"implicit declaration of <some function>" error

[Ritesh Ranjan]

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.