System freeze on kernel execution

Hello to everyone,
I have a problem with this piece of code:

int f_aes_cipher(EVP_CIPHER_CTX *ctx, unsigned char *out_arg,
						const unsigned char *in_arg, unsigned int nbytes) {
	int result = d_true, error;
	size_t global_work_size[1] = {nbytes}, local_work_size[1] = {16};
	void *pointer;
	memcpy(out_arg, in_arg, nbytes);
	v_cl_out_buffer = clCreateBuffer(v_cl_context, (CL_MEM_READ_WRITE|
									 nbytes, out_arg, &error);
	if ((error = clSetKernelArg(v_cl_kernel, 1, sizeof(v_cl_out_buffer),
								(void *)&v_cl_out_buffer)) == CL_SUCCESS) {
		if (v_space_initialized == d_false) {
			v_cl_key_buffer = clCreateBuffer(v_cl_context, CL_MEM_READ_ONLY,
											 const_b, NULL, &error);
			clEnqueueWriteBuffer(v_cl_command_queue, v_cl_key_buffer,
								 CL_TRUE, 0, const_b, expanded_key, 0,
								 NULL, NULL);
			if (((error = clSetKernelArg(v_cl_kernel, 0,
										 (void *)&v_cl_key_buffer))
				 != CL_SUCCESS) ||
				((error = clSetKernelArg(v_cl_kernel, 2,
									 (void *)&const_a))
				 != CL_SUCCESS)) {
				d_debug("can't hook parameters to the kernel");
				result = d_false;
			v_space_initialized = d_true;
	if (error == CL_SUCCESS) {
		if ((error = clEnqueueNDRangeKernel(v_cl_command_queue,
											v_cl_kernel, 1, NULL,
											0, NULL, NULL))
			 == CL_SUCCESS) {
			pointer = clEnqueueMapBuffer(v_cl_command_queue,
										 v_cl_out_buffer, CL_TRUE,
										 CL_MAP_READ, nbytes, 0, 0, NULL,
										 NULL, NULL);
			memcpy(out_arg, pointer, nbytes);
			clEnqueueUnmapMemObject(v_cl_command_queue, v_cl_out_buffer,
									pointer, 0, NULL, NULL);
	return result;

This piece of code is called many times (in my personal test case only two times) with a different nbytes value (but nbytes%16 is always 0), a different pointer to an in_arg string (initial_in_arg+last_nbytes foreach step) and a different pointer to an already allocated out_arg (out_arg has the same dimension of the in_arg parameter and work in the same way).
expanded_key and const_a are global variables and they don’t change their content.
My kernel is very simple (it’s just for test):

__kernel void example(__global uint *rkey, __global uint *odata, uint steps) {
	int executions;
	unsigned int *pointer[4], box[4];
	unsigned char *character;
	character = (unsigned char *)odata[0];
	character[0] = 'T';
	character[1] = 'H';
	character[2] = 'I';
	character[3] = 'S';
	character = (unsigned char *)odata[1];
	character[0] = ' ';
	character[1] = 'I';
	character[2] = 'S';
	character[3] = ' ';
	character = (unsigned char *)odata[2];
	character[0] = 'A';
	character[1] = ' ';
	character[2] = 'T';
	character[3] = 'E';
	character = (unsigned char *)odata[3];
	character[0] = 'S';
	character[1] = 'T';
	character[2] = '!';
	character[3] = '!';

Now, when I execute this piece of code on GPU everything freeze (the entire system, Mountain Lion) and I’ve to force the shutdown pressing the power button for 4/5 seconds.
No logs are written in this time and I’ve got no relevant informations from my system. So, my question is: what’s wrong with this? I can’t test my kernel or my kernel’s loader code if everything freeze!
Thank you so much for your patience!

Your problem lies here:

character = (unsigned char *)odata[0];

This code takes the integer stored at location 0 in odata and casts it to a pointer to an unsigned char. I think you wanted to use

character = (unsigned char *)&odata[0];

This code casts the address of location 0 to an unsigned char pointer.

Woops, so sorry! You’re absolutely right, thank you so much; I will “fix” this later but, it’s normal that my system completely freeze for this reason?

It is absolutely normal for this to cause your system to freeze. Your original code would access some part of GPU memory and write to it. This could overwrite some piece of data that is used by a rendering call or the driver and from there the behaviour is completely unpredictable.