Variable Scope and functions in C

By: Emiley J Emailed: 1768 times Printed: 2515 times    

Latest comments
By: rohit kumar - how this program is work
By: Kirti - Hi..thx for the hadoop in
By: Spijker - I have altered the code a
By: ali mohammed - why we use the java in ne
By: ali mohammed - why we use the java in ne
By: mizhelle - when I exported the data
By: raul - no output as well, i'm ge
By: Rajesh - thanx very much...
By: Suindu De - Suppose we are executing

The functions and external variables that make up a C program need not all be compiled at the same time; the source text of the program may be kept in several files, and previously compiled routines may be loaded from libraries. Among the questions of interest are
  • How are declarations written so that variables are properly declared during compilation?
  • How are declarations arranged so that all the pieces will be properly connected when the program is loaded?
  • How are declarations organized so there is only one copy?
  • How are external variables initialized?
Let us discuss these topics by reorganizing the calculator program into several files. As a practical matter, the calculator is too small to be worth splitting, but it is a fine illustration of the issues that arise in larger programs.

The scope of a name is the part of the program within which the name can be used. For an automatic variable declared at the beginning of a function, the scope is the function in which the name is declared. Local variables of the same name in different functions are unrelated. The same is true of the parameters of the function, which are in effect local variables.

The scope of an external variable or a function lasts from the point at which it is declared to the end of the file being compiled. For example, if main, sp, val, push, and pop are defined in one file, in the order shown above, that is,

   main() { ... }

   int sp = 0;
   double val[MAXVAL];

   void push(double f) { ... }

   double pop(void) { ... }
then the variables sp and val may be used in push and pop simply by naming them; no further declarations are needed. But these names are not visible in main, nor are push and pop themselves.

On the other hand, if an external variable is to be referred to before it is defined, or if it is defined in a different source file from the one where it is being used, then an extern declaration is mandatory.

It is important to distinguish between the declaration of an external variable and its definition. A declaration announces the properties of a variable (primarily its type); a definition also causes storage to be set aside. If the lines

   int sp;
   double val[MAXVAL];
appear outside of any function, they define the external variables sp and val, cause storage to be set aside, and also serve as the declarations for the rest of that source file. On the other hand, the lines
   extern int sp;
   extern double val[];
declare for the rest of the source file that sp is an int and that val is a double array (whose size is determined elsewhere), but they do not create the variables or reserve storage for them.

There must be only one definition of an external variable among all the files that make up the source program; other files may contain extern declarations to access it. (There may also be extern declarations in the file containing the definition.) Array sizes must be specified with the definition, but are optional with an extern declaration.

Initialization of an external variable goes only with the definition.

Although it is not a likely organization for this program, the functions push and pop could be defined in one file, and the variables val and sp defined and initialized in another. Then these definitions and declarations would be necessary to tie them together:

  in file1:

      extern int sp;
      extern double val[];

      void push(double f) { ... }

      double pop(void) { ... }
  in file2:
      int sp = 0;
      double val[MAXVAL];
Because the extern declarations in file1 lie ahead of and outside the function definitions, they apply to all functions; one set of declarations suffices for all of file1. This same organization would also bee needed if the definition of sp and val followed their use in one file.

C Home | All C Tutorials | Latest C Tutorials

Sponsored Links

If this tutorial doesn't answer your question, or you have a specific question, just ask an expert here. Post your question to get a direct answer.



Bookmark and Share

Comments(0)


Be the first one to add a comment

Your name (required):


Your email(required, will not be shown to the public):


Your sites URL (optional):


Your comments:



More Tutorials by Emiley J
Password must include both numeric and alphabetic characters - Magento
What is Hadoop?
Returning multiple values from a web service
Tomcat and httpd configured in port 8080 and 80
Java Webservices using Netbeans and Tomcat
Java WebService connected to Database
How to Deploy a Java Web Service
Call a webservice in Java
Java WebService - Create your first web service in Java
package javax.jws does not exist
Getting Started with Android
HTML5 Location - getCurrentPosition() in HTML5
HTML5 Canvas - Using Canvas in HTML5
HTML5 - Introduction
HTML5 Video - Handling video in HTML5

More Tutorials in C
Sum of the elements of an array in C
Printing a simple histogram in C
Sorting an integer array in C
Find square and square root for a given number in C
Simple arithmetic calculations in C
Command-line arguments in C
Calculator in C
Passing double value to a function in C
Passing pointer to a function in C
Infix to Prefix And Postfix in C
while, do while and for loops in C
Unicode and UTF-8 in C
Formatting with printf in C
if, if...else and switch statements in C with samples
Statements in C

More Latest News
Most Viewed Articles (in C )
Open, Creat, Close, Unlink system calls sample program in C
UNIX read and write system calls sample program in C
Using memset(), memcpy(), and memmove() in C
Character Counting sample program in C
register Variables in C
Listing Files and Directories sample program in C
Sum of the elements of an array in C
The Birth and history of C Programming Language
Functions in C
Data Types and Sizes in C
Variable Scope and functions in C
Pointers and Function Arguments in C
Pointers to Functions example in C
Basics of Structures in C
Variable-length Argument Lists sample program in C
Most Emailed Articles (in C)
Arguments - Call by Value in C
Increment and Decrement Operators in C
The if else statement in C
Do while Loops in C
Printing a simple histogram in C
The Birth and history of C Programming Language
Basics of C
Getting Started with C
Variables and Arithmetic Expressions in C
The for statement in C
Symbolic Constants using #define in C
File Copying in C
Character Counting sample program in C
Line Counting sample program in C
Word Counting sample program in C