static

The static storage class serves different purposes, depending on the location of the declaration in the file:

To confine the identifier to that translation unit only (scope=file).

/* No other translation unit can use this variable. */
static int i;

/* Same; static is attached to the function type of f, not the return type int. */
static int f(int n);

To save data for use with the next call of a function (scope=block):

void foo()
{
    static int a = 0; /* has static storage duration and its lifetime is the
                       * entire execution of the program; initialized to 0 on 
                       * first function call */ 
    int b = 0; /* b has block scope and has automatic storage duration and 
                * only "exists" within function */
    
    a += 10;
    b += 10; 

    printf("static int a = %d, int b = %d\n", a, b);
}

int main(void)
{
    int i;
    for (i = 0; i < 5; i++)
    {
        foo();
    }

    return 0;
}

This code prints:

static int a = 10, int b = 10
static int a = 20, int b = 10
static int a = 30, int b = 10
static int a = 40, int b = 10
static int a = 50, int b = 10

Static variables retain their value even when called from multiple different threads.

Used in function parameters to denote an array is expected to have a constant minimum number of elements and a non-null parameter:

/* a is expected to have at least 512 elements. */
void printInts(int a[static 512])
{
    size_t i;
    for (i = 0; i < 512; ++i)
        printf("%d\n", a[i]);
}

The required number of items (or even a non-null pointer) is not necessarily checked by the compiler, and compilers are not required to notify you in any way if you don’t have enough elements. If a programmer passes fewer than 512 elements or a null pointer, undefined behavior is the result. Since it is impossible to enforce this, extra care must be used when passing a value for that parameter to such a function.

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