| <?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 --> |
| |
| <refentry id="sd_bus_message_read" |
| xmlns:xi="http://www.w3.org/2001/XInclude"> |
| |
| <refentryinfo> |
| <title>sd_bus_message_read</title> |
| <productname>systemd</productname> |
| </refentryinfo> |
| |
| <refmeta> |
| <refentrytitle>sd_bus_message_read</refentrytitle> |
| <manvolnum>3</manvolnum> |
| </refmeta> |
| |
| <refnamediv> |
| <refname>sd_bus_message_read</refname> |
| <refname>sd_bus_message_readv</refname> |
| <refname>sd_bus_message_peek_type</refname> |
| |
| <refpurpose>Read a sequence of values from a message</refpurpose> |
| </refnamediv> |
| |
| <refsynopsisdiv> |
| <funcsynopsis> |
| <funcsynopsisinfo>#include <systemd/sd-bus.h></funcsynopsisinfo> |
| |
| <funcprototype> |
| <funcdef>int <function>sd_bus_message_read</function></funcdef> |
| <paramdef>sd_bus_message *<parameter>m</parameter></paramdef> |
| <paramdef>const char *<parameter>types</parameter></paramdef> |
| <paramdef>...</paramdef> |
| </funcprototype> |
| |
| <funcprototype> |
| <funcdef>int <function>sd_bus_message_readv</function></funcdef> |
| <paramdef>sd_bus_message *<parameter>m</parameter></paramdef> |
| <paramdef>const char *<parameter>types</parameter></paramdef> |
| <paramdef>va_list <parameter>ap</parameter></paramdef> |
| </funcprototype> |
| |
| <funcprototype> |
| <funcdef>int <function>sd_bus_message_peek_type</function></funcdef> |
| <paramdef>sd_bus_message *<parameter>m</parameter></paramdef> |
| <paramdef>char *<parameter>type</parameter></paramdef> |
| <paramdef>const char **<parameter>contents</parameter></paramdef> |
| </funcprototype> |
| </funcsynopsis> |
| </refsynopsisdiv> |
| |
| <refsect1> |
| <title>Description</title> |
| |
| <para><function>sd_bus_message_read()</function> reads a sequence of fields from the D-Bus message object |
| <parameter>m</parameter> and advances the read position in the message. The type string |
| <parameter>types</parameter> describes the types of items expected in the message and the field arguments |
| that follow. The type string may be <constant>NULL</constant> or empty, in which case nothing is read. |
| </para> |
| |
| <para>The type string is composed of the elements described in |
| <citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
| i.e. basic and container types. It must contain zero or more single "complete types". The type string is |
| <constant>NUL</constant>-terminated.</para> |
| |
| <para>For each type specified in the type string, one or more arguments need to be specified after the |
| <parameter>types</parameter> parameter, in the same order. The arguments must be pointers to appropriate |
| types (a pointer to <type>int8_t</type> for a <literal>y</literal> in the type string, a pointer to |
| <type>int32_t</type> for an <literal>i</literal>, a pointer to <type>const char*</type> for an |
| <literal>s</literal>, ...) which are set based on the values in the message. As an exception, in case of |
| array and variant types, the first argument is an "input" argument that further specifies how the message |
| should be read. See the table below for a complete list of allowed arguments and their types. Note that, |
| if the basic type is a pointer (e.g., <type>const char *</type> in the case of a string), the argument is |
| a pointer to a pointer, and also the pointer value that is written is only borrowed and the contents must |
| be copied if they are to be used after the end of the messages lifetime. If the type is |
| <literal>h</literal> (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.</para> |
| |
| <para>Each argument may also be <constant>NULL</constant>, in which case the value is read and ignored. |
| </para> |
| |
| <table> |
| <title>Item type specifiers</title> |
| |
| <tgroup cols='5'> |
| <colspec colname='specifier' /> |
| <colspec colname='constant' /> |
| <colspec colname='description' /> |
| <colspec colname='type1' /> |
| <colspec colname='type2' /> |
| <thead> |
| <row> |
| <entry>Specifier</entry> |
| <entry>Constant</entry> |
| <entry>Description</entry> |
| <entry>Type of the first argument</entry> |
| <entry>Types of the subsequent arguments, if any</entry> |
| </row> |
| </thead> |
| |
| <tbody> |
| <xi:include href="sd_bus_message_read_basic.xml" xpointer="xpointer(//table[@id='format-specifiers']//tbody/*)" /> |
| |
| <row> |
| <entry><literal>a</literal></entry> |
| <entry><constant>SD_BUS_TYPE_ARRAY</constant></entry> |
| <entry>array</entry> |
| <entry><type>int</type>, which specifies the expected length <parameter>n</parameter> of the array</entry> |
| <entry><parameter>n</parameter> sets of arguments appropriate for the array element type</entry> |
| </row> |
| |
| <row> |
| <entry><literal>v</literal></entry> |
| <entry><constant>SD_BUS_TYPE_VARIANT</constant></entry> |
| <entry>variant</entry> |
| <entry>signature string</entry> |
| <entry>arguments appropriate for the types specified by the signature</entry> |
| </row> |
| |
| <row> |
| <entry><literal>(</literal></entry> |
| <entry><constant>SD_BUS_TYPE_STRUCT_BEGIN</constant></entry> |
| <entry>array start</entry> |
| <entry morerows="1" namest="type1" nameend="type2">arguments appropriate for the structure elements</entry> |
| </row> |
| <row> |
| <entry><literal>)</literal></entry> |
| <entry><constant>SD_BUS_TYPE_STRUCT_END</constant></entry> |
| <entry>array end</entry> |
| </row> |
| |
| <row> |
| <entry><literal>{</literal></entry> |
| <entry><constant>SD_BUS_TYPE_DICT_ENTRY_BEGIN</constant></entry> |
| <entry>dictionary entry start</entry> |
| <entry morerows="1">arguments appropriate for the first type in the pair</entry> |
| <entry morerows="1">arguments appropriate for the second type in the pair</entry> |
| </row> |
| <row> |
| <entry><literal>}</literal></entry> |
| <entry><constant>SD_BUS_TYPE_DICT_ENTRY_END</constant></entry> |
| <entry>dictionary entry end</entry> |
| </row> |
| </tbody> |
| </tgroup> |
| </table> |
| |
| <para>If objects of the specified types are not present at the current position in the message, an error |
| is returned.</para> |
| |
| <para>The <function>sd_bus_message_readv()</function> is equivalent to the |
| <function>sd_bus_message_read()</function>, except that it is called with a <literal>va_list</literal> |
| instead of a variable number of arguments. This function does not call the <function>va_end()</function> |
| macro. Because it invokes the <function>va_arg()</function> macro, the value of <parameter>ap</parameter> |
| is undefined after the call.</para> |
| |
| <para><function>sd_bus_message_peek_type()</function> determines the type of the next element in |
| <parameter>m</parameter> to be read by <function>sd_bus_message_read()</function> or similar functions. |
| On success, the type is stored in <parameter>type</parameter>, if it is not <constant>NULL</constant>. |
| If the type is a container type, the type of its elements is stored in <parameter>contents</parameter>, |
| if it is not <constant>NULL</constant>. If this function successfully determines the type of the next |
| element in <parameter>m</parameter>, it returns a positive integer. If there are no more elements to be |
| read, it returns zero.</para> |
| </refsect1> |
| |
| <refsect1> |
| <title>Return Value</title> |
| |
| <para>On success, these functions return a non-negative integer. On failure, they return a negative |
| errno-style error code.</para> |
| |
| <xi:include href="sd_bus_message_read_basic.xml" xpointer="errors" /> |
| </refsect1> |
| |
| <xi:include href="libsystemd-pkgconfig.xml" /> |
| |
| <refsect1> |
| <title>Examples</title> |
| |
| <para>Read a single basic type (a 64-bit integer): |
| </para> |
| |
| <programlisting>sd_bus_message *m; |
| int64_t x; |
| |
| sd_bus_message_read(m, "x", &x);</programlisting> |
| |
| <para>Read a boolean value:</para> |
| |
| <programlisting>sd_bus_message *m; |
| int x; /* Do not use C99 'bool' type here, it's typically smaller |
| in memory and would cause memory corruption */ |
| |
| sd_bus_message_read(m, "b", &x);</programlisting> |
| |
| <para>Read all types of integers:</para> |
| |
| <programlisting>uint8_t y; |
| int16_t n; |
| uint16_t q; |
| int32_t i; |
| uint32_t u; |
| int32_t x; |
| uint32_t t; |
| double d; |
| |
| sd_bus_message_read(m, "ynqiuxtd", &y, &n, &q, &i, &u, &x, &t, &d);</programlisting> |
| |
| <para>Read a structure composed of a string and a D-Bus path:</para> |
| |
| <programlisting>const char *s, *p; |
| |
| sd_bus_message_read(m, "(so)", &s, &p); |
| </programlisting> |
| |
| <para>Read a variant, with the real type "gt" (signature, unsigned integer): |
| </para> |
| |
| <programlisting>const char *s; |
| uint64_t *v; |
| |
| sd_bus_message_read(m, "v", "gt", &s, &v);</programlisting> |
| |
| <para>Read a dictionary containing three pairs of type {integer=>string}: |
| </para> |
| |
| <programlisting>int i, j, k; |
| const char *s, *t, *u; |
| |
| sd_bus_message_read(m, "a{is}", 3, &i, &s, &j, &t, &k, &u); |
| </programlisting> |
| |
| <para>Read a single file descriptor, and duplicate it in order to keep it open after the message is |
| freed.</para> |
| |
| <programlisting>sd_bus_message *m; |
| int fd, fd_copy; |
| |
| sd_bus_message_read(m, "h", &fd); |
| fd_copy = fcntl(fd, FD_DUPFD_CLOEXEC, 3);</programlisting> |
| </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_read_basic</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
| <citerefentry><refentrytitle>sd_bus_message_skip</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
| <citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
| <citerefentry><refentrytitle>sd_bus_message_enter_container</refentrytitle><manvolnum>3</manvolnum></citerefentry> |
| </para> |
| </refsect1> |
| |
| </refentry> |