Questions about 02_variable_addition_reference.cu

[tangyipeng100]

I have tested this code. When I remove the blockIdx, such as the code as follows:

__global__ void myfirstkernel(void) {
	//blockIdx.x gives the block number of current kernel
	//printf("Hello!!!I'm thread in block: %d\n", blockIdx.x);
	printf("Hello!!!I'm thread in block: %d\n");
}

int main(void) {
	//A kernel call with 16 blocks and 1 thread per block
	myfirstkernel << <16, 1 >> > ();
	//Function used for waiting for all kernels to finish
	cudaDeviceSynchronize();
	printf("All threads are finished!\n");
	return 0;
}

I can not get the kernel running results as the book shows. Just as:

If add blockIdx back, as printf("Hello!!!I'm thread in block: %d\n", blockIdx.x) then I get the right results.
Why?