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man: extend documentation on sd_bus_add_match a bit()

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Explain briefly how the concept of "sd_bus_slot" works.

This recently came up on the mailing list, hence let's document this for
the next time.
This commit is contained in:
Lennart Poettering 2017-05-18 18:24:17 +02:00 committed by Zbigniew Jędrzejewski-Szmek
parent cb4d8b2d05
commit 6b7af82122
1 changed files with 25 additions and 18 deletions
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43
man/sd_bus_add_match.xml
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@ -73,26 +73,34 @@
<title>Description</title>
<para>
<function>sd_bus_add_match()</function> adds a match rule used to dispatch
incoming messages. The syntax of the rule passed in
<parameter>match</parameter> is described in the
<ulink url="https://dbus.freedesktop.org/doc/dbus-specification.html">D-Bus Specification</ulink>.
<function>sd_bus_add_match()</function> installs a match rule for incoming messages received on the specified bus
connection object <parameter>bus</parameter>. The syntax of the match rule expression passed in
<parameter>match</parameter> is described in the <ulink
url="https://dbus.freedesktop.org/doc/dbus-specification.html">D-Bus Specification</ulink>. The specified handler
function <parameter>callback</parameter> is called for eaching incoming message matching the specified
expression, the <parameter>userdata</parameter> parameter is passed as-is to the callback function.
</para>
<para>
The message <parameter>m</parameter> passed to the callback is only
borrowed, that is, the callback should not call
<citerefentry><refentrytitle>sd_bus_message_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
on it. If the callback wants to hold on to the message beyond the lifetime
of the callback, it needs to call
<citerefentry><refentrytitle>sd_bus_message_ref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
to create a new reference.
On success, and if non-<constant>NULL</constant>, the <parameter>slot</parameter> return parameter will be set to
a slot object that may be used as a reference to the installed match, and may be utilized to remove it again at a
later time with
<citerefentry><refentrytitle>sd_bus_slot_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>. If
specified as <constant>NULL</constant> the lifetime of the match is bound to the lifetime of the bus object itself, and the match
cannot be removed independently.
</para>
<para>
If an error occurs during the callback invocation, the callback should
return a negative error number. If it wants other callbacks that match the
same rule to be called, it should return 0. Otherwise it should return a
The message <parameter>m</parameter> passed to the callback is only borrowed, that is, the callback should not
call <citerefentry><refentrytitle>sd_bus_message_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry> on
it. If the callback wants to hold on to the message beyond the lifetime of the callback, it needs to call
<citerefentry><refentrytitle>sd_bus_message_ref</refentrytitle><manvolnum>3</manvolnum></citerefentry> to create
a new reference.
</para>
<para>
If an error occurs during the callback invocation, the callback should return a negative error number. If it
wants other callbacks that match the same rule to be called, it should return 0. Otherwise it should return a
positive integer.
</para>
</refsect1>
@ -101,9 +109,8 @@
<title>Return Value</title>
<para>
On success, <function>sd_bus_add_match()</function> returns 0 or a
positive integer. On failure, it returns a negative errno-style error
code.
On success, <function>sd_bus_add_match()</function> returns 0 or a positive integer. On failure, it returns a
negative errno-style error code.
</para>
</refsect1>
@ -112,7 +119,7 @@
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>
</para>
</refsect1>