C Storage Classes.

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

So what Storage Classes are available?

auto

register

static

extern

typedef

auto - storage class

   void main()
   {
       int Count;
       auto int Month;    
   }

register - Storage Class

   {
       register int  Miles;    
   }

register should only be used for variables that require quick access - such as counters. It should also be noted that defining register goes not mean that the variable will be stored in a register. It means that it MIGHT be stored in a register - depending on hardware and implimentation restrictions.

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);
    }

Global static variables can be 'seen' by all functions within the current r source file. At link time, global static variables will not be seen by other object modules that are brought in by the loader.

static can also be defined within a function. If this is performed, the variable is initalised at compilation and retains its value between calls. Because it is initialsed at compilation, 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

 extern int count;		    

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

 int count = 5;    

 main()
 {
     write();
 }

Source 1
Source 2

The compile command will look something like.

	gcc source1.c source2.c -o program

See Also: