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

auto is the default storage class for local variables. 
	{
	    int Count;
	    auto int Month;
	}
The example above defines two variables with the same storage class. auto can only be used within functions, i.e. local variables.

register - Storage Class

register is used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and cant have the unary '&' operator applied to it (as it does not have a memory location). 
	{
	  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

Click here for static functions

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

extern defines a global variable that is visable to ALL object modules. When you use 'extern' the variable cannot be initalized as all it does is point the variable name at a storage location that has been previously defined. 


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


	extern int count;			int count=5;

        write()					main()
        {					{
          printf("count is %d\n", count);	  write();
        }					}
Count in 'source 1' will have a value of 5. If source 1 changes the value of count - source 2 will see the new value. Here are some example source files.

Source 1
Source 2

The compile command will look something like.

	gcc source1.c source2.c -o program

See Also:

Data types.

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Martin Leslie