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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
SPDX-License-Identifier: LGPL-2.1-or-later
Copyright © 2016 Julian Orth
-->
<refentry id="sd_bus_message_read_basic">
<refentryinfo>
<title>sd_bus_message_read_basic</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>sd_bus_message_read_basic</refentrytitle>
<manvolnum>3</manvolnum>
</refmeta>
<refnamediv>
<refname>sd_bus_message_read_basic</refname>
<refpurpose>Read a basic type from a message</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include <systemd/sd-bus.h></funcsynopsisinfo>
<funcprototype>
<funcdef>int <function>sd_bus_message_read_basic</function></funcdef>
<paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
<paramdef>char <parameter>type</parameter></paramdef>
<paramdef>void *<parameter>p</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<function>sd_bus_message_read_basic()</function> reads a basic type from a
message and advances the read position in the message. The set of basic
types and their ascii codes passed in <parameter>type</parameter> are
described in the <ulink
url="https://dbus.freedesktop.org/doc/dbus-specification.html">D-Bus
Specification</ulink>.
</para>
<para>
If <parameter>p</parameter> is not <constant>NULL</constant>, it should contain a pointer to an
appropriate object. For example, if <parameter>type</parameter> is <constant>'y'</constant>, the object
passed in <parameter>p</parameter> should have type <type>uint8_t *</type>. If
<parameter>type</parameter> is <constant>'s'</constant>, the object passed in <parameter>p</parameter>
should have type <type>const char **</type>. Note that, if the basic type is a pointer (e.g.,
<type>const char *</type> in the case of a string), the pointer is only borrowed and the contents must
be copied if they are to be used after the end of the message's lifetime. Similarly, during the
lifetime of such a pointer, the message must not be modified. If <parameter>type</parameter> is
<constant>'h'</constant> (UNIX file descriptor), the descriptor is not duplicated by this call and the
returned descriptor remains in possession of the message object, and needs to be duplicated by the
caller in order to keep an open reference to it after the message object is freed (for example by
calling <literal>fcntl(fd, FD_DUPFD_CLOEXEC, 3)</literal>). See the table below for a complete list of
allowed types.
</para>
<table id='format-specifiers'>
<title>Item type specifiers</title>
<tgroup cols='4'>
<colspec colname='specifier' />
<colspec colname='constant' />
<colspec colname='description' />
<colspec colname='ctype' />
<thead>
<row>
<entry>Specifier</entry>
<entry>Constant</entry>
<entry>Description</entry>
<entry>Expected C Type</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>y</literal></entry>
<entry><constant>SD_BUS_TYPE_BYTE</constant></entry>
<entry>8bit unsigned integer</entry>
<entry><type>uint8_t *</type></entry>
</row>
<row>
<entry><literal>b</literal></entry>
<entry><constant>SD_BUS_TYPE_BOOLEAN</constant></entry>
<entry>boolean</entry>
<entry><type>int *</type> (NB: not <type>bool *</type>)</entry>
</row>
<row>
<entry><literal>n</literal></entry>
<entry><constant>SD_BUS_TYPE_INT16</constant></entry>
<entry>16bit signed integer</entry>
<entry><type>int16_t *</type></entry>
</row>
<row>
<entry><literal>q</literal></entry>
<entry><constant>SD_BUS_TYPE_UINT16</constant></entry>
<entry>16bit unsigned integer</entry>
<entry><type>uint16_t *</type></entry>
</row>
<row>
<entry><literal>i</literal></entry>
<entry><constant>SD_BUS_TYPE_INT32</constant></entry>
<entry>32bit signed integer</entry>
<entry><type>int32_t *</type></entry>
</row>
<row>
<entry><literal>u</literal></entry>
<entry><constant>SD_BUS_TYPE_UINT32</constant></entry>
<entry>32bit unsigned integer</entry>
<entry><type>uint32_t *</type></entry>
</row>
<row>
<entry><literal>x</literal></entry>
<entry><constant>SD_BUS_TYPE_INT64</constant></entry>
<entry>64bit signed integer</entry>
<entry><type>int64_t *</type></entry>
</row>
<row>
<entry><literal>t</literal></entry>
<entry><constant>SD_BUS_TYPE_UINT64</constant></entry>
<entry>64bit unsigned integer</entry>
<entry><type>uint64_t *</type></entry>
</row>
<row>
<entry><literal>d</literal></entry>
<entry><constant>SD_BUS_TYPE_DOUBLE</constant></entry>
<entry>IEEE 754 double precision floating-point</entry>
<entry><type>double *</type></entry>
</row>
<row>
<entry><literal>s</literal></entry>
<entry><constant>SD_BUS_TYPE_STRING</constant></entry>
<entry>UTF-8 string</entry>
<entry><type>const char **</type></entry>
</row>
<row>
<entry><literal>o</literal></entry>
<entry><constant>SD_BUS_TYPE_OBJECT_PATH</constant></entry>
<entry>D-Bus object path string</entry>
<entry><type>const char **</type></entry>
</row>
<row>
<entry><literal>g</literal></entry>
<entry><constant>SD_BUS_TYPE_SIGNATURE</constant></entry>
<entry>D-Bus signature string</entry>
<entry><type>const char **</type></entry>
</row>
<row>
<entry><literal>h</literal></entry>
<entry><constant>SD_BUS_TYPE_UNIX_FD</constant></entry>
<entry>UNIX file descriptor</entry>
<entry><type>int *</type></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
If there is no object of the specified type at the current position in the
message, an error is returned.
</para>
</refsect1>
<refsect1>
<title>Return Value</title>
<para>
On success, <function>sd_bus_message_read_basic()</function> returns a positive integer.
If the end of the currently opened array has been reached, it returns 0.
On failure, it returns a negative errno-style error code.
</para>
<refsect2 id='errors'>
<title>Errors</title>
<para>Returned errors may indicate the following problems:</para>
<variablelist>
<varlistentry>
<term><constant>-EINVAL</constant></term>
<listitem><para>Specified type string is invalid or the message parameter is
<constant>NULL</constant>.</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-ENXIO</constant></term>
<listitem><para>The message does not contain the specified type at current position.
</para></listitem>
</varlistentry>
<varlistentry>
<term><constant>-EBADMSG</constant></term>
<listitem><para>The message cannot be parsed.</para></listitem>
</varlistentry>
</variablelist>
</refsect2>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_bus_message_append_basic</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_bus_message_skip</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_bus_message_read</refentrytitle><manvolnum>3</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
|
