Global variables

Global variables are defined or declared outside every function scope: 

   extern int x;    // A global variable defined in another program file 

   int y = 4;       // A global variable

   void f( )
   {
      int y = 7;

      x = y + 3;
   }

   int main( int argc, char* argv[] )            
   {
      f( );
      printf( "%d  %d\n", x, y );
   }

Global variables are accessible (= can be used) in statements in any C program file

The (lexical) scope of global variables

The lexical scope (or scope) of a global variable is: 

   extern int x;    // A global variable defined in another program file

   int y = 4;       // A global variable         <- Scope of y starts here
                                                
   void f( )                                    
   {                                            
      int y = 7;     // Overshadows global var y
                                                
      x = y + 3;  // Uses y = 7                 
   }

   int main( int argc, char* argv[] )           
   {                                            
      f( );                                     
      printf( "%d  %d\n", x, y ); // Uses y = 4 
   }                                            
                                                 <- Scope of y ends here

• Scope starts at the definition or declaration
• Scope ends at the end of the C program file
• Global variables can be overshadowed by a local variable with the same variable name

Local declarations of global variables

Declarations of global variables can be written inside a function: 

   #include <stdio.h>

   int x;     

   void f( )
   {

      extern int y;    // Local declaration of global variable y 

      x = y + 3;
   }

   int y = 4;   // Global variable 

   int main( int argc, char* argv[] )            
   {
      f( );
      printf( "%d  %d\n", x, y );
   }

Scoping rule:   a local declaration follows the same scoping rules of local variables !

DEMO: demo/C/set1/scope-declare.c

Local declarations of global variables

Example that shows that a localized declaration obeys the scoping rule for local variables: 

   #include <stdio.h>

   int x;     

   void f( )
   {
      {
      extern int y;    // Scope of the declaration  
      } 
      x = y + 3; // Error: y is undeclared !
   }

   int y = 4;   // Global variable 

   int main( int argc, char* argv[] )            
   {
      f( );
      printf( "%d  %d\n", x, y );
   }

The scope of declaration of variable y ended in the inner scope !!

DEMO: demo/C/set1/scope-declare.c (edit and re-compile)

Scoping errors when using global variables

  • There are 2 common causes of scoping errors when you access global variables:

      1. The global variable has not (yet) been defined or declared when you used the global variable

          • In this case, you will receive a "variable undefined" compile error

      2. The global variable has been overshadowed by a local variable (with the same name) when you used the global variable

          • In this case, you will use a wrong variable

  • I will show you an example of each case next

Scoping errors when using global variables

Example where a global variable has not (yet) been defined or declared when you used it: 

   extern int x;    // A global variable defined in another program file


   void f( )
   {
      x = y + 3;  // Use of y outside it's scope    <---- Compile error
   }

   int y = 4;   // Global variable       // Scope of y starts here

   int main( int argc, char* argv[] )   
   {                                       
      printf( "%d  %d\n", x, y );       
   }

When  y was used, the variable definition was not yet processed !
Remember that the C-compiler is 1-pass compiler !!!

DEMO: demo/C/set1/scoping-err2.c (edit and re-compile)

Scoping errors when using global variables

Example where a global variable is overshadowed by a local variable: 

   extern int x;    // A global variable defined in another program file
   int y = 4;       // A global variable         <- Scope of y starts here
                                                
   void f( )                                    
   {                                            
      int y = 7;     // Overshadows global var y
                                                
      x = y + 3;  // Uses y = 7                  // Uses the local variable
   }

   int main( int argc, char* argv[] )           
   {                                            
      f( );                                     
      printf( "%d  %d\n", x, y ); // Uses y = 4 
   }                                            
                                                 <- Scope of y ends here

If the intention was to use the global variable (int y = 4), then the program is in error.