C linking error for multiple definition

Question

I have a .h file which I was intending to use only for storing all the information string that will be displayed in my program. In my info.h:

#ifndef __INFO_H
#define __INFO_H

char *info_msg = "This is version 1.0 of NMS.";

//all other strings used by view.c and controller.c

#endif

Then in my view.h I have as follows:

//view.h
#ifndef __VIEW_H
#define __VIEW_H   

#include "info.h"
//other stuff like method declaration etc.
#endif

My controller.h is using view.h:

//controller.h
#ifndef __CONTROLLER_H
#define __CONTROLLER_H   

#include "view.h"
#include "model.h"
//other stuff line method declaration etc.
#endif

main.c:

 #include "controller.h"
 int main()
 {
    //stuff
 }

view.c:

#include "view.h"

char esc,up,down,right,left;   
void change_character_setting(char pesc, char pup, char pdown, char pright, char pleft)
{      
  esc = pesc;
  up = pup;
  down = pdown;
  right = pright;
  left = pleft;
}


void print_warning()
{
 printf("%s \n",info_msg);
}

When I attempt to create the executable the linker complains:

/tmp/ccqylylw.o:(.data+0x0): multiple definition of `info_msg'
/tmp/cc6lIYhS.o:(.data+0x0): first defined here

I am not sure why it would see two definitions since I am using the protection block. I tried to google here but nothing specific showed up. Can someone explain how it is seeing multiple definitions? How do I achieve something as simple in Java to use one single file for all text manipulation in C?

NEVER assign variables in a header file. Declare them as extern in the header and define them in a source file. The compiler creates one instance for each include (e.g. in each compilation unit/source file). This creates errors at link time. — bash0r, Aug 25 '13 at 18:17
Thanks. Now I am getting the following error:/tmp/ccFMbvlv.o: In function `print_warning': view.c:(.text+0x44b): undefined reference to `info_msg' — as3rdaccount, Aug 25 '13 at 18:23
And the same error for all other strings in the file even after just externed all of them in the header and created a separate source file. — as3rdaccount, Aug 25 '13 at 18:24
I think you aren't showing everything that is relevant, because this should work. bash0r is right about using externs. .h files should be about defining stuff, not instantiating stuff. — Jiminion, Aug 25 '13 at 18:25
Updated the post. Still getting the same error after removing __ as suggested in the other answer. — as3rdaccount, Aug 25 '13 at 18:35
@ArunavDev Did you read the *entire* answer? The mere mentioning of underscore usage as violating the standard was a footnote, and I'll gladly remove it if it helps you read the meat of the solution to your problem, which you will find by reading the answer from the top-down rather than the bottom-up. — WhozCraig, Aug 26 '13 at 07:09

WhozCraig · Accepted Answer · 2013-09-17T22:02:05.263

5

You're compiling a global variable called info_msg into each source file that includes info.h either directly or pulled in from some other header. At link-time, the linker finds all these info_msg identifiers (one in each object file compiled) and doesn't know which one to use.

Change you're header to be:

#ifndef PROJ_INFO_H
#define PROJ_INFO_H

extern const char *info_msg;  // defined in info.cpp

#endif

And assuming you have an info.cpp (if not you can place this in any .cpp file, but that one would be the most natural location to maintain it):

// info.cpp
#include "info.h"

const char *info_msg = "This is version 1.0 of NMS.";

Note: Be careful when declaring preprocessor symbols and identifiers as to your placement of underscores. According to the C99 standard:

C99 §7.1.3/1

All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.

All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

edited Sep 17 '13 at 22:02

answered Aug 25 '13 at 18:24

WhozCraig

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Anything *containing* `__` is reserved. – chris Aug 25 '13 at 18:27
@chris whoa. reference please. (I don't doubt you, I just wanna read it). thanks. – WhozCraig Aug 25 '13 at 18:28
[reserved identifier](http://stackoverflow.com/questions/228783/what-are-the-rules-about-using-an-underscore-in-a-c-identifier) – chris Aug 25 '13 at 18:28
@chris you are *the* man. sir. thanks. (and updating answer, danka for the link). – WhozCraig Aug 25 '13 at 18:29
The 'containing double underscore' thing is a C++ only point though, no? This is a C question... – Crowman Sep 17 '13 at 21:46
@PaulGriffiths i'll double check the standard, you are likely quite right. I rarely dive into C except this board. I'll surely take a look. – WhozCraig Sep 17 '13 at 21:48
1

@PaulGriffiths sure enough you're correct, and I'll be updating the text accordingly. Double-`_` is more lenient in C than C++. – WhozCraig Sep 17 '13 at 22:01

C linking error for multiple definition

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