Why don't I get 'Multiple Definitions' of enum in C?

Question

Let's say I have three files:

//m.h
const int RED = 1;
//m.h ends here

//f1.c
#include "m.h"
//f1.c ends here


//f2.c
#include "m.h"
int main() {return 0;}
//f2.c ends here

Compiling each one separately will work, but gcc -Wall f1.o f2.o -o prog will produce: multiple definition of 'RED'

Now if I replace the const with:

//m.h
enum {RED=1} colors;
//m.h ends here

I'll be able to compile prog and use RED as a const and won't get any multiple definition error.

Why is the behavior with enums different from the one visible when you have global variables or structs with the same name in different files?

Answer 1

An enumeration will not create a value in memory, which means there is no address added to the symbol table when linking.

A const variable will have an address in the compiled object, with a symbol name. When you try to link the two object files together, they each have the same symbol name "RED" pointing to different addresses, which is what causes your conflict.

Answer 2

Enumeration constants are not objects, they have only a value but no allocation inside the executable. So in your second example, RED is just a definition of a value in each of the files. On the other hand color is not what you think. It is a "tentative definition" of an object named color. If you don't use it, it will not be allocated and everything seems fine. But it only seems so.

In your first example you have a const-qualified object named RED in each of the files. Since it is initialized this is a definition and the symbol is generated in each of the .o files. Thus the complaint by the linker.