+++ /dev/null
-<?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">
-
-<!--
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-
- Copyright 2012 Lennart Poettering
-
- systemd is free software; you can redistribute it and/or modify it
- under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 2.1 of the License, or
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- Lesser General Public License for more details.
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- along with systemd; If not, see <http://www.gnu.org/licenses/>.
--->
-
-<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_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;
-}</programlisting>
-
- <para>Waiting with <function>poll()</function> (this
- example lacks all error checking for the sake of
- simplicity):</para>
-
- <programlisting>#include <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);
-}</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