
<para>
    We use the generic type name gentype to indicate the built-in 
    OpenCL C scalar or vector integer or floating-point data types or any user defined type built 
    from these scalar and vector data types can be used as the type for the arguments to the pipe 
    functions listed in this section.
    The half scalar and vector types can only be used if the 
    <citerefentry><refentrytitle>cl_khr_fp16</refentrytitle></citerefentry> 
    extension is supported. The <type>double</type> scalar 
    and vector types can only be used if double precision is supported.
</para>

<bridgehead>General information about pipes</bridgehead>

<para>
    A pipe is identified by specifying the <code>pipe</code> 
    keyword with a type. The data type specifies the 
    size of each packet in the pipe. The <code>pipe</code> 
    keyword is a type modifier. When it is applied to 
    another type <code>T</code>, the result is a pipe type 
    whose elements (or packets) are of type <code>T</code>. The packet 
    type <code>T</code> may be any supported OpenCL C scalar 
    and vector integer or floating-point data types, or 
    a user-defined type built from these scalar and vector data types.
</para>

<para>
    The 
    <citerefentry href="qualifiers"><refentrytitle>read_only</refentrytitle></citerefentry> 
    (or <code>__read_only</code>) and 
    <citerefentry href="qualifiers"><refentrytitle>write_only</refentrytitle></citerefentry>
    (or <code>__write_only</code>) qualifiers
    must be used with the pipe qualifier when a pipe 
    is a parameter of a kernel or of a user-defined
    function to identify if a pipe can be read from 
    or written to by a kernel and its callees and
    enqueued child kernels. If no qualifier is specified, 
    <citerefentry href="qualifiers"><refentrytitle>read_only</refentrytitle></citerefentry> 
    is assumed.
</para>

<para>
    A kernel cannot read from and write to the same pipe object. Using the 
    <citerefentry href="qualifiers"><refentrytitle>read_write </refentrytitle></citerefentry>
    (or <code>__read_write</code>) qualifier with the pipe qualifier is a compilation error.
</para>

<para>
    The macro <constant>CLK_NULL_RESERVE_ID</constant> refers to an invalid reservation ID.
</para>

<para>
    NOTE: The 
    <citerefentry><refentrytitle>read_pipe</refentrytitle></citerefentry> and 
    <citerefentry><refentrytitle>write_pipe</refentrytitle></citerefentry> 
    functions that take a reservation ID as an argument can 
    be used to read from or write to a packet index. These built-ins can be used to read from or write 
    to a packet index one or multiple times. If a packet index that is reserved for writing is not 
    written to using the 
    <citerefentry><refentrytitle>write_pipe</refentrytitle></citerefentry> 
    function, the contents of that packet in the pipe are undefined. 
    <citerefentry><refentrytitle>commit_read_pipe</refentrytitle></citerefentry> and 
    <citerefentry><refentrytitle>work_group_commit_read_pipe</refentrytitle></citerefentry>
    remove the entries reserved for reading from the pipe. 
    <citerefentry><refentrytitle>commit_write_pipe</refentrytitle></citerefentry> and 
    <citerefentry><refentrytitle>work_group_commit_write_pipe</refentrytitle></citerefentry> 
    ensures that the entries reserved for writing are all added in-order 
    as one contiguous set of packets to the pipe.
</para>

<para>
    There can only be <constant>CL_DEVICE_PIPE_MAX_ACTIVE_RESERVATIONS</constant> 
    (refer to the list of possible values for <varname>parame_name</varname>
    for <citerefentry><refentrytitle>clGetDeviceInfo</refentrytitle></citerefentry>(table 4.3)) 
    reservations active (i.e. reservation IDs that have been reserved 
    but not committed) per work-item or work-group for a 
    pipe in a kernel executing on a device.
</para>

<para>
    Work-item based reservations made by a work-item are ordered in the pipe as they are ordered in 
    the program. Reservations made by different work-items that belong to the same work-group 
    can be ordered using the work-group barrier function. The order of work-item based 
    reservations that belong to different work-groups is implementation defined.
</para>

<para>
    Work-group based reservations made by a work-group are ordered in the pipe as they are ordered 
    in the program. The order of work-group based reservations by different work-groups is
    implementation defined.
</para>

<bridgehead>Restrictions</bridgehead>

<para>
    Pipes can only be passed as arguments to a function 
    (including kernel functions). The C 
    <citerefentry><refentrytitle>operators</refentrytitle></citerefentry> 
    (refer to section 6.3 of the OpenCL 2.0 specification) cannot be used with 
    variables declared with the <code>pipe</code> qualifier.
</para>

<para>
    The <code>pipe</code> qualifier cannot be used with 
    variables declared inside a kernel, a structure or 
    union field, a pointer type, an array, global variables 
    declared in program scope or the return type of a function.
</para>

<para>
    The following behavior is undefined:
</para>

<para>
    <itemizedlist>
        <listitem>
            <!-- [sic] spec refers to reserve_pipe, which does not exist -->
            A kernel fails to call 
            <function>reserve_pipe</function>
            before calling 
            <citerefentry><refentrytitle>read_pipe</refentrytitle></citerefentry> or 
            <citerefentry><refentrytitle>write_pipe</refentrytitle></citerefentry> that take a 
            reservation ID.
        </listitem>

        <listitem>
            A kernel calls 
            <citerefentry><refentrytitle>read_pipe</refentrytitle></citerefentry>, 
            <citerefentry><refentrytitle>write_pipe</refentrytitle></citerefentry>, 
            <citerefentry><refentrytitle>commit_read_pipe</refentrytitle></citerefentry> or 
            <citerefentry><refentrytitle>commit_write_pipe</refentrytitle></citerefentry> with 
            an invalid reservation ID.
        </listitem>

        <listitem>
            A kernel calls 
            <citerefentry><refentrytitle>read_pipe</refentrytitle></citerefentry> or 
            <citerefentry><refentrytitle>write_pipe</refentrytitle></citerefentry> 
            with an valid reservation ID but with an <varname>index</varname>
            that is not a value from 0 ... <varname>num_packets</varname> - 1 
            specified to the corresponding call to 
            <function>reserve_pipe</function>.
        </listitem>

        <listitem>
            A kernel calls 
            <citerefentry><refentrytitle>read_pipe</refentrytitle></citerefentry> or 
            <citerefentry><refentrytitle>write_pipe</refentrytitle></citerefentry> 
            with a reservation ID that has already been committed (i.e. a 
            <citerefentry><refentrytitle>commit_read_pipe</refentrytitle></citerefentry> or 
            <citerefentry><refentrytitle>commit_write_pipe</refentrytitle></citerefentry>
            with this reservation ID has 
            already been called).
        </listitem>

        <listitem>
            A kernel fails to call 
            <citerefentry><refentrytitle>commit_read_pipe</refentrytitle></citerefentry> 
            for any reservation ID obtained by a prior call 
            to <citerefentry><refentrytitle>reserve_read_pipe</refentrytitle></citerefentry>.
        </listitem>

        <listitem>
            A kernel fails to call 
            <citerefentry><refentrytitle>commit_write_pipe</refentrytitle></citerefentry> 
            for any reservation ID obtained by a prior call 
            to <citerefentry><refentrytitle>reserve_write_pipe</refentrytitle></citerefentry>.
        </listitem>

        <listitem>
            The contents of the reserved data packets in the pipe are undefined if the kernel does not 
            call <citerefentry><refentrytitle>write_pipe</refentrytitle></citerefentry> 
            for all entries that were reserved by the corresponding call to 
            <function>reserve_pipe</function>.
        </listitem>

        <listitem>
            Calls to 
            <citerefentry><refentrytitle>read_pipe</refentrytitle></citerefentry> 
            that takes a reservation ID and 
            <citerefentry><refentrytitle>commit_read_pipe</refentrytitle></citerefentry> or 
            <citerefentry><refentrytitle>write_pipe</refentrytitle></citerefentry> that 
            takes a reservation ID and 
            <citerefentry><refentrytitle>commit_write_pipe</refentrytitle></citerefentry> 
            for a given reservation ID must be called 
            by the same kernel that made the reservation using 
            <citerefentry><refentrytitle>reserve_read_pipe</refentrytitle></citerefentry> or 
            <citerefentry><refentrytitle>reserve_write_pipe</refentrytitle></citerefentry>. 
            The reservation ID cannot be passed to another kernel including 
            child kernels.
        </listitem>

    </itemizedlist>
</para>

<!-- ================================ EXAMPLE  -->

    <refsect2 id="example1">
        <title>
            Example
        </title>

        <para>
In the following example
        </para>

        <informaltable frame="none">
            <tgroup cols="1" align="left" colsep="0" rowsep="0">
                <colspec colname="col1" colnum="1" />
                <tbody>
                    <row>
                        <entry>
kernel void
foo (read_only pipe fooA_t pipeA, 
     write_only pipe fooB_t pipeB)
{
...
}
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>

        <para>
            <code>pipeA</code> is a read-only pipe object, 
            and <code>pipeB</code> is a write-only pipe object.
        </para>
    </refsect2>

<!-- 7-Nov-2015, API ver 2.1 rev 19; Ext ver 2.1 rev 10; C lang ver 2.0 rev 30 -->