CODEWITHSUNDEEP
cheaderlinker
as3rdaccount
as3rdaccount

Reputation: 3951

C linking error for multiple definition

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?

Upvotes: 4

Views: 2231

Answers (1)

WhozCraig
WhozCraig

Reputation: 66254

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.

Upvotes: 5

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