CODEWITHSUNDEEP
cvisual-studiocompiler-errorsheader-files
Ewan
Ewan

Reputation: 357

Header File being included twice

I am relatively new to C (as I don't want to use C++, or at least just yet) and I'm not sure how to fix my include error I am having.

I have a header file containing the constant value of 1000 and is called Test.

const int Test = 1000;

I have this file included in 2 files - Myfile.c and Myfile2.c each including the file as such:

#include "MyHeader.h"

My project will not build/compile and as I have found out it is including the header twice which is not allowed as I am declaring my variable "Test" twice. Upon research I found this on Wikipedia: http://bit.ly/10wPraP

I used this "Include Guard"

Example:

#ifndef MY_HEADER
#define MY_HEADER
 const int Test = 1000;
#endif

and I have also tried the pre-processor(?) command pragma once.

#pragma once

However, my program still will not build. I now get error saying that the varibale "Test" is already defined in MyFile.obj.

I thought this might be a Visual Studio-ism as I am using that but both my 2010 Express C++ and VS2003 Professional wont build this. I have tried cleaning the project within Visual Studio and I am not sure what else to do.

Am I being very silly and missing something obvious here and that is why it isn't work?

I am used to C# and "using" with namespaces rather than includes. May my setting on VS to only compile C code be effecting this?

Upvotes: 3

Views: 7288

Answers (4)

MonkeyWithDarts
MonkeyWithDarts

Reputation: 775

Useful Reference:

The problem is with the way that header files are processed when you #include them: Header files are literally copied and pasted into the body of your C files. This means that Myfile.c and Myfile2.c both have their own declarations of an int named Test - essentially creating two different versions of the one variable. The linker then complains about having two different variables with the same name.

The solution is to put the const int Test = 1000; in one of your C files, and to add extern const int Test; to MyHeader.h. This way, the variable is declared only once, and all files are aware of the one variable because the extern directive tells them that another file has the variable Test they are looking for.

MyHeader.h

extern const int Test;

Myfile.c (for instance)

#include "MyHeader.h"
...
const int Test = 1000;

Myfile2.c

#include "MyHeader.h"
...
<use Test>

Upvotes: 3

rashok
rashok

Reputation: 13494

Define that variable in any one of the .c file and declare that as extern in the header files.

#ifndef MY_HEADER
#define MY_HEADER
 extern const int Test;
#endif

In Myfile.c define the variable

const int Test = 1000;

Upvotes: 0

djechlin
djechlin

Reputation: 60848

This is correct. You have two source files that are defining Test. You should only define this once. Since header files get included all over the place, they should generally only declare variables, not define them. e.g.

header:

const int Test;

Exactly one c file:

const int Test = 1000;

Upvotes: 0

Kerrek SB
Kerrek SB

Reputation: 477640

Include guards have nothing to do with it. You need to separate the declaration from the definition and have only one definition (this is called the "one definition rule", ODR):

header.h:

extern const int n;

one source file:

#include "header.h"

const int n = 1000;

all other source files:

#include "header.h"

Upvotes: 9

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