The auto Storage-Class Specifier
The auto storage-class specifier declares an automatic variable, a variable with a local lifetime. An auto variable is visible only in the block in which it is declared. Declarations of auto variables can include initializers, as discussed in Initialization. Since variables with auto storage class are not initialized automatically, you should either explicitly initialize them when you declare them, or assign them initial values in statements within the block. The values of uninitialized auto variables are undefined. (A local variable of auto or register storage class is initialized each time it comes in scope if an initializer is given.)
An internal static variable (a static variable with local or block scope) can be initialized with the address of any external or static item, but not with the address of another auto item, because the address of an auto item is not a constant.