Somehow I'm violating the one definition rule

Question

I am getting errors with the linker such as:

osd.o(.ndata+0x514):C:\Documents and Settings\Thomas\My Documents\PIC\dsPIC33FJ128GP802\On Screen Display\osd.c: multiple definition of `video_buff_vis_num'
main.o(.ndata+0x0):C:\Documents and Settings\Thomas\My Documents\PIC\dsPIC33FJ128GP802\On Screen Display\main.c: first defined here
osd.o(.ndata+0x515):C:\Documents and Settings\Thomas\My Documents\PIC\dsPIC33FJ128GP802\On Screen Display\osd.c: multiple definition of `video_buff_draw_num'
main.o(.ndata+0x1):C:\Documents and Settings\Thomas\My Documents\PIC\dsPIC33FJ128GP802\On Screen Display\main.c: first defined here
osd.o(.ndata+0x516):C:\Documents and Settings\Thomas\My Documents\PIC\dsPIC33FJ128GP802\On Screen Display\osd.c: multiple definition of `vid_format'
main.o(.ndata+0x2):C:\Documents and Settings\Thomas\My Documents\PIC\dsPIC33FJ128GP802\On Screen Display\main.c: first defined here
osd.o(.ndata+0x518):C:\Documents and Settings\Thomas\My Documents\PIC\dsPIC33FJ128GP802\On Screen Display\osd.c: multiple definition of `vid_line'
main.o(.ndata+0x4):C:\Documents and Settings\Thomas\My Documents\PIC\dsPIC33FJ128GP802\On Screen Display\main.c: first defined here

This is bugging me, because in the source code I have include guards around the only place these definitions could come from.

#ifndef OSD_H 
#define OSD_H 

// code here, including definitions for the above

#endif // OSD_H 

This is getting on my nerves. I've cleaned, rebuilt, and tried again. I even started a new project from scratch with the same files and I'm getting exactly the same problem! Would anyone please enlighten me on why this isn't working! :)

Compiling with PIC-GCC v3.23 (a version of GCC for PIC24F/H and dsPIC30F/33F microcontrollers.)

Let me know if anyone wants to see the sources of more files. I didn't want to overcrowd this page.

Answer 1

If you include this header in more than 1 .c file you will have multiple definitions. And you have main.c and osd.c.

The .h is the right place for functions and extern data declarations. But for a variable you will have to pick one of the source files. the #defined guards don't change that.

Also see this question. And this answer describes the standard pattern .

And a little more explanation/analysis:

1) Each .c source file is compiled independently. The guards only protect from reading a header file twice during 1 compile.

2) The error you are getting is a linker (not compiler) error.

When your header defines a variable, the compiler will treat it as a definition in each separate run. The linker will detect the multiple instances.

Answer 2

What does your include file contain?

 int video_buff_vis_num;
 extern int video_buff_vis_num;

The former or the latter? The former doesn't define the variable but allocates memory for it while the latter just says "somewhere is a variable with that name".

You must have only one place where it is allocated but you can have many definitions.