Lesson 8.6. Global variables

Definition

A global variable is a variable declared outside all functions. Its scope extends from its declaration to the end of the program, its memory allocation too. A global variable permanently allocates the memory space intended for it.

As far as possible, the use of global variables should be avoided.

In large projects, global variables can be modified by functions.
These variables constantly occupy memory space that could be freed up when the variable is no longer used.

Prefer local variables and parameter passed to functions instead of global variables.

If within a function a variable coexists with a global variable of the same name, it is the local variable that is considered in the function.

Example

Here is an example of a program that uses a global variable:

int i=0; // Global variable i

void fct(void);

int main(void) {
  for ( ;i<5;i++)   // The loop uses i
    fct ();
  return 0;
}

void fct(void) {
  printf ("i= %d\n", i);  // The function also uses i
}

Try online

The variable i is a global variable that is readable and writable in the main() and in the fct() function.

Exercise

The following program is provided which contains 4 global variables. Correct the program to remove all global variables by replacing them with local variables and functions arguments.

# include <stdio.h>

// Global variables TO BE DELETED
int a,b,c,sum;

// Function that ask for an integer
void input (void);

int main(void)
{
  // Ask the first integer
  input(); 
  a=c; 

  // Ask the second integer
  input();
  b=c;

  // Display the sum
  sum=a+b;
  printf ("The sum is %d\n",sum);
  return 0 ;
}

// Function that ask for an integer
void input (void)
{
  printf ("Enter an integer: ");
  scanf ("%d",&c);
}

Do the exercise

Solution

Quiz

Global variables are...

to avoid

to be favored

a bad programming habit

to be replaced by local variables whenever possible

Check

The scope of a global variable is ...

the first function after its declaration.

the whole program

all the code that follows its declaration

the function in which the variable is declared

Check

The memory allocated for a global variable ...

is minimized

occupies the memory space permanently

is released when the main() function ends

is kept, even if the program is terminated

Check

What does the following program display?

#include <stdio.h>
int var = 7;

int main(void) {
  int var = 12;
  printf ("var = %d",var);
  return 0;
}

var = 7

var = 12

var = 712

the program does not compile

Check Bravo! There are two variables named var: a local and a global. Local variables overwrite global variables.

Lesson 8.5 Summary Lesson 8.7

Lesson 8.6. Global variables

Definition

Example

Exercise

Quiz

See also