Difference between Declaration, Definition & Initialisation of a Variable

In C programming, the terms declaration, definition, and initialisation refer to different stages of a variable's lifecycle.

Understanding the difference is key to writing correct and efficient code.

🔹 1. Declaration

• What it is: Tells the compiler that a variable exists, but doesn't allocate memory (unless it's also a definition).

• Why it's used: Usually for external variables (defined elsewhere).

• Syntax:

  extern int x;

• Memory allocated? ❌ No

• Value assigned? ❌ No

  
  

🔹 2. Definition

• What it is: Allocates memory for the variable. If not declared before, it's also treated as a declaration.

• Why it's used: To actually create the variable.

• Syntax:

  int x; // Definition (and declaration, if not done before)

• Memory allocated? ✅ Yes

• Value assigned? ❌ No (value is indeterminate if uninitialised)

  
  

🔹 3. Initialisation

• What it is: Assigning an initial value to a variable at the time of definition.

• Why it's used: To start the variable with a known value.

• Syntax:

  int x = 10;

• Memory allocated? ✅ Yes

• Value assigned? ✅ Yes

  
  

✅ Summary Table

#Embedded #Basics #Programming #AutoEConnectRecommended #InterviewPrep