<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
- "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
<refentry id="sd_journal_get_fd">
- <refentryinfo>
- <title>sd_journal_get_fd</title>
- <productname>systemd</productname>
-
- <authorgroup>
- <author>
- <contrib>Developer</contrib>
- <firstname>Lennart</firstname>
- <surname>Poettering</surname>
- <email>lennart@poettering.net</email>
- </author>
- </authorgroup>
- </refentryinfo>
-
- <refmeta>
- <refentrytitle>sd_journal_get_fd</refentrytitle>
- <manvolnum>3</manvolnum>
- </refmeta>
-
- <refnamediv>
- <refname>sd_journal_get_fd</refname>
- <refname>sd_journal_reliable_fd</refname>
- <refname>sd_journal_process</refname>
- <refname>sd_journal_wait</refname>
- <refname>SD_JOURNAL_NOP</refname>
- <refname>SD_JOURNAL_APPEND</refname>
- <refname>SD_JOURNAL_INVALIDATE</refname>
- <refpurpose>Journal change notification
- interface</refpurpose>
- </refnamediv>
-
- <refsynopsisdiv>
- <funcsynopsis>
- <funcsynopsisinfo>#include <systemd/sd-journal.h></funcsynopsisinfo>
-
- <funcprototype>
- <funcdef>int <function>sd_journal_get_fd</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>
-
- <funcprototype>
- <funcdef>int <function>sd_journal_process</function></funcdef>
- <paramdef>sd_journal* <parameter>j</parameter></paramdef>
- </funcprototype>
-
- <funcprototype>
- <funcdef>int <function>sd_journal_wait</function></funcdef>
- <paramdef>sd_journal* <parameter>j</parameter></paramdef>
- <paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef>
- </funcprototype>
-
- </funcsynopsis>
- </refsynopsisdiv>
-
- <refsect1>
- <title>Description</title>
-
- <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
- <function>sd_journal_reliable_fd()</function>,
- below.</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>After each POLLIN 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
- 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_reliable_fd()</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>,
- <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
- code.</para>
-
- <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>
-
- <para><function>sd_journal_process()</function> and
- <function>sd_journal_wait()</function> return one of
- <literal>SD_JOURNAL_NOP</literal>,
- <literal>SD_JOURNAL_APPEND</literal> or
- <literal>SD_JOURNAL_INVALIDATE</literal> on success or
- a negative errno-style error code. If
- <literal>SD_JOURNAL_NOP</literal> is returned the
- journal didn't change since the last invocation. If
- <literal>SD_JOURNAL_APPEND</literal> is returned new
- entries have been appended to the end of the
- journal. If <literal>SD_JOURNAL_INVALIDATE</literal>
- 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 <literal>SD_JOURNAL_APPEND</literal> it is
- sufficient to simply continue reading at the previous
- end of the journal.</para>
- </refsect1>
-
- <refsect1>
- <title>Notes</title>
-
- <para>The <function>sd_journal_get_fd()</function>,
- <function>sd_journal_reliable_fd()</function>,
- <function>sd_journal_process()</function> and
- <function>sd_journal_wait()</function> interfaces are
- available as shared library, which can be compiled and
- linked to with the
- <literal>libsystemd-journal</literal>
- <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
- file.</para>
- </refsect1>
-
- <refsect1>
- <title>Examples</title>
-
- <para>Iterating through the journal, in a live view tracking all changes:</para>
-
- <programlisting>#include <stdio.h>
+ <refentryinfo>
+ <title>sd_journal_get_fd</title>
+ <productname>systemd</productname>
+
+ <authorgroup>
+ <author>
+ <contrib>Developer</contrib>
+ <firstname>Lennart</firstname>
+ <surname>Poettering</surname>
+ <email>lennart@poettering.net</email>
+ </author>
+ </authorgroup>
+ </refentryinfo>
+
+ <refmeta>
+ <refentrytitle>sd_journal_get_fd</refentrytitle>
+ <manvolnum>3</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+ <refname>sd_journal_get_fd</refname>
+ <refname>sd_journal_get_events</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>
+ <refpurpose>Journal change notification
+ interface</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+ <funcsynopsis>
+ <funcsynopsisinfo>#include <systemd/sd-journal.h></funcsynopsisinfo>
+
+ <funcprototype>
+ <funcdef>int <function>sd_journal_get_fd</function></funcdef>
+ <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+ </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_get_timeout</function></funcdef>
+ <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+ <paramdef>uint64_t *<parameter>timeout_usec</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_journal_process</function></funcdef>
+ <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_journal_wait</function></funcdef>
+ <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+ <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>
+
+ <refsect1>
+ <title>Description</title>
+
+ <para><function>sd_journal_get_fd()</function> 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
+ <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 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.
+ <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 wait for. This function will
+ return a combination of <constant>POLLIN</constant> and
+ <constant>POLLOUT</constant> and similar to fill into the
+ <literal>.events</literal> field of <varname>struct
+ pollfd</varname>.</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
+ <constant>CLOCK_MONOTONIC</constant> for timing out
+ <function>poll()</function> in <varname>timeout_usec</varname>.
+ See
+ <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+ for details about <constant>CLOCK_MONOTONIC</constant>. If there
+ is no timeout to wait for, this will fill in <constant>(uint64_t)
+ -1</constant> 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 <varname>msec</varname> 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 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_get_timeout()</function> and
+ <function>sd_journal_process()</function> is
+ <function>sd_journal_wait()</function>. It will synchronously wait
+ until the journal gets changed. The maximum time this call sleeps
+ may be controlled with the <parameter>timeout_usec</parameter>
+ parameter. Pass <constant>(uint64_t) -1</constant> to wait
+ indefinitely. Internally this call simply combines
+ <function>sd_journal_get_fd()</function>,
+ <function>sd_journal_get_events()</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>
+ <title>Return Value</title>
+
+ <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 <constant>POLLIN</constant>,
+ <constant>POLLOUT</constant> and suchlike on success or a negative
+ errno-style error code.</para>
+
+ <para><function>sd_journal_reliable_fd()</function> returns a
+ positive integer if the file descriptor returned by
+ <function>sd_journal_get_fd()</function> 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
+ <constant>SD_JOURNAL_NOP</constant>,
+ <constant>SD_JOURNAL_APPEND</constant> or
+ <constant>SD_JOURNAL_INVALIDATE</constant> on success or a
+ negative errno-style error code. If
+ <constant>SD_JOURNAL_NOP</constant> is returned, the journal did
+ not change since the last invocation. If
+ <constant>SD_JOURNAL_APPEND</constant> is returned, new entries
+ have been appended to the end of the journal. If
+ <constant>SD_JOURNAL_INVALIDATE</constant>, 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 <constant>SD_JOURNAL_APPEND</constant>, it is
+ sufficient to simply continue reading at the previous end of the
+ journal.</para>
+ </refsect1>
+
+ <refsect1>
+ <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 available as
+ a shared library, which can be compiled and linked to with the
+ <constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+ file.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>Examples</title>
+
+ <para>Iterating through the journal, in a live view tracking all
+ changes:</para>
+
+ <programlisting>#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;
+ 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;
}</programlisting>
-
<para>Waiting with <function>poll()</function> (this
- example lacks all error checking for the sake of
- simplicity):</para>
+ <para>Waiting with <function>poll()</function> (this
+ example lacks all error checking for the sake of
+ simplicity):</para>
- <programlisting>#include <sys/poll.h>
+ <programlisting>#include <poll.h>
#include <systemd/sd-journal.h>
int wait_for_changes(sd_journal *j) {
- struct pollfd pollfd;
- pollfd.fd = sd_journal_get_fd(j);
- pollfd.events = POLLIN;
- poll(&pollfd, 1, sd_journal_reliable_fd(j) > 0 ? -1 : 2000);
- return sd_journal_process(j);
-}
- </programlisting>
- </refsect1>
-
-
- <refsect1>
- <title>See Also</title>
-
- <para>
- <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
- <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>
- </para>
- </refsect1>
+ 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);
+}</programlisting>
+ </refsect1>
+
+ <refsect1>
+ <title>See Also</title>
+
+ <para>
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <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>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+ </para>
+ </refsect1>
</refentry>
~ianmdlvl / elogind.git / blobdiff