Use a variable before it's declaration

Hi, all: Here is a snippet of linux-v3.4 which drives me crazy. Because init_task hasn't been declared yet when init_thread_union is initializing. Why is there no compiling error? Any hint would be helpful. union thread_union init_thread_union __init_task_data = { INIT_THREAD_INFO(init_task) }; /* * Initial task structure. * * All other task structs will be allocated on slabs in fork.c */ struct task_struct init_task = INIT_TASK(init_task); -- Rock Lee