Just clarify one thing, the static variables within a function are not allocated to the function or thread's stack, instead, they are allocated to BSS area, just like other global variables. so it won't affect their function too much.

if Volatile used then the compiler dont consider to optimize that variable . Assume that a variable changed at the time of running . But some compiler optimized .to avoid that we use volatile. Static is the keyword which is used to initialized to once that means extra burden for a compiler to keep the variable .and permanantly allocate space in stack.if you diclare static array then stack overflow will occur.