Go to definition of a C function in a different file in Notepad++

Try File Switcher plugin for Notepad++, you can download it directly from the Plugin manager. Just press Ctrl+Shift+O then search your file name.

Set up a User Defined Command:

1. Select the Run menu then the Run... menuitem
2. Enter charmap
3. Click Save...
4. Enter a name to identify it, e.g. &charmap (the ampersand lets you specify C as the accelerator key, so Alt+R then C activate it, unless you have another command with the same accelerator key).
5. Optionally specify a keyboard shortcut to trigger it, e.g. ALT and Num +
6. Click OK

Now whenever you want to enter a character, either use the Run menu or shortcut/accelerator key to open Windows Character Map, and either pick one or more characters or search for it/them using its Unicode name in "Search for:", copy to the clipboard, close Character Map and paste in Notepad++.

Note that you have to be in a unicode format for the characters to display in Notepad++. This is done by going to the Format menu and selecting "Encode in UTF-8" or similar. Now you can paste in Greek letters, subscripts, etc. into Notepad++.