__CONSTANT(3clc)                                  OpenCL Manual                                 __CONSTANT(3clc)

NAME

       __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 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.

NOTES

       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 readonly variables can be
       accessed by all (global) work-items of the kernel during its execution. Pointers to the __constant
       address space are allowed as arguments to functions (including kernel functions) and for variables
       declared inside functions.

       All string literal storage shall be in the __constant address space.

       NOTE: Each argument to a kernel that is a pointer to the __constant address space is counted separately
       towards the maximum number of such arguments, defined as CL_DEVICE_MAX_CONSTANT_ARGS are described in the
       table for clGetDeviceInfo(3clc).

       Variables in the program scope or the outermost scope of kernel functions can be declared in the
       __constant address space. These variables are required to be initialized and the values used to
       initialize these variables must be a compile time constant. Writing to such a variable results in a
       compile-time error.

       Implementations are not required to aggregate these declarations into the fewest number of constant
       arguments. This behavior is implementation defined.

       Thus portable code must conservatively assume that each variable declared inside a function or in program
       scope allocated in the __constant address space counts as a separate constant argument.

   Example
       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];

       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.

SPECIFICATION

       OpenCL Specification[1]

SEE ALSO

       global(3clc), local(3clc), private(3clc)

AUTHORS

       The Khronos Group

COPYRIGHT

       Copyright © 2007-2011 The Khronos Group Inc.
       Permission is hereby granted, free of charge, to any person obtaining a copy of this software and/or
       associated documentation files (the "Materials"), to deal in the Materials without restriction, including
       without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
       copies of the Materials, and to permit persons to whom the Materials are furnished to do so, subject to
       the condition that this copyright notice and permission notice shall be included in all copies or
       substantial portions of the Materials.

NOTES

        1. OpenCL Specification
           page 222, section 6.5 - Address Space Qualifiers

The Khronos Group                                  01/14/2021                                   __CONSTANT(3clc)