C language linkage: Which types of linkages are recognized for identifiers in C—considering scope and visibility across translation units?

Introduction / Context:In C, linkage describes whether multiple declarations refer to the same entity across scopes and translation units. Understanding linkage is key to organizing headers and source files, avoiding multiple-definition errors, and intentionally restricting visibility of symbols.

Given Data / Assumptions:

• We are discussing identifiers with potential external visibility (functions, global objects) and those restricted to a translation unit or block.
• Storage-class specifiers like 'static' and 'extern' affect linkage.
• Block-scope variables generally have no linkage.

Concept / Approach:

• External linkage: The identifier refers to the same entity across translation units (e.g., non-static functions/globals).
• Internal linkage: The identifier is limited to a single translation unit (e.g., 'static' global objects or functions).
• No linkage: Each appearance refers to a distinct entity (e.g., block-scope variables, function parameters, typedef names).

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• Pairs or singletons omit one or two categories defined by the standard.
• Only External/Internal: ignores block-scope identifiers with no linkage.

Common Pitfalls:

• Confusing storage duration (automatic/static) with linkage (external/internal/none).
• Assuming 'extern' always defines; it usually only declares for objects.

Final Answer: