Re: [Xenomai-help] shm_open, ftruncate

[Lionel Perrin <perrin@domain.hid>]

Gilles Chanteperdrix a écrit :
> Lionel Perrin wrote:
>  > // 	/* the following lines shouldn't be commented but xenomai... */
>  > // 	else
>  > // 	{
>  > // 		/* a new shm_file has been created, we need to truncate it */
>  > // 		if (ftruncate(h_shm, nbvalues * sizeof(double))==-1)
>  > // 		{
>  > // 			printf("truncate failed\n");
>  > // 			goto close_and_unlink;
>  > // 		}
>  > // 	}
>
> Do you still have an issue with ftruncate ? 
>
> Note that it is better to always call ftruncate even in a process that
> is not creating the shared memory, this avoid the race condition where
> the process that created the shared memory is about to truncate it and
> is interrupted by the second process which did not create it but want to
> mmap it.
>
>  > 	mem->h_mut = (pthread_mutex_t *)malloc(sizeof(pthread_mutex_t));
>  > 	pthread_mutex_init(mem->h_mut, NULL);
>
>
> Note that if you want to share the mem structure between several
> processes, you should put the mutex on the shared memory, doing for
> example:
>
> typedef struct {
>     pthread_mutex_t mutex;
>     int nbaccess;
>     double values[0]
> } myshm_t;
>
> myshm_t *myshm;
>
> myshm = (myshm_t *) mmap(...);
>
> mem->h_mut = &myshm->mutex;
> mem->ptr = &myshm->values;
>
> pthread_mutex_init(mem->h_mut, NULL);
>
> Xenomai should detect that you are initializing a mutex that was already
> initialized and return EBUSY that you can safely ignore.
>   
Thanks for your help, all work as i wish now...
You're right, i've done a terrible mistake with the mutex... I'm a bit 
ashamed ;)