Built-in variables

CUDA provides 5 built-in variables: gridDim denotes the dimension of grid, and blockDim denotes the dimension of block; their types are dim3. blockIdx and threadIdx identify the block index within the grid and thread index within the block respectively, and their types are uint3. warpSize is int type, and identifies the warp size in threads, and it should be 32 for all compute capabilities.

References:
CUDA C Programming Guide.