If you are editing a C program using vim, then create a tag file using ctags
:!ctags . [ . for the current file ]
:!ctags * [ * for all the .c and .h files, if you are in a src directory]
Now, the next time you stumble across a function name,variable name and you want to know its declaration, then just do a CNRL + ]. Thats it! Quite useful.
Another feature of Vim is Auto-completion. for example
#include<expat.h> has number of XML related declarations typing the first few characters of variable and then doing CNTL+N
should provide you list for the auto-completion. Interesting, huh?
I could have never typed the above properly (without losing my sense, CNTL+N helped me)
...creating a tags file is the first thing I do when browsing a program
- Bram Moolenar in Vim: Seven habits of effective text editing.