fileio: add brief explanations for flags

[thirdparty/systemd.git] / man / sd_bus_message_read_basic.xml

<?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+

  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 &lt;systemd/sd-bus.h&gt;</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 messages lifetime. Similarly, during the lifetime of such a pointer, the
      message must not be modified. 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 0 or
      a positive integer. 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>