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<?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 2010 Lennart Poettering

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-->

<refentry id="systemd.special">

        <refentryinfo>
                <title>daemon</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>daemon</refentrytitle>
                <manvolnum>7</manvolnum>
        </refmeta>

        <refnamediv>
                <refname>daemon</refname>
                <refpurpose>Writing and Packaging System Daemons</refpurpose>
        </refnamediv>

        <refsect1>
                <title>Description</title>

                <para>A daemon is a service process that runs in the
                background and supervises the system or provides
                functionality to other processes. Traditionally,
                daemons are implemented following a scheme originating
                in SysV Unix. Modern daemons should follow a simpler
                yet more powerful scheme here called "new-style"
                daemons, as implemented by systemd. </para>

                <refsect2>
                        <title>SysV Daemons</title>

                        <para>When a traditional SysV daemon
                        starts, it should execute the following steps
                        as part of the initialization. Note that these
                        steps are unnecessary for new-style daemons,
                        and should only be implemented if compatibility
                        with SysV is essential.</para>

                        <orderedlist>
                                <listitem><para>Close all open file
                                descriptors except STDIN, STDOUT,
                                STDERR (i.e. the first three file
                                descriptors 0, 1, 2). This ensures
                                that no accidentally passed file
                                descriptor stays around in the daemon
                                process. On Linux this is best
                                implemented by iterating through
                                <filename>/proc/self/fd</filename>,
                                with a fallback of iterating from file
                                descriptor 3 to the value returned by
                                getrlimit() for
                                RLIMIT_NOFILE.</para></listitem>

                                <listitem><para>Reset all signal
                                handlers to their default. This is
                                best done by iterating through the
                                available signals up to the limit of
                                _NSIG and resetting them to
                                SIG_DFL.</para></listitem>

                                <listitem><para>Reset the signal mask
                                using sigprocmask().</para></listitem>

                                <listitem><para>Call fork(),
                                to create a background
                                process.</para></listitem>

                                <listitem><para>In the child, call
                                setsid() to detach from any terminal
                                and create an independent
                                session.</para></listitem>

                                <listitem><para>In the child, call
                                fork() again, to ensure the daemon can
                                never re-aquire a terminal
                                again.</para></listitem>

                                <listitem><para>Call exit() in the
                                first child, so that only the second
                                child (the actual daemon process)
                                stays around. This ensures that the
                                daemon process is reparented to
                                init/PID 1, as all daemons should
                                be.</para></listitem>

                                <listitem><para>In the daemon process,
                                connect <filename>/dev/null</filename>
                                to STDIN, STDOUT,
                                STDERR.</para></listitem>

                                <listitem><para>In the daemon process,
                                reset the umask to 0, so that the file
                                modes passed to open(), mkdir() and
                                suchlike directly control the access
                                mode of the created files and
                                directories.</para></listitem>

                                <listitem><para>In the daemon process,
                                change the current directory to the
                                root directory (/), in order to avoid
                                that the daemon involuntarily
                                blocks mount points from being
                                unmounted.</para></listitem>

                                <listitem><para>In the daemon process,
                                drop privileges, if possible and
                                applicable.</para></listitem>

                                <listitem><para>From the daemon
                                process notify the original process
                                started that initialization is
                                complete. This can be implemented via
                                an unnamed pipe or similar
                                communication channel that is created
                                before the first fork() and available
                                in both processes.</para></listitem>

                                <listitem><para>Call exit() in the
                                original process. The process that
                                invoked the daemon must be able to
                                rely that this exit() happens after
                                initialization is complete and all
                                external communication channels
                                established and
                                accessible.</para></listitem>
                        </orderedlist>

                        <para>The BSD daemon() function should not be
                        used, as it does only a subset of these steps.</para>

                        <para>A daemon that needs to provide
                        compatibility with SysV systems should
                        implement the scheme pointed out
                        above. However, it is recommended to make this
                        behaviour optional and configurable via a
                        command line argument, to ease debugging as
                        well as to simplify integration into systems
                        using systemd.</para>
                </refsect2>

                <refsect2>
                        <title>New-Style Daemons</title>

                        <para>Modern services for Linux should be
                        implemented as new-style daemons. This makes it
                        easier to supervise and control them at
                        runtime and simplifies their
                        implementation.</para>

                        <para>For developing a new-style daemon none
                        of the initialization steps recommended for
                        SysV daemons need to be implemented. New-style
                        init systems such as systemd make all of them
                        redundant. Moreover, since some of these steps
                        interfere with process monitoring, file
                        descriptor passing and other functionality of
                        the init system it is recommended not to
                        execute them when run as new-style
                        service.</para>

                        <para>It is recommended for new-style daemons
                        to implement the following:</para>

                        <orderedlist>
                                <listitem><para>If SIGTERM is
                                received, shut down the daemon and
                                exit cleanly.</para></listitem>

                                <listitem><para>If SIGHUP is received,
                                reload the configuration files, if
                                this applies.</para></listitem>

                                <listitem><para>Provide a correct exit
                                code from the main daemon process, as
                                this is used by the init system to
                                detect service errors and problems. It
                                is recommended to follow the exit code
                                scheme as defined in LSB
                                recommendations for SysV init scripts
                                (http://refspecs.freestandards.org/LSB_3.1.1/LSB-Core-generic/LSB-Core-generic/iniscrptact.html).</para></listitem>

                                <listitem><para>As much as possible,
                                rely on systemd's functionality to
                                limit the access of the daemon to
                                files, services and other
                                resources. i.e. rely on systemd's
                                resource limit control instead of
                                implementing your own, rely on
                                systemd's privilege dropping code
                                instead of implementing it in the
                                daemon, and similar.</para></listitem>

                                <listitem><para>If possible and
                                applicable expose the daemon's control
                                interface via the D-Bus IPC system and
                                grab a bus name as last step of
                                initialization.</para></listitem>

                                <listitem><para>If D-Bus is used, make
                                your daemon bus-activatable, via
                                supplying a D-Bus service activation
                                configuration file. This has multiple
                                advantages: your daemon may be started
                                lazily on-demand; it may be started in
                                parallel to other daemons requiring it
                                -- which maximizes parallelization and
                                boot-up speed; your daemon can be
                                restarted on failure, without losing
                                any bus requests, as the bus queues
                                requests for activatable
                                services.</para></listitem>

                                <listitem><para>If your daemon
                                provides services to other local
                                processes or remote clients via a
                                socket, it should be made
                                socket-activatable following the
                                scheme pointed out below. Like D-Bus
                                activation this enables on-demand
                                starting of services as well as it
                                allows improved parallelization of
                                service start-up. Also, for state-less
                                protocols (such as syslog, DNS) a
                                daemon implementing socket-based
                                activation can be restarted without
                                losing a single
                                request.</para></listitem>

                                <listitem><para>If applicable a daemon
                                should notify the init system about
                                startup completion or status
                                updates via the sd_notify()
                                interface.</para></listitem>

                                <listitem><para>Instead of using the
                                syslog() call to log directly to the
                                system logger, a new-style daemon may
                                choose to simply log to STDERR via
                                fprintf(), which is then forwarded to
                                syslog by the init system. If log
                                priorities are necessary these can be
                                encoded by prefixing individual log
                                lines with strings like "&lt;4&gt;"
                                (for log priority 4 "WARNING" in the
                                syslog priority scheme), following a
                                similar style as the Linux kernel's
                                printk() priority system. In fact, using
                                this style of logging also enables the
                                init system to optionally direct all
                                application logging to the kernel log
                                buffer (kmsg), as accessible via
                                dmesg.</para></listitem>

                        </orderedlist>
                </refsect2>

                <refsect2>
                        <title>Bus Activation</title>
                </refsect2>

                <refsect2>
                        <title>Socket Activation</title>
                </refsect2>

                <refsect2>
                        <title>Writing Service Files</title>
                </refsect2>

                <refsect2>
                        <title>Installing Service Files</title>
                </refsect2>

        </refsect1>


        <refsect1>
                <title>See Also</title>
                <para>
                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>sd_listen_fds</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>