Consider a file-scope definition, unsigned i = 1, and a block-scope declaration, extern unsigned i. The declaration names an existing object rather than creating the block's separate unsigned i. This is external linkage: declarations in different translation units can designate the same object or function, with the linker establishing their correspondence. TAKEAWAY: linkage belongs to identifiers, not to the objects they represent. section 13.2
Shadowing an extern variable
In the example, file-scope unsigned i is initialized to 1, while main declares a new unsigned i initialized to 2. Inside the if block, extern unsigned i refers back to the existing file-scope object; the else branch refers to the block-scope object. Scope and linkage therefore answer different questions: scope controls which declaration is visible, while linkage can connect declarations across object files. section 13.2
Listing 13.2. An example of shadowing with an extern variable
1 #include <stdio.h>
2
3 unsigned i = 1;
4
5 int main(void) {
6 unsigned i = 2; /* A new object */
7 if (i) {
8 extern unsigned i; /* An existing object */
9 printf("%u\n", i);
10 } else {
11 printf("%u\n", i);
12 }
13 }
!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":1},{\"line\":0,\"ch\":2}],[{\"line\":2,\"ch\":18},{\"line\":2,\"ch\":19}],[{\"line\":9,\"ch\":0},{\"line\":9,\"ch\":1}],[{\"line\":10,\"ch\":0},{\"line\":10,\"ch\":1}],[{\"line\":11,\"ch\":0},{\"line\":11,\"ch\":1}],[{\"line\":12,\"ch\":0},{\"line\":12,\"ch\":1}],[{\"line\":0,\"ch\":5},{\"line\":0,\"ch\":13}],[{\"line\":1,\"ch\":1},{\"line\":1,\"ch\":2}],[{\"line\":5,\"ch\":20},{\"line\":5,\"ch\":21}],[{\"line\":11,\"ch\":1},{\"line\":11,\"ch\":2}],[{\"line\":2,\"ch\":1},{\"line\":2,\"ch\":2}],[{\"line\":12,\"ch\":1},{\"line\":12,\"ch\":2}],[{\"line\":2,\"ch\":5},{\"line\":2,\"ch\":13}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":15}],[{\"line\":7,\"ch\":16},{\"line\":7,\"ch\":24}],[{\"line\":3,\"ch\":1},{\"line\":3,\"ch\":2}],[{\"line\":4,\"ch\":1},{\"line\":4,\"ch\":2}],[{\"line\":4,\"ch\":5},{\"line\":4,\"ch\":8}],[{\"line\":8,\"ch\":11},{\"line\":8,\"ch\":14}],[{\"line\":10,\"ch\":11},{\"line\":10,\"ch\":14}],[{\"line\":4,\"ch\":9},{\"line\":4,\"ch\":13}],[{\"line\":4,\"ch\":14},{\"line\":4,\"ch\":18}],[{\"line\":5,\"ch\":1},{\"line\":5,\"ch\":2}],[{\"line\":2,\"ch\":5},{\"line\":2,\"ch\":13}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":15}],[{\"line\":7,\"ch\":16},{\"line\":7,\"ch\":24}],[{\"line\":6,\"ch\":1},{\"line\":6,\"ch\":2}],[{\"line\":6,\"ch\":7},{\"line\":6,\"ch\":9}],[{\"line\":7,\"ch\":1},{\"line\":7,\"ch\":2}],[{\"line\":7,\"ch\":9},{\"line\":7,\"ch\":24}],[{\"line\":8,\"ch\":1},{\"line\":8,\"ch\":2}],[{\"line\":8,\"ch\":9},{\"line\":8,\"ch\":15}],[{\"line\":10,\"ch\":9},{\"line\":10,\"ch\":15}],[{\"line\":9,\"ch\":0},{\"line\":9,\"ch\":2}],[{\"line\":9,\"ch\":9},{\"line\":9,\"ch\":13}],[{\"line\":10,\"ch\":0},{\"line\":10,\"ch\":2}],[{\"line\":8,\"ch\":9},{\"line\":8,\"ch\":15}],[{\"line\":10,\"ch\":9},{\"line\":10,\"ch\":15}],[{\"line\":11,\"ch\":0},{\"line\":11,\"ch\":2}],[{\"line\":12,\"ch\":0},{\"line\":12,\"ch\":2}]]"}
!@%STYLE%@!
Inspect the complete C example showing a block-scope extern declaration alongside a shadowing local variable. section 13.2
