C Storage Classes

# 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

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;

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

static variables can be 'seen' within all functions in this source file. At link time, the static variables defined here will not be seen by the object modules that are brought in.

'static' can also be defined within a function! If this is done the variable is initalised at run time but is not reinitalized when the function is called. This is serious stuff - tread with care.

    	{
    	    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);
            }

Now, '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 but 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.

C++ has overloaded static, here are the details.

## 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();
    }					}
| test

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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