The keyword static doesn't have a single clear use. The meaning depends on context.

• It can specify static storage duration, indicating that an object exists for the duration of the program's execution.

• It can specify internal linkage, indicating that an object with static storage, or a function, is not visible outside of its translation unit.

• It can indicate a minimum size for an arrays passed to a function.

The following skeleton of code shows four uses of static:

static int counter;

static void func(int sz, double arr[static 10])
{
  static double sum;
}

sum has block scope because it is declared inside a block statement. static gives it static storage duration, so it is created once at the start of the program's execution, and deleted when program execution ends. Without it, sum would have automatic storage duration, so a new object would be created on each entry to the function, and destroyed when the function returned.

counter has file scope, so it would already have static storage duration even without static, but the extra keyword means that counter has internal rather than external linkage. When static is applied to a function, such as func, the function has internal rather than external linkage.

The static that appears in the parameter list of func indicates that the second argument (assigned to arr) must be a pointer to the first of at least 10 elements of an array, or the behaviour would be undefined. (This feature is only available from C99.)