Address space qualifier.
OpenCL implements the following disjoint address spaces: __global, __local, __constant, and __private 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.
The address space names without the __prefix i.e. global, local, constant and private may be substituted for the corresponding address space names with the __prefix.
The generic address space name for arguments to a function in a program, or local variables of a function is __private. All function arguments shall be in the __private address space.
functionQualifiers(3clc) function arguments declared to be a pointer of a type can point to one of the following address spaces only: __global, __local or __constant. A pointer to address space A can only be assigned to a pointer to the same address space A. Casting a pointer to address space A to a pointer to address space B is illegal.
Function arguments of type otherDataTypes(3clc), otherDataTypes(3clc), otherDataTypes(3clc), otherDataTypes(3clc), otherDataTypes(3clc), and otherDataTypes(3clc) refer to image memory objects allocated in the __global address space.
The __global or global address space name is used to refer to memory objects (buffer or image objects) allocated from the global memory pool.
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.
The actual size of the array memory object is determined when the memory object is allocated via appropriate API calls in the host code.
If an image object is attached to an argument declared with this qualifier, the argument must be declared as type otherDataTypes(3clc) for a 2D image object, as type otherDataTypes(3clc) for a 3D image object, as type otherDataTypes(3clc) for a 2D image array object, as type otherDataTypes(3clc) for a 1D image object, as type otherDataTypes(3clc) for a 1D image buffer object, and as type otherDataTypes(3clc) for a 1D image array object. The elements of an image object cannot be directly accessed. Built-in functions to read from and write to an image object are provided.
The const qualifier can also be used with the __global qualifier to specify a read-only buffer memory object.
There is no generic address space name for program scope variables. All program scope variables must be declared in the __constant address space. For example:
| // declares a pointer p in the __private address space that // points to an int object in address space __global __global int *p; // declares an array of 4 floats in the __private address space. float x[4]; |
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There is no address space for function return values. Using an address space qualifier in a function return type declaration will generate a compilation error, unless the return type is declared as a pointer type and the qualifier is used on the points-to address space.
For example:
| __private int f() { ... } // should generate an error __local int *f() { ... } // allowed __local int * __private f() { ... }; // should generate an error. |
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| __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. |
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local(3clc), constant(3clc), private(3clc)
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OpenCL Specification
page 222, section 6.5 - Address Space Qualifiers