restrictions
restrictions
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Restrictions

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- The use of pointers is somewhat restricted. The following rules apply:

  - Arguments to __kernel functions declared in a program that are pointers must be declared with the __global, __constant, or __local qualifier.

  - A pointer declared with the __constant, __local, or __global qualifier can only be assigned to a pointer declared with the __constant, __local, or __global qualifier respectively.

  - Pointers to functions are not allowed.

  - Arguments to __kernel functions in a program cannot be declared as a pointer to a pointer(s). Variables inside a function or arguments to non-kernel functions in a program can be declared as a pointer to a pointer(s).

- An image type ( image2d_t, image3d_t, image2d_array_t, image1d_t, image1d_buffer_t, or image1d_array_t) can only be used as the type of a function argument. An image function argument cannot be modified. Elements of an image can only be accessed using the Built-in Image Functions.

  An image type cannot be used to declare a variable, a structure or union field, an array of images, a pointer to an image, or the return type of a function. An image type cannot be used with the __private, __local and __constant address space qualifiers. The image3d_t type cannot be used with the __write_only access qualifier unless the cl_khr_3d_image_writes extension is enabled. An image type cannot be used with the __read_write access qualifer which is reserved for future use.

  The sampler type (sampler_t) can only be used as the type of a function argument or a variable declared in the program scope or the outermost scope of a kernel function. The behavior of a sampler variable declared in a non-outermost scope of a kernel function is implementation-defined. A sampler argument or variable cannot be modified.

  The sampler type cannot be used to declare a structure or union field, an array of samplers, a ponter to a sampler, or the return type of a function. The sampler type cannot be used with the __local and __global address space qualifiers.

- Bit-fields struct members are currently not supported.

- Variable length arrays and structures with flexible (or unsized) arrays are not supported.

- Variadic macros and functions are not supported.

- The library functions defined in the C99 standard headers `assert.h`, `ctype.h`, `complex.h`, `errno.h`, `fenv.h`, `float.h`, `inttypes.h`, `limits.h`, `locale.h`, `setjmp.h`, `signal.h`, `stdarg.h`, `stdio.h`, `stdlib.h`, `string.h`, `tgmath.h`, `time.h`, `wchar.h`, and `wctype.h` are not available and cannot be included by a program.

- The `auto` and `register` storage-class specifiers are not supported.

- Recursion is not supported.

- The function using the __kernel qualifier can only have return type void in the source code.

- Arguments to kernel functions in a program cannot be declared with the built-in scalar types bool, half, size_t, ptrdiff_t, intptr_t, and uintptr_t, or a struct and/or union that contain fields declared to be one of these built-in scalar types. The size in bytes of these types except half are implementation-defined and in addition can also be different for the OpenCL device and the host processor making it difficult to allocate buffer objects to be passed as arguments to a kernel declared as pointer to these types. half is not supported as half can be used as a storage format only (unless the cl_khr_fp16 extension is supported) and is not a data type on which floating-point arithmetic can be performed.

- Whether or not irreducible control flow is illegal is implementation defined.

- Arguments to kernel functions in a program cannot be declared to be of type event_t.

- Elements of a struct or union must belong to the same address space. Declaring a struct or union whose elements are in different address spaces is illegal.

- Arguments to kernel functions that are declared to be a struct or union do not allow OpenCL objects to be passed as elements of the struct or union.

- The type qualifiers `const`, `restrict`, and `volatile` as defined by the C99 specification are supported. These qualifiers cannot be used with image2d_t, image3d_t, image2d_array_t, image1d_t, image1d_buffer_t, and image1d_array_t types. Types other than pointer types shall not use the `restrict` qualifier.

- The event type (event_t) cannot be used as the type of a kernel function argument. The event type cannot be used to declare a program scope variable. The event type cannot be used to declare a structure or union field. The event type cannot be used with the __local, __constant, and __global address space qualifiers.

## Specification

OpenCL Specification

## Also see

Image Functions, Function Qualifiers

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