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diff --git a/src/libsystemd/sd_journal_get_fd.xml b/src/libsystemd/sd_journal_get_fd.xml new file mode 100644 index 0000000000..61293f7f99 --- /dev/null +++ b/src/libsystemd/sd_journal_get_fd.xml @@ -0,0 +1,332 @@ +<?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"> + +<!-- + This file is part of systemd. + + 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 + (at your option) any later version. + + systemd is distributed in the hope that it will be useful, but + WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public License + 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 is 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> |