liveBook · Manning

Linkage

As explained in Modern C, Third Edition

Linkage is the C property determining whether declarations in different scopes or translation units can designate the same entity.

Linkage determines whether declarations in different scopes or translation units can designate the same entity. External linkage lets identifiers in separate object files refer to one object or function; internal linkage keeps an identifier within one translation unit; identifiers without linkage do not connect declarations across scopes or files. The linker establishes correspondence for external-linkage identifiers. section 13.2

Listing 13.2 An example of shadowing with an extern variable
#include <stdio.h> 1 

unsigned i = 1;        

int main(void) { 2 
  unsigned i = 2;        /* A new object */
  if (i) {            3 
    extern unsigned i;   /* An existing object */ 
    printf("%u\n", i);  /* prints 1 */
  } else {
    printf("%u\n", i);  /* prints 2 */
  }
}

This example contrasts a block-scope declaration that creates a new object with an extern declaration that refers to the existing file-scope object: it prints 1 inside the conditional and 2 in the alternative branch. section 13.2

Keep these distinctions explicit:

  • Treat extern as a declaration mechanism, not as a guarantee of external linkage; it can refer to identifiers with either external or internal linkage. section 13.2
  • Use static at file or block scope to enforce internal linkage for a variable or function; do not confuse that keyword with static storage duration. section 13.2
  • Separate linkage from storage duration: static, automatic, allocated, and thread storage durations describe object lifetime rather than declaration correspondence. section 13.2
Table 13.1 Storage classes, scope, linkage of identifiers, and storage duration of the associated objects. Tentative indicates that a definition is implied only if there is no other definition with an initializer. Induced indicates that the linkage is internal if another declaration with internal linkage has been met prior to that declaration; otherwise, it is external. Compound literals follow similar rules with respect to the block in which they occur, if any.
Class Scope Definition Linkage Duration Lifetime
Initialized File Yes External Static Whole execution
extern, initialized File Yes External Static Whole execution
String literal Any Yes N/A Static Whole execution
static, initialized Any Yes Internal Static Whole execution
constexpr Any Yes Internal Static Whole execution
Uninitialized File Tentative External Static Whole execution
extern, uninitialized Any No Induced Static Whole execution
static, uninitialized Any Tentative Internal Static Whole execution
thread_local File Yes External Thread Whole thread
extern thread_local Any No External Thread Whole thread
static thread_local Any Yes internal Thread Whole thread
Non-VLA None
Non-VLA, auto Block Yes None Automatic Block of definition
register None
VLA Block Yes None Automatic From definition to
end of block
Function return Block Yes None Automatic To the end of
with array expression

The table compares scope, definition status, linkage, storage duration, lifetime, and initialization across storage classes and related declarations. section 13.2

How Modern C, Third Edition covers it

How deep it goes
Distinguishes external, internal, and no linkage, then separates linkage from storage duration and explains extern and static.
Examples
Uses extern shadowing, file-scope and block-scope identifiers, object files, library functions, and a storage-class comparison table.
What it assumes
Assumes familiarity with C declarations, scopes, functions, variables, and separate object files.
What you can do after
Evaluate whether declarations designate the same entity and distinguish linkage from storage duration.
How it connects
Connects scope, declarations, storage duration, lifetime, storage classes, translation units, object files, and the linker.
Cover of Modern C, Third Edition
liveBookmanning.com
Read Modern C, Third Edition on liveBook Get the book at manning.com

Also explained in

Compare how 2 books explain linkage →

Related concepts

This explainer was generated by Manning's knowledge graph from the text of Modern C, Third Edition. The section links point to the parts of the book it draws on.