C Storage Classes.

C has a concept of 'Storage classes' which are used to define the scope (visability) and life time of variables and/or functions.

So what Storage Classes are available?

auto

register

static

extern

typedef

auto - storage class

	{
	    int Count;
	    auto int Month;
	}

register - Storage Class

	{
	  register int  Miles;
	}

static - Storage Class

static is the default storage class for global variables. The two variables below (count and road) both have a static storage class.

     static int Count;
     int Road;

     main()
     {
       printf("%d\n", Count);
       printf("%d\n", Road);
     }

'static' can also be defined within a function. If this is done, the variable is initalised at compilation time and retains its value between calls. Because it is initialsed at compilation time, the initalistation value must be a constant. This is serious stuff - tread with care.

     void Func(void)
     {
       static Count=1;
     }

Here is an example

There is one very important use for 'static'. Consider this bit of code.

     char *Func(void);

     main()
     {
       char *Text1;
       Text1 = Func();
     }

     char *Func(void)
     {
       char Text2[10]="martin";
       return(Text2);
     }

'Func' returns a pointer to the memory location where 'Text2' starts BUT Text2 has a storage class of auto and will disappear when we exit the function and could be overwritten by something else. The answer is to specify:

     static char Text[10]="martin";

The storage assigned to 'Text2' will remain reserved for the duration if the program.

extern - storage Class

 	Source 1				Source 2
        --------				--------


	extern int count;			int count=5;

        write()					main()
        {					{
          printf("count is %d\n", count);	  write();
        }					}

Source 1
Source 2

The compile command will look something like.

	gcc source1.c source2.c -o program

See Also: