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A declaration can include type qualifiers to advise the compiler
about how the variable will be used. There are three different
qualifiers, const, volatile and restrict. They
pertain to different issues, so you can use more than one together.
For instance, const volatile describes a value that the
program is not allowed to change, but might have a different value
each time the program examines it. (This might perhaps be a special
hardware register, or part of shared memory.)
If you are just learning C, you can skip this chapter.