DEFINE_MUTEX(mutexname);
// of
DECLARE_MUTEX(mutexname);
void some_init(void)
{
  mutex_init(mutexname);
}

struct semaphore test;

DECLARE_MUTEX(test);

#include <linux/completion.h>

struct completion messagesAvailable; //voor lezers om te slapen als er geen berichten zijn

/File operation functies
int rn_open(struct inode *inode, struct file *filp){
    return 0;
}

ssize_t rn_read(struct file *filp, char __user *userBuff, size_t count, loff_t *offp){
    printk( KERN_INFO  "waiting for: %p\n", &messagesAvailable );
    wait_for_completion( &messagesAvailable);   

    copy_to_user( userBuff, "b", 1);
    (*offp) += 1;

    return 1;
}

ssize_t rn_write(struct file *filp, const char __user *userBuff, size_t count, loff_t *offp){
    complete_all( &messagesAvailable);
    printk( KERN_INFO  "inited complete for: %p\n", &messagesAvailable );
    return count;
}

//deze functies worden doorgelinked door gebruikers van het device
struct file_operations fops =
{
    .open = rn_open,
    .release = rn_release,
    .read = rn_read,
    .write = rn_write,
};

// Module ingeladen door de kernel
static int hello_init(void)
{
    int result;

    printk( KERN_ALERT "Kernelmodule completion: init_module() \n" );

    init_completion(&messagesAvailable);

    result  = register_chrdev(0, "completion", &fops);

    if( result < 0) {
        printk(KERN_WARNING "Error creating device\n");
        return result;
    }
    
    device = result;

    return 0;
}

complete_all( &messagesAvailable);
init_completion( &messagesAvailable );