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?

Obviously the name `HOST_NAME` is also defined in one of the headers you include. Put the definition of `HOST_NAME` in a namespace. — 101010, Jul 03 '14 at 15:59
I thought of that too but I used other names e.g. LOL and had the same issue. — Billy Grande, Jul 03 '14 at 16:00
Why not make `HOST_NAME` const so it will get internal linkage? — Kerrek SB, Jul 03 '14 at 16:02
@BillyGrande: `HOST_NAME` is not const. It points to const data. You're looking for `const char * const HOST_NAME`. — Bill Lynch, Jul 03 '14 at 16:04

Wojtek Surowka · Accepted Answer · 2019-11-11T00:37:51.943

39

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.

edited Nov 11 '19 at 00:37

answered Jul 03 '14 at 16:07

Wojtek Surowka

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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!! :) – Billy Grande Jul 03 '14 at 16:12
3

Actually it was the other way round - your pointer was non-const pointer to const data. I made it const pointer to const data. – Wojtek Surowka Jul 03 '14 at 16:13
For how many years I have been programming, the const char* means the pointer will be constant and not the data. – Sauer Voussoir Jan 25 '23 at 09:18
@SauerVoussoir sorry to say but you are wrong. You can find detailed explanation at https://stackoverflow.com/questions/1143262/what-is-the-difference-between-const-int-const-int-const-and-int-const – Wojtek Surowka Jan 26 '23 at 00:24
Oh my bad, I did not see you corrected @BillyGrande since you did not mentioned him – Sauer Voussoir Jan 26 '23 at 20:55

score 2 · Answer 2 · edited Jul 03 '14 at 16:10

2

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.

edited Jul 03 '14 at 16:10

adpeace

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answered Jul 03 '14 at 16:02

Sujith Gunawardhane

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No header guards does not prevent it. – Sujith Gunawardhane Jul 03 '14 at 16:05
Is that a problem? They are two different cpp files. – Billy Grande Jul 03 '14 at 16:05
@SujithGunawardhane: You're correct. I misread the question as having one cpp file and 2 header files. – Bill Lynch Jul 03 '14 at 16:06
Yes, If you define some variable in a header file it goes to all the cpp files which included. – Sujith Gunawardhane Jul 03 '14 at 16:07
1

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. – Sujith Gunawardhane Jul 03 '14 at 16:09

score 2 · Answer 3 · answered Jul 03 '14 at 16:07

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.

Multiple definition of a const char*

main.cpp

connection.cpp

connection.hpp

3 Answers3