Multiple definition of ...

[optomation]

After 2 years of (happy) idle time, I find myself in the necessity of migrating my old OpenAT projects (based on VS6) to this new Eclipse-based environment. I have solved all the issues related to changes in OpenAT libraries, and all my c files now compile. The problem arises when linking:

For every variable I declare in my header files I recieve the error message
multiple definitions of …
I have read the thread [url]Migrating to M2M Studio from VS 2008] and came up with the explanation:

You should never define variables in headers files - for precisely this reason!
You should only ever declare variables in header files

Well, in my header files, all variables are simply declared, never defined.

[optomation]

By some reason I was not allowed to include a sample code in the previous post. Here is a sample of the lines in my headers that are giving me such a ‘headache’.

s8	hData;
bool	bPortReady;
bool	bLastReadingOK;
bool	bProgramPaused;

[Flex]

That way should look your .c file, not the header. You should have:
//xx.c
s8 hData=0;
bool bPortReady=FALSE;
bool bLastReadingOK=FALSE;
bool bProgramPaused=FALSE;

//xx.h
extern s8 hData;
extern bool bPortReady;
extern bool bLastReadingOK;
extern bool bProgramPaused;

[awneil]

@optomation

Well, in my header files, all variables are simply declared, never defined

Not according to the sample which you posted:

@optomation

Here is a sample of the lines in my headers that are giving me such a ‘headache’.

s8	hData;
bool	bPortReady;
bool	bLastReadingOK;
bool	bProgramPaused;

Those are definitions!

Read that FAQ again: c-faq.com/decl/decldef.html

[optomation]

I wish it was that easy!

That’s not the case. I’ve even commented out any line that used some of such variables, so in fact the only reference across my project to those variables is their declaration in the .h file, and still is marked as “multiple-declared”.

Thanks for the reply anyway.

[optomation]

@awneil

Those are definitions!

Read that FAQ again: c-faq.com/decl/decldef.html

awneil, accordong to the link you provided:

For example, these are declarations:
extern int i;
extern int f();

and these are definitions:
int i = 0;
int f()
{
return 1;
}

… my code are declarations (not definitions, the variables are not initialized). On the other hand, if you insist on my sample code being definitions, then: how should I declare a shared variable?

[awneil]

Read that link again, more carefully:

For example, these are declarations:
extern int i;
extern int f();

and these are definitions:
int i = 0;
int f()
{
return 1;
}

The ‘extern’ is the key!

An initialisation is sufficient to make a definition - but it is not necessary.

As Flex told you, your header file should contain just the extern declarations; eg,

@file.h

// External Declarations
extern s8 hData;
extern bool bPortReady;
extern bool bLastReadingOK;
extern bool bProgramPaused;

If you think about the purpose of header files, this should be obvious: the header is telling the compiler that these variables will be defined somewhere externally to the current compilation unit.

You must then provide definitions in exactly one of your ‘C’ source files; eg,

@file.c

// Data Definitions
s8 hData;
bool bPortReady;
bool bLastReadingOK;
bool bProgramPaused;

The definition is the thing which actually causes resources to be committed

if you insist on my sample code being definitions

I do - as does the definition of the ‘C’ programming language!

[optomation]

Now I am beginning to understand things!

1.- Developer Studio is using a C++ compiler to compile my C files. Can that be changed?
2.- It is not enough to correct the .h files: the GUI is so “clever” that it won’t mark affected .o files as outdated, so changes made to .h files will have no impact on final result.

Thanks for your patience, awneil; in my old C-way thinking, “extern int a;” and “int a;” at file-scope are equivalent.

[awneil]

@optomation

1.- Developer Studio is using a C++ compiler to compile my C files.

No, that’s not true.

It’s using GCC which, like the VS6 compiler, can do both ‘C’ and C++.
It should be automatically assuming that .c files are to be compiled as ANSI ‘C’

2.- It is not enough to correct the .h files: the GUI is so “clever” that it won’t mark affected .o files as outdated, so changes made to .h files will have no impact on final result.

This is a known bug: Dependencies not generated properly

Doing a ‘Clean’ will rebuild using the modified files (be sure that the changes have been saved!)

in my old C-way thinking, “extern int a;” and “int a;” at file-scope are equivalent.

No, that has never been the case!

[optomation]

@C Language Reference Manual by Silicon Graphics

If the storage class specifier is missing from a declaration, it is assumed to be extern unless the declaration is of an object and occurs inside a function, in which case it is assumed to be auto.

You can read the whole chapter here:
http://menehune.opt.wfu.edu/Kokua/More_SGI/007-0701-150/sgi_html/ch07.html#Z35064

This is not the reference I use, but I am pretty sure I read the same thing in my books 20 years ago when I started programming in C.

[awneil]

But that’s declarations - you had definitions!

[Flex]

optomation take it like a man and move forward, no need to continue, problem solved after all

[JasonDiplomat]

There’s also the possibility of labelling variables in header files as “static”, however I prefer to use constructs such as the following:

//bearer.h
#ifndef BEARER_H_
#define BEARER_H_

#ifdef BEARER_C
  #define EXTR_BEARER
#else
  #define EXTR_BEARER extern
#endif


#ifndef BEARER_H_
#define BEARER_H_

#ifdef BEARER_C
  #define EXTR_BEARER
#else
  #define EXTR_BEARER extern
#endif

//stuff

EXTR_BEARER <type> <name>;

// ------------------------------------------------------------------------------------------------

#endif	//BEARER_H_

//bearer.cpp
#define BEARER_C
#include "bearer.h"

[awneil]

@JasonDiplomat

There’s also the possibility of labelling variables in header files as “static”

What would be the point of that??

See: 'static' in headers - what's all that about??!

[blackyblack]

There’s also the possibility of labelling variables in header files as “static”

Too verbose I think. It is shorter to use hard coded ‘extern’ in headers and definitions in .c files. And much better not to use global variables at all.

[awneil]

@JasonDiplomat

There’s also the possibility of labelling variables in header files as “static”

@blackyblack

Too verbose I think.

Sorry, don’t get you there - in what way “verbose”?

[blackyblack]

Too much code for nothing. Sorry for bad english.

[awneil]

@blackyblack

Too much code for nothing.

Yes - I think that’s what the ‘C’ FAQ means when it says,

@'C' FAQ

It is possible to use preprocessor tricks … but it’s not clear if this is worth the trouble
c-faq.com/decl/decldef.html

As for the use of static, that’s not just a matter of no benefit - it is (potentially) plain wrong.

[josepvr]

Maybe one file includes himself.
Or fileA includes fileB that includes fileA…

[awneil]

@josepvr

Maybe one file includes himself.

No - if anything, that would cause Compiler errors - these are Linker errors!