How do I use keyword completion that searches included files and headers in Vim?

Explanation

Vim's <C-x><C-i> completion mode searches for keyword matches not just in the current buffer, but also in all files reachable via include directives (e.g., #include in C/C++, import in Python). This lets you complete function names, types, and constants defined in headers without leaving insert mode.

How it works

• <C-x> enters Vim's sub-completion mode from insert mode
• <C-i> selects "include-file completion" — searches the current file and all files it includes, recursively
• The include option tells Vim how to recognize include statements (default handles C-style #include "..." and #include <...>)
• The path option is used to resolve include file locations
• Once the menu appears, use <C-n> / <C-p> to navigate, <CR> or <C-y> to accept

Example

In a C file:

#include <stdio.h>

With the cursor in insert mode after typing pri, pressing <C-x><C-i> opens a completion menu with printf, pread, pwrite, and other symbols from stdio.h and its transitive includes.

Tips

• Run :set include? to see the current include pattern; customize it for your language with :set include=...
• Run :set path? to verify Vim can find your header directories; add paths with :set path+=/usr/local/include
• To make <C-n> also search includes by default, add i to the complete option: :set complete+=i
• For just the current file (no includes), use <C-x><C-n> instead
• Similar completions: <C-x><C-]> for ctag-based completion, <C-x><C-k> for dictionary words