Scope Rules

A scope in any programming is a region of the program where a defined variable can have its existence and beyond that variable cannot be accessed.

 There are three places where variables can be declared in C programming language:

       1) Inside a function or a block which is called local variables, 
       2) Outside of all functions which is called global variables.
       3) In the definition of function parameters which is called formal parameters.

 Let us explain 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. Following is the example using local variables. 

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 Variable :

                 Global variables are defined outside of a function, usually on top of the program. The global variables will hold their value 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. Following is the example using global and local variables:

#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 value of local variable inside a function will take preference. Following 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 parameter :

               Function parameters, so called formal parameters, are treated as local variables within that function and they will take preference over the 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 variable : 

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 its memory location.