External linkage

 

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In the set of translation units and libraries that constitutes an entire program, each declaration of a particular identifier with external linkage denotes the same object or function.

If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined exactly as if it were declared with the storage-class specifier extern. So, by default, all functions have external linkage.

If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external. So, by default, all file scoped objects have external linkage.

Code in other translation units can access objects and functions with external linkage.

Example

Identifiers a and f have external linkage. Other translation units may access them.

Run this code

#include <stdio.h>
 
int a = 1;
 
void f (void) {printf("from function f()\n");}
 
int main(void)
{
    f();
 
    return 0;
}
/* end of this translation unit */

Possible output:

from function f()