sdk/2.0/docs/man/qualifierInc.xml

<!-- from section 6.5 - common stuff for global, local, constant, and private -->

    <para>
      OpenCL implements the following disjoint named address 
      spaces: <function>__global</function>,
      <function>__local</function>, <function>__constant</function>, and
      <function>__private</function>. The address space qualifier may be used in variable
      declarations to specify the region of memory that is used to allocate the object. The
      C syntax for type qualifiers is extended in OpenCL 
      to include an address space name as a valid type qualifier.
      If the type of an object is qualified by an address 
      space name, the object is allocated in the 
      specified address name; otherwise, the object is 
      allocated in the generic address space.
    </para>

    <para>
      The address space names without the  __  prefix i.e. <function>global</function>,
      <function>local</function>, <function>constant</function> 
      and <function>private</function>
      may be substituted for the corresponding address 
      space names with the  __  prefix.
    </para>

    <para>
        The address space name for arguments to a function 
        in a program, or local variables of a function 
        is <function>__private</function>. All function 
        arguments shall be in the <function>__private</function> address space.
        The address space for a variable at program scope 
        or a static variable inside a function can either 
        be <function>__global</function> or 
        <function>__constant</function>, but defaults to 
        <function>__global</function> if not specified.
    </para>

    <para>
        OpenCL 2.0 adds support for an unnamed <function>generic</function> 
        address space. Pointers that are declared 
        without pointing to a named address space point to the 
        <function>generic</function> address space. Before 
        referring to the region pointed to, the pointer 
        must be associated with a named address space.
        Functions may be written with arguments and return values that point to the 
        <function>generic</function> address space.
    </para>

    <para>
        kernel function arguments declared to be a pointer or 
        an array of a type must point to one of 
        the named address spaces <function>__global</function>, 
        <function>__local</function> or <function>__constant</function>.
    </para>

    <para>
        The named address spaces are a subset of the generic 
        address space except for the <function>constant</function>
        address space.
    </para>

    <para>
        A pointer to address space A can only be assigned to 
        a pointer to the same address space A or a 
        pointer to the <function>generic</function> address 
        space. Casting a pointer to address space A to a pointer to 
        address space B is illegal if A and B are named 
        address spaces and A is not the same as B.
    </para>

    <para>
        The <function>__global</function>, <function>__constant</function>, 
        <function>__local</function>, 
        <function>__private</function>, <function>__generic</function>, 
        <function>global</function>, 
        <function>constant</function>, <function>local</function>, 
        <function>private</function> and 
        <function>generic</function> names are reserved for use as address space 
        qualifiers and shall not be used otherwise.
    </para>

    <para>
        The size of pointers to different address spaces 
        may differ. It is not correct to assume
        that, for example, <code>sizeof(__global int *)</code> 
        always equals <code>sizeof(__local int *)</code>.
    </para>

<!-- 25-Dec-2013, rev. 19 -->