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20.10 auto and register

For historical reasons, you can write auto or register 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.