C modifier

when variable has those modifier:
n/a variable outside function variable inside function
default default as global, could combine with extern default as auto, local scope
auto can not use outside function local scope
register can not use outside function local scope but faster access
static internal linkage, global to translation unit, can not combine with extern persistent vairable to function
static can also be used for function, when a function use static it means it can not be used outside its translation unit, such as declared in another file.

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