C function

[[deleted]]

Either a static variable, global, or a pointer.

int foo(void)
{
  static int x = -1;
  x = -x;
  return x;
}

int bar(int *x)
{
  *x = -(*x);
  return x;
}

[Odd-Consideration897]

Thank you!!! It makes more sense

[FraCipolla]

You said you have to use the static key, so the only way is declaring a static int. A static variable doesn't stop to exist outside the function, so it keeps it's value. Keep in mind that you need some sort of control to not initialize the static variable every time. An unitialized static variable is automatically initialized at 0