Multiple definition of a const char*

Question

I get the above message linker error for a global

const char* HOST_NAME = "127.0.0.1";

I don't think that I have compiled some files twice but here's my definition of the files anyway.

main.cpp

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include "connection.hpp"

connection.cpp

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <sys/socket.h>
#include <sys/types.h>
#include <netdb.h>
#include <arpa/inet.h>
#include "connection.hpp"

connection.hpp

#ifndef __connection__
#define __connection__
#include <unistd.h>
#include <netinet/in.h>

const int BUFFSIZE = sysconf(_SC_PAGESIZE);             //Define page size
const char* HOST_NAME = "127.0.0.1";                    //Local host
//Definition of a class
#endif

Any help?

Flophouse

Obviously the name HOST_NAME is also defined in one of the headers you include. Put the definition of HOST_NAME in a namespace. – Bowens

I thought of that too but I used other names e.g. LOL and had the same issue. – Flophouse

Why not make HOST_NAME const so it will get internal linkage? – Minutes

I think it's const already – Flophouse

@BillyGrande: HOST_NAME is not const. It points to const data. You're looking for const char * const HOST_NAME. – Rahm

Answer 1

You use wrong declaration for your string. You need to make your string a constant, since constants may be defined in several compilation units. This is why compiler does not report the same error for BUFFSIZE: BUFFSIZE is const, so it may be defined several times in different compilation units. But HOST_NAME is not const, so it is reported. HOST_NAME will be const if you change its declaration to

const char* const HOST_NAME = "127.0.0.1"; 

Then the error should disappear.

---

[C++11: 3.5/3]: A name having namespace scope (3.3.6) has internal linkage if it is the name of

• a variable, function or function template that is explicitly declared static; or,
• a variable that is explicitly declared const or constexpr and neither explicitly declared extern nor previously declared to have external linkage; or
• a data member of an anonymous union.

This effectively makes the constant "local" to each translation unit in which it is defined, removing the opportunity for conflict.

Dyadic

Ohhh, now I get it. The pointer was const but not the data. So now we make the data const as well and it works. Thanks a lot guys!! :) – Flophouse

Actually it was the other way round - your pointer was non-const pointer to const data. I made it const pointer to const data. – Dyadic

For how many years I have been programming, the const char* means the pointer will be constant and not the data. – Crystlecs

@SauerVoussoir sorry to say but you are wrong. You can find detailed explanation at #1143762 – Dyadic

Oh my bad, I did not see you corrected @BillyGrande since you did not mentioned him – Crystlecs

Answer 2

don't think that I have compiled some files twice

Nevertheless that's exactly what happened. You have compiled connection.hpp several times, each time you have # included it into some translation unit.

Either add static to the declaration, or add extern to it, delete the = somestring portion, and provide a definition in exactly one source file.

Stereotypy

Answer 3

You have included "connection.hpp" to both connection.cpp and main.cpp. Therefore it (const char* HOST_NAME = "127.0.0.1";) is defined in 2 cpp files.

Maugre

No header guards does not prevent it. – Maugre

Is that a problem? They are two different cpp files. – Flophouse

@SujithGunawardhane: You're correct. I misread the question as having one cpp file and 2 header files. – Rahm

Yes, If you define some variable in a header file it goes to all the cpp files which included. – Maugre

You can get this solved by defining "const char* HOST_NAME = "127.0.0.1";" in one cpp file and "extern const char* HOST_NAME;" in the other cpp file. Then you can access it from both cpp files. – Maugre