Getting error Symbol current_process multiply defined, confused on what error means and how to fix?

Question

I'm just trying to get my code to compile, but I keep running into errors:

.\Objects\Lab4.axf: Error: L6200E: Symbol current_process multiply defined (by lab4_t0.o and 3140_concur.o).
.\Objects\Lab4.axf: Error: L6200E: Symbol current_process multiply defined (by process.o and 3140_concur.o).
.\Objects\Lab4.axf: Error: L6200E: Symbol process_queue multiply defined (by lab4_t0.o and 3140_concur.o).
.\Objects\Lab4.axf: Error: L6200E: Symbol process_queue multiply defined (by process.o and 3140_concur.o).

I really don't understand the error message.

First, I'm not sure if process.o is referring to process.h or process.c (so I don't know which file to focus on). Same goes for 3140_concur.o which has 3140_concur.c and 3140_concur.h.

Secondly, I'm not sure what "multiply defined" means. I figure that means I instantiate current_process & process_queue twice, but honestly I can't see how because I only "define" current_process and process_queue in process.h:

process_t * current_process; 
process_t * process_queue;

I don't even see how lab4_t0.c could be defining process_queue either so I'm confused by why these "multiply defined" error messages are popping up.

ALTHOUGH, I have a feeling its because in3140_concur.h I #include "process.h" AND in process.h I define process_queue and current_process AND I also #include "3140_concur.h". Same goes for lab4_t0.c (I #include "process.h"). But the reason I do that is b/c process.c uses methods from 3140_concur.h and also lab4_t0.c uses methods from from process.h.

So if my hunch is correct, and that's the reason I'm getting the errors, how can I properly modularize my code (aka what should I #include and not #include) so that I don't get these errors?

If someone could type the code I need to do to remove these errors that would be great!

Note: If you were wondering why I created process.h and why I didn't just put everything in process.h in 3140_concur.h file, the reason for that is that it caused errors w/ lock.c that I don't want to go into here.

Brief summary of my code

lock.c

#include "lock.h"
extern process_t * current_process; 
extern process_t * process_queue;

lock.h

#ifndef __LOCK_H_INCLUDED__
#define __LOCK_H_INCLUDED__
#include "3140_concur.h"
#include "shared_structs.h"
void l_init(lock_t* l);
void l_lock(lock_t* l);
void l_unlock(lock_t* l);
#endif /* __LOCK_H_INCLUDED */

3140_concur.c

#include "3140_concur.h"
#include <stdlib.h>

3140_concur.h

#ifndef __3140_CONCUR_H__
#define __3140_CONCUR_H__

#include <stdlib.h>
#include <fsl_device_registers.h>
#include "process.h"

void process_blocked (void);

void process_terminated (void);

unsigned int * process_stack_init (void (*f)(void), int n);

void process_stack_free (unsigned int *sp, int n);

void process_begin (void);

#endif

process.h

#include <stdlib.h>
#include <fsl_device_registers.h>
#include "3140_concur.h"

struct process_state;
typedef struct process_state process_t;

unsigned int * process_select (unsigned int * cursp);

process_t * current_process; 
process_t * process_queue;

void process_start (void);

int process_create (void (*f)(void), int n);

process.c

(I do process_t* process_queue = NULL; & process_t* current_process = NULL; b/c I want them to be NULL before any functions are called)

#include "3140_concur.h" 
#include "shared_structs.h"
#include <stdlib.h> 
#include <fsl_device_registers.h>

process_t* process_queue = NULL;
process_t* current_process = NULL;

lab4_t0.c

#include "process.h"
#include "utils.h"
#include "lock.h"

lock_t l;

Any help would be greatly appreciated!

Answer 1

Adding extern to my defintion of current_process and process_queue in process.h removed my "multiply defined" errors (thanks to @JonathanLeffler)

Now I'm getting a different error, but at least I resolved this one :)