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For historical reasons, you can write
before a local variable declaration.
auto merely emphasizes
that the variable isn’t static; it changes nothing.
register suggests to the compiler storing this variable in a
register. However, GNU C ignores this suggestion, since it can
choose the best variables to store in registers without any hints.
It is an error to take the address of a variable declared
register, so you cannot use the unary ‘&’ operator on it.
If the variable is an array, you can’t use it at all (other than as
the operand of
sizeof), which makes it rather useless.