Using pthread on Linux 2.4 - getting “incomplete type” compilation error

Question

I'm working on Linux 2.4 (doing h.w for my O.S course), I want to use pthread to implement a reader-writer lock. In rw_lock.c I have:

#include <pthread.h>
#include <stdlib.h>
#include "rw_lock.h"

struct readers_writers_t
{
    int prio;
    int number_of_readers;
    pthread_cond_t no_readers;
    int number_of_writers;
    int number_of_waiting_writers;
    pthread_cond_t no_writers;
    pthread_mutex_t lock;
};

[functions...]

in rw_lock.h I have:

typedef struct readers_writers_t readers_writers;

In another C file (implementation of linked list) I have :

#include "rw_lock.h"

struct LinkedList
{
    ListNode* head;
    ListNode* tail;
    readers_writers rwLock;
};

(and more functions,includes etc').

I get (one) compilation error :

"rwLock has incomplete type".

I have no idea why I get this error (or how to fix it...).

help is appreciated,thanks!

Answer 1

Your linked list C file does not know about the struct readers_writers_t, it's defined within your rw_lock.c file.

1. You should just move your struct readers_writers_t out of rw_lock.c and into your rw_lock.h file.

2. Or perhaps you don't want your linked list to know about that struct. In that case, you should define the readers_writers rwLock as a pointer (readers_writers *rwLock) rather. Another alternative is to make your readers_writers typedef a pointer: typedef struct readers_writers_t *readers_writers This requires that functions in rw_lock.c allocates space for the actual struct, as that's the only file that knows about the actual struct.

Answer 2

You need to move the structure defintion from the source file to the header file.

rw_lock.h should be:

#ifndef  SOME_UNIQUE_STRING_MY_RW_LOCK_H
#define  SOME_UNIQUE_STRING_MY_RW_LOCK_H

#include <pthread.h>

struct readers_writers_t
{
    int prio;
    int number_of_readers;
    pthread_cond_t no_readers;
    int number_of_writers;
    int number_of_waiting_writers;
    pthread_cond_t no_writers;
    pthread_mutex_t lock;
};
typedef struct readers_writers_t readers_writers;

#endif