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C Global Variables

In C programming language, the global variables are those variables that are defined outside all functions, usually at the top of a program. Global variables hold their values throughout the lifetime of a program, and they can be accessed inside any of the functions defined for the program.

If a function accesses and modifies the value of a global variable, then the updated value is available for other function calls.

If a variable is defined in a certain file, then you can still access it inside another code module as a global variable by using the extern keyword. The extern keyword can also be used to access a global variable instead of a local variable of the same name.

The declaration of a global variable in C is similar to the declaration of the normal (local) variables but global variables are declared outside of the functions.

Consider the following syntax to declare a global variable:

data_type variable_name;

// main or any function
int main()
{
}

The following program shows how global variables are used in a program:

#include <stdio.h>

/* global variable declaration */
int g = 10;

int main(){

   /* local variable declaration */
   int a;

   /* actual initialization */
   a = g * 2;
   printf("Value of a = %d, and g = %d\n", a, g);
   
   return 0;
}

When you run this code, it will produce the following output −

Value of a = 20, and g = 10

Global variables are accessible in all the functions in a C program. If any function updates the value of a global variable, then its updated value will subsequently be available for all the other functions.

The following example demonstrates the example of accessing global variables in C language:

#include <stdio.h>

/* global variable declaration */
int g = 10;

int function1();
int function2();

int main(){

   printf("Value of Global variable g = %d\n", g);
   
   function1();
   printf("Updated value of Global variable g = %d\n", g);
   
   function2();
   printf("Updated value of Global variable g = %d\n", g);
   
   return 0;
}

int function1(){
   g = g + 10;
   printf("New value of g in function1(): %d\n", g);
   return 0;
}

int function2(){  
   printf("The value of g in function2(): %d\n", g);
   g = g + 10;
   return 0;
}

Run the code and check its output −

Value of Global variable g = 10
New value of g in function1(): 20
Updated value of Global variable g = 20
The value of g in function2(): 20
Updated value of Global variable g = 30

Global variables are available to only those functions that are defined after their declaration. Global variables are declared outside of any function, so they can be accessed by all functions within the same file by default.

In this example, we declared a global variable (x) before the main() function. There is another global variable y that is declared after the main() function but before function1(). In such a case, the variable y, even thought it is a global variable, is not available for use in the main() function, as it is declared afterwards. As a result, you get an error.

#include <stdio.h>

/* global variable declaration */
int x = 10;

int function1();

int main(){

   printf("value of Global variable x= %d y=%d\n", x, y);
   function1();
   return 0;
}

int y = 20;

int function1(){
   printf ("Value of Global variable x = %d y = %d\n", x, y);
}

When you run this code, it will produce an error −

Line no 11: error: 'y' undeclared (first use in this function)
   11 | printf ("Value of Global variable x = %d y = %d\n", x, y);
      |                                                     ^

If you want to access a global variable when a local variable with same name is also there in the program, then you should use the extern keyword.

In this C Program, we have a global variable and a local variable with the same name (x). Now, let's see how we can use the keyword "extern" to avoid confusion −

#include <stdio.h>

// Global variable x
int x = 50;

int main(){

   // Local variable x
   int x = 10;{
      extern int x;
      printf("Value of global x is %d\n", x);
   }

   printf("Value of local x is %d\n", x);

   return 0;
}

When you run this code, it will produce the following output −

Value of global x is 50
Value of local x is 10

Global variables can simplify the programming logic. They can be accessed across functions, and you need not use a parameter-passing technique to pass variables from one function to another. However, it is not wise or efficient to have too many global variables in a C program, as the memory occupied by these variables is not released till the end of the program.

Using global declaration is not considered a good programming practice because it doesn't implement a structured approach. Global declarations are also not advised from a security point of view, as they are accessible to all the functions. Finally, using global declarations can make a program difficult to debug, maintain, and scale up.

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