Scope rules in C | My CS Tutorial

Scope rules of variable in C programming | Scope rules | C variable scope | Scope of variable in C

In C every variable defined in scope. You can define scope as the section or region of a program where a variable has its existence; moreover, that variable cannot be used or accessed beyond that region.

A scope in any programming is a region of the program where a defined variable
can have its existence and beyond that variable it cannot be accessed. There are
three places where variables can be declared in C programming language:
 Inside a function or a block which is called local variables,
 Outside of all functions which is called global variables.
 In the definition of function parameters which are called formal
parameters.
Let us understand what are local and global variables, and formal parameters.

Local Variables

Variables that are declared inside a function or block are called local variables.
They can be used only by statements that are inside that function or block of
code. Local variables are not known to functions outside their own. The following
example shows how local variables are used. Here all the variables a, b, and c
are local to main() function.

#include <stdio.h>
int main ()
{
 /* local variable declaration */
 int a, b;
 int c;
 /* actual initialization */
 a = 10;
 b = 20;
 c = a + b;
 printf ("value of a = %d, b = %d and c = %d\n", a, b, c);
 return 0;
}

Global Variables

Global variables are defined outside a function, usually on top of the program.
Global variables hold their values throughout the lifetime of your program and
they can be accessed inside any of the functions defined for the program.
A global variable can be accessed by any function. That is, a global variable is
available for use throughout your entire program after its declaration. The
following program shows how global variables are used in a program.

#include <stdio.h>
/* global variable declaration */
int g;
int main ()
{
 /* local variable declaration */
 int a, b;
 /* actual initialization */
 a = 10;
 b = 20;
 g = a + b;
 printf ("value of a = %d, b = %d and g = %d\n", a, b, g);
 return 0;
}


A program can have same name for local and global variables but the value of
local variable inside a function will take preference. Here is an example:

#include <stdio.h>
/* global variable declaration */
int g = 20;

int main ()
{
 /* local variable declaration */
 int g = 10;
 printf ("value of g = %d\n", g);
 return 0;
}
When the above code is compiled and executed, it produces the following result:
value of g = 10

Formal Parameters

Formal parameters are treated as local variables with-in a function and they take
precedence over global variables. Following is an example:


#include <stdio.h>
/* global variable declaration */
int a = 20;
int main ()
{
 /* local variable declaration in main function */
 int a = 10;
 int b = 20;
 int c = 0;
 printf ("value of a in main() = %d\n", a);
 c = sum( a, b);
 printf ("value of c in main() = %d\n", c);
 return 0;
}
/* function to add two integers */
int sum(int a, int b)
{
 printf ("value of a in sum() = %d\n", a);
 printf ("value of b in sum() = %d\n", b);
 return a + b;
}

When the above code is compiled and executed, it produces the following result:
value of a in main() = 10
value of a in sum() = 10
value of b in sum() = 20
value of c in main() = 30

Initializing Local and Global Variables

When a local variable is defined, it is not initialized by the system, you must
initialize it yourself. Global variables are initialized automatically by the system
when you define them, as follows:

Data Type         Initial Default Value

int                          0
char                     '\0'
float                       0
double                   0
pointer                NULL


It is a good programming practice to initialize variables properly, otherwise your
program may produce unexpected results, because uninitialized variables will
take some garbage value already available at their memory location.

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Created by-- HARSH CHAUHAN