__constant

Address Space Qualifier.

__constant constant

Description

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 arguments to a __kernel function shall be in the __private address space.

__kernel 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.

The __kernel function arguments declared to be of type image2d_t or image3d_t always point to the __global address space. All program scope variables must be declared in the __constant address space.

The __constant or constant address space name is used to describe variables allocated in global memory and which are accessed inside a kernel(s) as read-only variables. These read- only variables can be accessed by all (global) work-items of the kernel during its execution. This qualifier can be used with arguments to functions (including __kernel functions) that are declared as pointers, or with local variables inside a function declared as pointers, or with global variables. Global variables declared in the program source with the __constant qualifier are required to be initialized.

Writes to variables declared with the __constant address space qualifier in the OpenCL program source should be a compile-time error.

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];

Specification

OpenCL Specification

Also see

__global, __local, __private

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