"implicit declaration of <some function>" error

I keep getting "implicit declaration of <some function>" error quite often. So I'm assuming I am not adding the correct header files when I am writing the module. eg. I wanted to create a kernel thread using ktheard_create/kthread_run, but couldn't find which header to include. While I was looking for solutions I stumbled upon ctags. It mostly gets the job done, but not always. I couldn't find the proper header to use for open(inode,file); even though ctags pointed me to <linux/fs.h>, I still get the implicit declaration error. Is there any way of finding exactly which headers to include when one is trying to use a function? A detailed explanation would be really great.