vim keyword completion

#include<stdio.h>
int main()
{
    sca //here i press control+N, it doesnot complete to scanf
}

set path+=/usr/include/**
set path+=/my_include_dir/include
set path+=/my_include_dir/srclib/apr/**
set path+=/my_other_include_dir/srclib/apr-util/**

** means all sub-directories.
*  means all contained directories
.  means all files in the directory of the found file
+= means prepend to existing path (iirc)

:checkpath

:checkpath!

^x ^o  = "omnicomplete"
^x ^i  = "included-files completion"
^x ^f  = "path completion"
^x ^l  = "Complete this line using one that looks the same"

:help ins-completion