Vim's "path" variable isn't changing or updating with modeline setting (finding header file in different directory)

Question

I'm trying to do something relatively simple: Editing a file in src and pointing Vim to the header file located in include. My .vimrc file has set modeline on, and the first line in my file is: // vim: set path += ../include (I've also tried include, include;, and several other variations). When I look at the path variable:

echo &path gives me .,/usr/include,,; the path doesn't change regardless of what I put in the modeline.

echo path gives me nothing.

What am I doing incorrectly here?

Answer 1

There are basically, two forms of modelines. Let me copy in the most important parts here together with an example:

 There are two forms of modelines.  The first form:
   [text{white}]{vi:vim:ex:}[white]{options}
 [...]
 Examples:
   vi:noai:sw=3 ts=6
 [...]
 The second form (this is compatible with some versions of Vi):
   [text{white}]{vi:vim:|Vim:ex:}[white]se[t] {options}:[text]
 [...]
 Examples:
   /* vim: set ai tw=75: */

Since you have been using the :set command, this means you are trying to create the latter form. There are two things to note here: First, you cannot use whitespace in the option value, otherwise Vim will parse those as separate options. The second is, you need to add a trailing : to indicate where the modeline ends.

This means, in your case, the complete line with the modeline should look like this:

// vim: set path+=../include:

Here is a simple tip, I use when creating a modeline. After reloading the file (:e), you can check the value of the options from the modeline by using :verbose set path?. Vim will output something like:

  path=.,,,../include
  Last set from modeline line 8