clSetKernelArg

Used to set the argument value for a specific argument of a kernel.

cl_int clSetKernelArg ( cl_kernel kernel,
  cl_uint arg_index,
  size_t arg_size,
  const void *arg_value)

Parameters

kernel

A valid kernel object.

arg_index

The argument index. Arguments to the kernel are referred by indices that go from 0 for the leftmost argument to n - 1, where n is the total number of arguments declared by a kernel.

arg_value

A pointer to data that should be used as the argument value for argument specified by arg_index. The argument data pointed to by arg_value is copied and the arg_value pointer can therefore be reused by the application after clSetKernelArg returns. The argument value specified is the value used by all API calls that enqueue kernel (clEnqueueNDRangeKernel and clEnqueueTask) until the argument value is changed by a call to clSetKernelArg for kernel.

If the argument is a memory object (buffer, image or image array), the arg_value entry will be a pointer to the appropriate buffer, image or image array object. The memory object must be created with the context associated with the kernel object. If the argument is a buffer object, the arg_value pointer can be NULL or point to a NULL value in which case a NULL value will be used as the value for the argument declared as a pointer to __global or __constant memory in the kernel. If the argument is declared with the __local qualifier, the arg_value entry must be NULL. If the argument is of type sampler_t, the arg_value entry must be a pointer to the sampler object.

If the argument is declared to be a pointer of a built-in scalar or vector type, or a user defined structure type in the global or constant address space, the memory object specified as argument value must be a buffer object (or NULL). If the argument is declared with the __constant qualifier, the size in bytes of the memory object cannot exceed CL_DEVICE_MAX_CONSTANT_BUFFER_SIZE and the number of arguments declared as pointers to __constant memory cannot exceed CL_DEVICE_MAX_CONSTANT_ARGS.

The memory object specified as argument value must be a 2D image object if the argument is declared to be of type image2d_t. The memory object specified as argument value must be a 3D image object if argument is declared to be of type image3d_t. The memory object specified as argument value must be a 1D image object if the argument is declared to be of type image1d_t. The memory object specified as argument value must be a 1D image buffer object if the argument is declared to be of type image1d_buffer_t. The memory object specified as argument value must be a 1D image array object if argument is declared to be of type image1d_array_t. The memory object specified as argument value must be a 2D image array object if argument is declared to be of type image2d_array_t.

For all other kernel arguments, the arg_value entry must be a pointer to the actual data to be used as argument value.

arg_size

Specifies the size of the argument value. If the argument is a memory object, the size is the size of the buffer or image object type. For arguments declared with the __local qualifier, the size specified will be the size in bytes of the buffer that must be allocated for the __local argument. If the argument is of type sampler_t, the arg_size value must be equal to sizeof(cl_sampler). For all other arguments, the size will be the size of argument type.

Notes

A kernel object does not update the reference count for objects such as memory, sampler objects specified as argument values by clSetKernelArg. Users may not rely on a kernel object to retain objects specified as argument values to the kernel.

Implementations shall not allow cl_kernel objects to hold reference counts to cl_kernel arguments, because no mechanism is provided for the user to tell the kernel to release that ownership right. If the kernel holds ownership rights on kernel args, that would make it impossible for the user to tell with certainty when he may safely release user allocated resources associated with OpenCL objects such as the cl_mem backing store used with CL_MEM_USE_HOST_PTR.

An OpenCL API call is considered to be thread-safe if the internal state as managed by OpenCL remains consistent when called simultaneously by multiple host threads. OpenCL API calls that are thread-safe allow an application to call these functions in multiple host threads without having to implement mutual exclusion across these host threads i.e. they are also re-entrant-safe.

All OpenCL API calls are thread-safe except clSetKernelArg. clSetKernelArg is safe to call from any host thread, and is safe to call re-entrantly so long as concurrent calls operate on different cl_kernel objects. However, the behavior of the cl_kernel object is undefined if clSetKernelArg is called from multiple host threads on the same cl_kernel object at the same time. Please note that there are additional limitations as to which OpenCL APIs may be called from OpenCL callback functions -- please see section 5.9.

There is an inherent race condition in the design of OpenCL that occurs between setting a kernel argument and using the kernel with clEnqueueNDRangeKernel or clEnqueueTask. Another host thread might change the kernel arguments between when a host thread sets the kernel arguments and then enqueues the kernel, causing the wrong kernel arguments to be enqueued. Rather than attempt to share cl_kernel objects among multiple host threads, applications are strongly encouraged to make additional cl_kernel objects for kernel functions for each host thread.

Errors

clSetKernelArg returns CL_SUCCESS if the function is executed successfully. Otherwise, it returns one of the following errors:

  • CL_INVALID_KERNEL if kernel is not a valid kernel object.
  • CL_INVALID_ARG_INDEX if arg_index is not a valid argument index.
  • CL_INVALID_ARG_VALUE if arg_value specified is not a valid value.
  • CL_INVALID_MEM_OBJECT for an argument declared to be a memory object when the specified arg_value is not a valid memory object.
  • CL_INVALID_SAMPLER for an argument declared to be of type sampler_t when the specified arg_value is not a valid sampler object.
  • CL_INVALID_ARG_SIZE if arg_size does not match the size of the data type for an argument that is not a memory object or if the argument is a memory object and arg_size != sizeof(cl_mem) or if arg_size is zero and the argument is declared with the __local qualifier or if the argument is a sampler and arg_size != sizeof(cl_sampler).
  • CL_INVALID_ARG_VALUE if the argument is an image declared with the read_only qualifier and arg_value refers to an image object created with cl_mem_flags of CL_MEM_WRITE or if the image argument is declared with the write_only qualifier and arg_value refers to an image object created with cl_mem_flags of CL_MEM_READ.
  • CL_OUT_OF_RESOURCES if there is a failure to allocate resources required by the OpenCL implementation on the device.
  • CL_OUT_OF_HOST_MEMORY if there is a failure to allocate resources required by the OpenCL implementation on the host.

Example

kernel void image_filter (int n, int m, __constant float *filter_weights, __read_only image2d_t src_image, __write_only image2d_t dst_image) { ... }

Argument index values for image_filter will be 0 for n, 1 for m, 2 for filter_weights, 3 for src_image and 4 for dst_image.

Specification

OpenCL Specification

Also see

clCreateKernel, clCreateKernelsInProgram, clReleaseKernel, clRetainKernel, clGetKernelInfo, clGetKernelWorkGroupInfo

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