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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



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