__LOCAL(3clc)                                     OpenCL Manual                                    __LOCAL(3clc)

NAME

       __local - Address Space Qualifier.

       __local local

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 __local or local address space name is used to describe variables that need to be allocated in local
       memory and are shared by all work-items of a work-group. Pointers to the __local address space are
       allowed as arguments to functions (including __kernel functions). Variables allocated in the __local
       address space can also be defined inside a __kernel function scope.

   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.

       // Examples of variables allocated in the __local
       address space // inside a __kernel function

       __kernel void my_func(...)  { __local float      a;
       // A single float allocated // in local address space

       __local float      b[10];   // An array of 10 floats //
       allocated in local address space.  if (...)  { //
       example of variable in __local address space but not //
       declared at __kernel function scope.  __local float c;
       // not allowed.  } }

       Variables allocated in the __local address space inside a __kernel function cannot be initialized.

           __kernel void my_func(...)
           {
           local float a = 1;    // not allowed __local float b; b = 1;                // allowed
           }

       NOTE: Variables allocated in the __local address space inside a __kernel function are allocated for each
       work-group executing the kernel.

SPECIFICATION

       OpenCL Specification[1]

SEE ALSO

       global(3clc), constant(3clc), private(3clc)

AUTHORS

       The Khronos Group

COPYRIGHT

       Copyright © 2007-2011y 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                                      __LOCAL(3clc)