sdk/2.1/docs/man/xhtml/global.html

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    <title>__global</title>
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    <div class="refentry">
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        <a xmlns="http://www.w3.org/1999/xhtml" id="__global"></a>
        <h1>__global</h1>
        <p>
            Address space qualifier.
        </p>
      </div>
      <div class="refsect2">
        <a id="synopsis"></a>
        <h3>
        </h3>
        <div class="informaltable">
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__global
global
                        </td>
              </tr>
            </tbody>
          </table>
        </div>
      </div>
      <div class="refsect1">
        <a id="description"></a>
        <h2>Description</h2>
        <p>
            The <code class="function">__global</code> or <code class="function">global</code> address
            space name is used to refer to memory objects (buffer or image objects)
            allocated from the global memory pool.
        </p>
        <p>
            A buffer memory object can be declared as a pointer to a scalar, vector
            or user-defined struct. This allows the kernel to read and/or write any
            location in the buffer.
        </p>
        <p>
            The actual size of the array memory object is determined when the memory
            object is allocated via appropriate API calls in the host code.
        </p>
        <p>
            As image objects are always allocated from the global 
            address space, the <code class="function">__global</code> or 
            <code class="function">global</code> qualifier should not be 
            specified for image types. The elements of an image object 
            cannot be directly accessed. Built-in functions to 
            read from and write to an image object are 
            provided.
        </p>
        <p>
            Variables defined at program scope and static 
            variables inside a function can also be declared 
            in the <code class="function">global</code> address space. 
            They can be defined with any valid OpenCL C data type except 
            for those in table 6.3. In particular, such 
            program scope variables may be of any user-defined 
            type, or a pointer to a user-defined type. In 
            the presence of shared virtual memory, these pointers 
            or pointer members should work as expected as 
            long as they are shared virtual memory pointers
            and the referenced storage has been mapped appropriately. 
            These variables in the <code class="function">global</code>
            address space have the same lifetime as the program, 
            and their values persist between calls to 
            any of the kernels in the program. These variables 
            are not shared across devices. They have 
            distinct storage.
        </p>
        <p>
            Program scope and static variables in the 
            <code class="function">global</code> address space may be initialized, but only 
            with constant expressions.
        </p>
        <p>
            The <code class="code">const</code> qualifier can also be used 
            with the <code class="function">__global</code> qualifier to
            specify a read-only buffer memory object.
        </p>
      </div>
      <div class="refsect1">
        <a id="notes"></a>
        <h2>Notes</h2>
        <h4><a id="id-1.6.2"></a>General information about address space qualifiers</h4>
        <p>
      OpenCL implements the following disjoint named address 
      spaces: <code class="function">__global</code>,
      <code class="function">__local</code>, <code class="function">__constant</code>, and
      <code class="function">__private</code>. 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.
    </p>
        <p>
      The address space names without the  __  prefix i.e. <code class="function">global</code>,
      <code class="function">local</code>, <code class="function">constant</code> 
      and <code class="function">private</code>
      may be substituted for the corresponding address 
      space names with the  __  prefix.
    </p>
        <p>
        The address space name for arguments to a function 
        in a program, or local variables of a function 
        is <code class="function">__private</code>. All function 
        arguments shall be in the <code class="function">__private</code> address space.
        The address space for a variable at program scope 
        or a static variable inside a function can either 
        be <code class="function">__global</code> or 
        <code class="function">__constant</code>, but defaults to 
        <code class="function">__global</code> if not specified.
    </p>
        <p>
        OpenCL 2.0 adds support for an unnamed <code class="function">generic</code> 
        address space. Pointers that are declared 
        without pointing to a named address space point to the 
        <code class="function">generic</code> 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 
        <code class="function">generic</code> address space.
    </p>
        <p>
        kernel function arguments declared to be a pointer or 
        an array of a type must point to one of 
        the named address spaces <code class="function">__global</code>, 
        <code class="function">__local</code> or <code class="function">__constant</code>.
    </p>
        <p>
        The named address spaces are a subset of the generic 
        address space except for the <code class="function">constant</code>
        address space.
    </p>
        <p>
        A pointer to address space A can only be assigned to 
        a pointer to the same address space A or a 
        pointer to the <code class="function">generic</code> 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.
    </p>
        <p>
        The <code class="function">__global</code>, <code class="function">__constant</code>, 
        <code class="function">__local</code>, 
        <code class="function">__private</code>, <code class="function">__generic</code>, 
        <code class="function">global</code>, 
        <code class="function">constant</code>, <code class="function">local</code>, and
        <code class="function">private</code> names are reserved for use as address space 
        qualifiers and shall not be used otherwise.
        The <code class="function">__generic</code> and <code class="function">generic</code> 
        names are reserved for future use.
    </p>
        <p>
        The size of pointers to different address spaces 
        may differ. It is not correct to assume
        that, for example, <code class="code">sizeof(__global int *)</code> 
        always equals <code class="code">sizeof(__local int *)</code>.
    </p>
      </div>
      <div class="refsect2">
        <a id="example1"></a>
        <h3>
            Examples
        </h3>
        <p>
      General qualifier examples follow:
    </p>
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// declares a pointer p in the private address space that
// points to an object in address space global
global int *p;

void foo (...)
{
    // declares an array of 4 floats in the private address space
    float x[4];
    ...
}
            </td>
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        </div>
        <p>
      Example:
    </p>
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                <td align="left">
private int f() { ... } // should generate an error
local int *f() { ... } // allowed
local int * private f() { ... }; // should generate an error.
            </td>
              </tr>
            </tbody>
          </table>
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                <td align="left">
global float4  *color;      // An array of float4 elements
typedef struct {
      float a[3];
      int   b[2];
} foo_t;
global foo_t   *my_info;    // An array of foo_t elements.
                        </td>
              </tr>
            </tbody>
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        <p>
        </p>
        <div class="informaltable">
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              <col align="left" class="col1" />
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                <td align="left">
global int foo;         // OK.
int foo;                // OK. Declared in the global address space
global uchar buf[512];  // OK.
global int baz = 12;    // OK. Initialization is allowed
static global int bat;  // OK. Internal linkage

global uchar bigbuf[CL_DEVICE_MAX_GLOBAL_VARIABLE_SIZE]; // OK.

static int foo;         // OK. Declared in the global address space
static global int foo;  // OK.

int *foo;               // OK. foo is allocated in global address space.
                        // pointer to foo in generic address space

void func(...)
{
   int *foo;            // OK. foo is allocated in private address space.
                        // foo points to a location in generic address space.
   ...
}

global int * global ptr;          // OK.
int * global ptr;                 // OK.
constant int *global ptr=&amp;baz;    // error since baz is in global address 
                                  // space.
global int * constant ptr = &amp;baz; // OK

// Pointers work. Also, initialization to a constant known at 
// program load time
global int *global baz_ptr = &amp;baz;

global image2d_t im;       // Error. Invalid type for program scope 
                           // variables

global event_t ev;   // Error. Invalid type for program scope variables

global int *bad_ptr; // Error. No implicit address space
                        </td>
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            </tbody>
          </table>
        </div>
      </div>
      <div class="refsect1">
        <a id="specification"></a>
        <h2>Specification</h2>
        <p>
            <img src="pdficon_small1.gif" />

            <a href="https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#namedest=addressSpaceQualifiers" target="OpenCL Spec">OpenCL Specification</a>
        </p>
      </div>
      <div class="refsect1">
        <a id="seealso"></a>
        <h2>Also see</h2>
        <p>
            <a class="citerefentry" href="local.html"><span class="citerefentry"><span class="refentrytitle">__local</span></span></a>,
            <a class="citerefentry" href="constant.html"><span class="citerefentry"><span class="refentrytitle">__constant</span></span></a>,
            <a class="citerefentry" href="private.html"><span class="citerefentry"><span class="refentrytitle">__private</span></span></a>,
            <a class="citerefentry" href="qualifiers.html"><span class="citerefentry"><span class="refentrytitle">qualifiers</span></span></a>
        </p>
      </div>
      <div xmlns="" class="refsect3" lang="en" xml:lang="en"><a xmlns="http://www.w3.org/1999/xhtml" id="Copyright"></a><h4 xmlns="http://www.w3.org/1999/xhtml"></h4><img xmlns="http://www.w3.org/1999/xhtml" src="KhronosLogo.jpg" /><p xmlns="http://www.w3.org/1999/xhtml"></p>Copyright © 2007-2015 The Khronos Group Inc.
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