What is the meaning of this warning, and how do I get rid of it?

Ron Ford wrote:

<snip>

The fact that I have to ask leaves me to believe that I have no use
for the 'volatile' function qualifier. [ ]

Qualify an object as volatile when you want to be sure that writes and
reads to it actually happen according to the abstract semantics of C.
If you need atomicity too, you can further use the sig_atomic_t
qualifier, though that everely restricts the guaranteed range of the
object.

<snip>