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

static is used as both a storage class specifier and a linkage specifier. As a linkage specifier it restricts the scope of an otherwise global variable or function to a single compilation unit. This allows, for example a compilation unit to have variables and functions with the same identifier names as other compilation units but without causing a clash, since such identifiers are 'hidden' from the linker. This is useful if you are creating a library for example and need internal 'helper' functions that must not cause a conflict with user code.

As a storage class specifier applied to a local variable, it has different semantics entirely, but your question seems to imply that you are referring to static linkage.