How do I autocomplete macro-defined identifiers from header files while in insert mode?

Explanation

<C-x><C-d> in insert mode triggers defined identifier completion — it searches for identifiers that match the partial word before the cursor by scanning #define statements in the current file and any files reachable through #include directives. This is distinct from generic keyword completion (<C-n>) because it targets only preprocessor-defined symbols.

How it works

• Pressing <C-x><C-d> opens a completion menu populated by #define identifiers
• Vim uses the define option pattern to identify what counts as a definition (default: ^\s*#\s*define)
• It searches the current file and all files scanned via #include (controlled by the include option)
• Navigate the popup with <C-n> / <C-p> to select the completion
• Closes automatically when you select or dismiss

Example

Given the header limits.h contains:

#define MAX_BUFFER_SIZE 4096
#define MAX_CONNECTIONS 100

In your source file, typing MAX_B and pressing <C-x><C-d> offers MAX_BUFFER_SIZE as a completion.

Tips

• Unlike <C-x><C-i> (keyword completion from includes), <C-x><C-d> is scoped to only #define macros
• Useful in C/C++ to complete constants, feature flags, and macro names without memorizing exact spelling
• Works without ctags or LSP — it uses Vim's built-in include scanning
• Customize which files to scan: :set include= and :set path+=...
• Reference: :help i_CTRL-X_CTRL-D