 0 if there might be a latency + involved.</para> <para><function>sd_journal_process()</function> and <function>sd_journal_wait()</function> return one of @@ -270,9 +308,22 @@ int main(int argc, char *argv[]) { 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, sd_journal_reliable_fd(j) > 0 ? -1 : 2000); + poll(&pollfd, 1, msec); return sd_journal_process(j); } </programlisting> @@ -286,7 +337,8 @@ int wait_for_changes(sd_journal *j) { <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>, - <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry> + <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>, + <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry> </para> </refsect1> |