Restrictions

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

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 __global, __private, __local and __constant address space qualifiers.

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 pointer 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.

Variadic macros and functions with the exception of printf and enqueue_kernel 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 return type of a kernel function must be void.

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.

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, image2d_depth_t, image2d_array_depth_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.

The clk_event_t, ndrange_t and reserve_id_t types cannot be used as arguments to kernel functions that get enqueued from the host. The clk_event_t and reserve_id_t types cannot be declared in program scope.

The behavior of applying the sizeof operator to the queue_t, clk_event_t, ndrange_t and reserve_id_t types is implementation-defined.

Kernels enqueued by the host must continue to have their arguments that are a pointer to a type declared to point to a named address space.

A function in an OpenCL program cannot be called main.

If the cl_khr_initialize_memory extension is enabled, if the context is created with CL CONTEXT MEMORY INITIALIZE KHR, appropriate memory locations as specified by the bit-field is initialized with zeroes, prior to the start of execution of any kernel. The driver chooses when, prior to kernel execution, the initialization of local and/or private memory is performed. The only requirement is there should be no values set from outside the context, which can be read during a kernel execution.