Why declare a variable or function static in C?

Question

I understand what static does, but not why we use it. Is it just for keeping the abstraction layer?

Answer 1

The keyword static has several uses; Outside of a function it simply limits the visibility of a function or variable to the compilation unit (.c file) the function or variable occurs in. That way the function or variable doesn't become global. This is a good thing, it promotes a kind of "need to know" principle (don't expose things that don't need to be exposed). Static variables of this type are zero initialized, but of course global variables are also zero initialized, so the static keyword is not responsible for zero initialization per se.

Variables can also be declared static inside a function. This feature means the variable is not automatic, i.e. allocated and freed on the stack with each invocation of the function. Instead the variable is allocated in the static data area, it is initialized to zero and persists for the life of the program. If the function modifies it during one invocation, the new modified value will be available at the next invocation. This sounds like a good thing, but there are good reasons "auto" is the default, and "static" variables within functions should be used sparingly. Briefly, auto variables are more memory efficient, and are essential if you want your function to be thread safe.