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  Concepts of Local and External Variables

Local and External Variables:

If we state, 

       f( ) {
               int x;
               ...
       }
       g( ) {
               int x;
               ...
       }

each x is local to its own routine-the x in f is not related to the x in g. (That is, local variables are also termed as ‘automatic’). Moreover each local variable in a routine emerges only whenever the function is called, and disappears whenever the function is exited. Local variables contain no memory from one call to the subsequent and should be explicitly initialized on each entry. There is a static storage class for forming local variables with memory.

As contrasting to local variables, external variables are stated external to all functions, and are potentially accessible to all functions. External storage for all the time remains in existence. To make variables external we have to state them externally to all functions, and, wherever we want to utilize them, make a declaration. 

       main( ) {
               extern int nchar, hist[ ];
               ...
               count( );
               ...
       }
       count( ) {
               extern int nchar, hist[ ];
               int i, c;
               ...
       }
       int     hist[129];      /* space for histogram */
       int     nchar;          /* character count */

Roughly speaking, any function which wishes to access an external variable should contain an extern declaration for it. The declaration is similar as others, apart from the added keyword extern. Moreover, there should somewhere be a definition of the external variables, external to all functions. External variables can be initialized; they are set to zero (0) if not explicitly initialized. In its simplest form, the initialization is completed by putting the value (that should be a constant) after the definition.

Function Arguments:

In the function strcopy, two string names as arguments then carry on to clobber both of them by incrementation. Therefore how come we do not lose the original strings in the function which called strcopy?

As we state before, ‘C is a call by value’ language: whenever you make a function call such as f(x), the value of x is passed, not its address. Therefore there's no way to modify x from inside f. If x is an array (char x[10]) this is not a problem, since x  is an address anyhow, and you are not trying to modify it, just what it addresses. This is the reason why strcopy works as it does. And it is convenient not to have to worry regarding making temporary copies of input arguments.

However what if x is scalar and you do want to modify it?  In that situation, you have to pass the address of x to f, and then utilize it as a pointer. Therefore for illustration, to interchange two integers, we should write: 

   flip(x, y)
      int *x, *y; {
           int temp;
           temp = *x;
           *x = *y;
           *y = temp;
   }

and to call flip, we encompass to pass the addresses of variables:

flip (&a, &b);

those interchange two integers.

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