Global Variables in a Programming Language

Global Variables in a Programming Language

Global Variables in Programming Languages


  • Global Variables are variables that are declared outside all subroutines or functions.
  • They are defined and initialized in a global scope which makes them accessible throughout the program.

Scope and Lifetime

  • The scope of a global variable extends across the entire code, meaning they can be used by any part of the program.
  • The lifetime of a global variable is for the entire runtime of the program. Once the global variable is defined, it remains in memory until the program terminates.

Properties and Usage

  • All functions and subroutines can access global variables and change their value.
  • This means if a subroutine alters the value of a global variable, this change will be reflected in all other parts of the program that use the variable.
  • Global variables can be used to store information that needs to be shared among different subroutines in the program.


  • Global variables make it easy to share data between subroutines.
  • They can be used to keep track of state or exchange information without using function arguments or return values.
  • Global variables can simplify code by eliminating the need to pass frequently used values as parameters to multiple functions.


  • Overuse of global variables can lead to code that is difficult to debug and maintain. If a bug is caused by the incorrect value of a global variable, it can be hard to identify where this value got changed.
  • Global variables can lead to name clashes. If a local variable has the same name as a global one, the local variable will overshadow the global variable, often leading to unexpected results.
  • They introduce the risk of data manipulation from anywhere in the program, which can lead to unpredictable side effects.
  • Programs that rely on global variables are not easily reusable and scalable. Changing the program can lead to unintentional side effects because of the dependence on global state.

Relationship between Local and Global Variables

  • A local variable with the same name as a global variable takes precedence within its local scope.
  • Despite their scope difference, local and global variables have the same naming conventions.
  • Understanding the behaviour of both local and global variables, as well as the interactions between them, is critical in writing efficient and bug-free programs.