C linkage example: In the program shown, where is variable 'a' declared and where is it defined? (extern int a; ... int a = 20;)

Introduction / Context:
The C language distinguishes between declaration (introducing a name and type) and definition (allocating storage or providing a body). This is especially important for global variables, which are often declared in headers and defined in exactly one source file. The snippet uses 'extern int a;' and later 'int a = 20;'. We must identify which line declares and which line defines the object 'a'.

Given Data / Assumptions:

• Global context contains 'extern int a;' inside main's scope (visible as a declaration) and later a global definition 'int a = 20;'.
• We are discussing a non-const, non-static integer with external linkage.
• Standard C rules for declarations/definitions apply.

Concept / Approach:

• 'extern int a;' is a declaration only: it promises that a definition exists elsewhere and does not allocate storage.
• 'int a = 20;' is a definition: it allocates storage and initializes the variable with 20.
• One program must have exactly one definition for 'a' with external linkage; multiple declarations are allowed.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• Swapping declaration/definition reverses roles and is incorrect.
• Claiming 'a is not defined' contradicts the explicit definition present.
• Stating both are declarations ignores that initialization with storage constitutes a definition.

Common Pitfalls:

• Placing a definition in a header and including it in multiple files causes multiple-definition linker errors.
• Confusing 'extern' on variables (suppresses definition) with 'extern' on functions (redundant for linkage).

Final Answer: