External Variables and Scope in C

By: Norman Chap Viewed: 153242 times    

The variables in main, such as line, longest, etc., are private or local to main. Because they are declared within main, no other function can have direct access to them. The same is true of the variables in other functions; for example, the variable i in getline is unrelated to the i in copy. Each local variable in a function comes into existence only when the function is called, and disappears when the function is exited. This is why such variables are usually known as automatic variables, following terminology in other languages. We will use the term automatic henceforth to refer to these local variables.

Because automatic variables come and go with function invocation, they do not retain their values from one call to the next, and must be explicitly set upon each entry. If they are not set, they will contain garbage.

As an alternative to automatic variables, it is possible to define variables that are external to all functions, that is, variables that can be accessed by name by any function. (This mechanism is rather like Fortran COMMON or Pascal variables declared in the outermost block.) Because external variables are globally accessible, they can be used instead of argument lists to communicate data between functions. Furthermore, because external variables remain in existence permanently, rather than appearing and disappearing as functions are called and exited, they retain their values even after the functions that set them have returned.

An external variable must be defined, exactly once, outside of any function; this sets aside storage for it. The variable must also be declared in each function that wants to access it; this states the type of the variable. The declaration may be an explicit extern statement or may be implicit from context. To make the discussion concrete, let us rewrite the longest-line program with line, longest, and max as external variables. This requires changing the calls, declarations, and bodies of all three functions.

   #include <stdio.h>

   #define MAXLINE 1000    /* maximum input line size */

   int max;                /* maximum length seen so far */
   char line[MAXLINE];     /* current input line */
   char longest[MAXLINE];  /* longest line saved here */

   int getline(void);
   void copy(void);

   /* print longest input line; specialized version */
   main()
   {
       int len;
       extern int max;
       extern char longest[];

       max = 0;
       while ((len = getline()) > 0)
           if (len > max) {
               max = len;
               copy();
           }
       if (max > 0)  /* there was a line */
           printf("%s", longest);
       return 0;
   }

   /* getline:  specialized version */
   int getline(void)
   {
       int c, i;
       extern char line[];

       for (i = 0; i < MAXLINE - 1
            && (c=getchar)) != EOF && c != '\n'; ++i)
                line[i] = c;
       if (c == '\n') {
           line[i] = c;
           ++i;
       }
       line[i] = '\0';
       return i;
   }

   /* copy: specialized version */
   void copy(void)
   {
       int i;
       extern char line[], longest[];

       i = 0;
       while ((longest[i] = line[i]) != '\0')
           ++i;
   }
The external variables in main, getline and copy are defined by the first lines of the example above, which state their type and cause storage to be allocated for them. Syntactically, external definitions are just like definitions of local variables, but since they occur outside of functions, the variables are external. Before a function can use an external variable, the name of the variable must be made known to the function; the declaration is the same as before except for the added keyword extern.

In certain circumstances, the extern declaration can be omitted. If the definition of the external variable occurs in the source file before its use in a particular function, then there is no need for an extern declaration in the function. The extern declarations in main, getline and copy are thus redundant. In fact, common practice is to place definitions of all external variables at the beginning of the source file, and then omit all extern declarations.

If the program is in several source files, and a variable is defined in file1 and used in file2 and file3, then extern declarations are needed in file2 and file3 to connect the occurrences of the variable. The usual practice is to collect extern declarations of variables and functions in a separate file, historically called a header, that is included by #include at the front of each source file. The suffix .h is conventional for header names. The functions of the standard library, for example, are declared in headers like <stdio.h>.

Since the specialized versions of getline and copy have no arguments, logic would suggest that their prototypes at the beginning of the file should be getline() and copy(). But for compatibility with older C programs the standard takes an empty list as an old-style declaration, and turns off all argument list checking; the word void must be used for an explicitly empty list.

You should note that we are using the words definition and declaration carefully when we refer to external variables in this section.``Definition'' refers to the place where the variable is created or assigned storage; ``declaration'' refers to places where the nature of the variable is stated but no storage is allocated.

By the way, there is a tendency to make everything in sight an extern variable because it appears to simplify communications - argument lists are short and variables are always there when you want them. But external variables are always there even when you don't want them. Relying too heavily on external variables is fraught with peril since it leads to programs whose data connections are not all obvious - variables can be changed in unexpected and even inadvertent ways, and the program is hard to modify. The second version of the longest-line program is inferior to the first, partly for these reasons, and partly because it destroys the generality of two useful functions by writing into them the names of the variables they manipulate.

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