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. 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.

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 the value of the CL_DEVICE_MAX_CONSTANT_ARGS device query.

Variables in the program scope can be declared in the __constant address space. Variables in 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.

See Also

Document Notes

For more information, see the OpenCL C Specification

This page is extracted from the OpenCL C Specification. Fixes and changes should be made to the Specification, not directly.

Copyright (c) 2014-2020 Khronos Group. This work is licensed under a Creative Commons Attribution 4.0 International License.