how to get the exhaustive list of auto completions while editing in .c or .cpp file.

[Anand S. Dhankshirur]

I have one more question
GVIM, provides the auto completions with the exhaustive list of all the 
possibilities.
while typing in .c file
(for example GS_AM_ will give all the auto completions for all the 
tokens strating with GS_AM.)
In emacs also M-/ gives the completions which are there in current buffer.
Furhter it gives the case insensitive one.(the cases are ignored).
But it will not give the completions from the TAGS file
It does not consider other files and buffers.
Is there any way to do it in emacs.?