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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
/* 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
/* 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
/* 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.