What is the C One-Definition Rule and How Does it Ensure Program Consistency?

The Nature of the One-Definition Rule in C

In C , the One-Definition Rule governs the behavior of programs by limiting multiple definitions of certain entities within a translation unit. Let's delve into the specifics of this rule:

Official Definition

Beyond the reference in The C Programming Language, 3rd ed., P. 9.2.3, the official definition of the rule can be found in the C standard within section 3.2:

"No translation unit shall contain more than one definition of any variable, function, class type, enumeration type or template."

Furthermore, the standard emphasizes that every non-inline function or object used in a program must have exactly one definition. If an inline function is employed, it must be defined in every translation unit where it's referenced.

Implications

This rule ensures that:

• Consistency: Multiple definitions can introduce inconsistencies in the program's behavior. The One-Definition Rule prevents such conflicts.
• Unique Entity Identity: Each variable, function, class type, and other defined entities have a unique identity, ensuring consistent behavior throughout the program.
• Linker Compatibility: The linker cannot correctly resolve references if multiple definitions exist. The rule allows for smooth linking and compilation.

Exceptions

The One-Definition Rule does not apply to:

• Inline functions, which must be defined in every translation unit they're used in.
• Implicitly defined functions (see sections 12.1, 12.4, and 12.8 of the C standard).

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