Q: What's the auto keyword good for?
A: Nothing; it's archaic. [footnote] (It's a holdover from C's typeless predecessor language B, where in the absence of keywords like int a declaration always needed a storage class.) See also question 20.37.
K&R1 Sec. A8.1 p. 193
ISO Sec. 126.96.36.199, Sec. 6.5.1
H&S Sec. 4.3 p. 75, Sec. 4.3.1 p. 76