openCL - difference in handling array and scalar - opencl

Question

I'm trying to dig into openCL. At the moment I'm asking myself why a scalar differs from array when it comes to transferring to GPU.
Below there are two inputs. A scalar and a array. Why is there such a big difference in transferring them to the GPU?
Thanks in advance!
// input size
int input_size = 4;
err = clSetKernelArg(kernel, 0, sizeof(unsigned int), &_input_size);
if (err != CL_SUCCESS) {
throw std::runtime_error("Failed to set kernel arguments!");
}
// input
int input[input_size];
input[0] = 1;
input[1] = 2;
input[2] = 3;
input[3] = 4;
cl_mem cl_input = clCreateBuffer(context, CL_MEM_READ_ONLY, sizeof(int) * _input_size, NULL, NULL);
if (!cl_input) {
throw std::runtime_error("Failed to allocate device memory!");
}
err = clSetKernelArg(kernel, 1, sizeof(cl_mem), &cl_input);
if (err != CL_SUCCESS) {
throw std::runtime_error("Failed to set kernel arguments!");
}
err = clEnqueueWriteBuffer(commands, cl_input, CL_TRUE, 0, sizeof(int) * _input_size, _input, 0, NULL, NULL);
if (err != CL_SUCCESS) {
throw std::runtime_error("Failed to write to source array!");
}
// output
int output_size = 4;
int output[output_size];
cl_mem cl_output = clCreateBuffer(context, CL_MEM_WRITE_ONLY, sizeof(int) * _output_size, NULL, NULL);
if (!cl_output) {
throw std::runtime_error("Failed to allocate device memory!");
}
err = clSetKernelArg(kernel, 2, sizeof(cl_mem), &cl_output);
if (err != CL_SUCCESS) {
throw std::runtime_error("Failed to set kernel arguments!" );
}

Answer 1

In OpenCL scalars are passed by value and are constants to each thread. Arrays are passed by reference (address) and can be used a input (read only) , output ( write only) or input/output ( read/write). Arrays require the cl_mem object be passed as the arguments. cl_mem is a meta object that describes the array. Scalars are so simple they just require their size as meta data.
So in you above example the kernel should look like ....
kernel void mykernel( const int arg0, global int * arg1, global int * arg2 ... ) {
int i = get_global_id(0);
// In every thread arg0 is constant value and cannot be changed
if (i < arg0)
arg2[i] = arg[1[i];
}