difference between static and volatile variable

Projects Programming
1

Hi
could someone please explain the difference between static and volatile variables please and when to use each. I'm looking to define a few variables for time inside a function and don't want to lose there values between calls to the function.
Thanks
Don

2

Static variables retain their value between function calls.

Volatile variables (which is not the opposite of static) are used when a variable is used both within an ISR (interrupt service routine) and outside it.

The opposite of static is really "auto". eg.

void foo ()
  {
  auto int i;
  }

However since auto is implied, most programmers leave it off, eg.

void foo ()
  {
  int i;
  }

In both cases the variable "i" does not retain its value from one call of foo to the next.

3

Nick
Thank you for the explanation of that.
Don

4

Note that the "auto" keyword is changing meaning, so that a variable is automatically given the data type of its initialiser, I guess because the old usage was redundant.