Address Space Qualifier.



Variables inside a kernel function not declared with an address space qualifier, all variables inside non-kernel functions, and all function arguments are in the __private or private address space.


General information about address space qualifiers

OpenCL implements the following disjoint named 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 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. The address space for a variable at program scope or a static variable inside a function can either be global or constant, but defaults to __global if not specified.

OpenCL 2.0 adds support for an unnamed generic address space. Pointers that are declared without pointing to a named address space point to the generic 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 generic address space.

kernel function arguments declared to be a pointer or an array of a type must point to one of the named address spaces global, local or __constant.

The named address spaces are a subset of the generic address space except for the constant address space.

A pointer to address space A can only be assigned to a pointer to the same address space A or a pointer to the generic 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.

The global, constant, local, private, generic, global, constant, local, and private names are reserved for use as address space qualifiers and shall not be used otherwise. The generic and generic names are reserved for future use.

The size of pointers to different address spaces may differ. It is not correct to assume that, for example, sizeof(global int *) always equals sizeof(local int *).


General qualifier examples follow:

// 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];


private int f() { ... } // should generate an error
local int *f() { ... } // allowed
local int * private f() { ... }; // should generate an error.

Also see