C Language Storage Classes static


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

  1. 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);
  2. 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++)
         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.

  1. 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.