Simple Linux char driver

Since the resources I found to learn are generally out-of-date, I'm having to read a lot of documentation, which makes the learning process somewhat haphazard. The module makes a simple character stack which can be read and written to by multiple processes.

```
#include
#include

#include
#include
#include
#include

#include

#include

#define MHELLO_STACK_SIZE 20

static int major;
static dev_t dev;
static struct cdev my_cdev;
static int count;
static DEFINE_MUTEX(lock);
static char stack[MHELLO_STACK_SIZE];
static int head;

static ssize_t hello_read(struct file f,char buf,size_t count,loff_t * offset)
{
unsigned int res;
printk(KERN_ALERT"R\n");

mutex_lock(&lock);

if(head= MHELLO_STACK_SIZE-1)
{
mutex_unlock(&lock);
return -ENOSPC;
}
head++;
res=copy_from_user(stack+head,buf,1);

mutex_unlock(&lock);
return 1;
}

static int hello_open(struct inode inode,struct file f)
{
mutex_lock(&lock);
count++;
printk(KERN_ALERT"O %d\n",count);
mutex_unlock(&lock);
return 0;
}
static int hello_release(struct inode inode,struct file f)
{
mutex_lock(&lock);
count--;
printk(KERN_ALERT"C %d\n",count);
mutex_unlock(&lock);
return 0;
}
const struct file_operations fops=
{
.owner=THIS_MODULE,
.read=hello_read,
.write=hello_write,
.open=hello_open,
.release=hello_release
};

static int hello_init(void)
{
int result=-1;

mutex_init(&lock);
mutex_lock(&lock);

count=0;
head=-1;

result=alloc_chrdev_region(&dev,0,1,"mhello");

if(result<0)
printk(KERN_ALERT"Can't allocate major.\n");

major=MAJOR(dev);
printk(KERN_ALERT"Hello, my major no. is %d.\n",major);

cdev_init(&my_cdev,&fops);
my_cdev.owner=THIS_MODULE;

cdev_add(&my_cdev,dev,1);

mutex_unlock(&lock);
return 0;
}
static void hello_exit(void)
{
mutex_lock(&lock);
unregister_chrdev_region(dev,1);
cdev_del(&my_cdev);
mutex_unlock(&

So, this isn't a full answer to your question, but maybe still relevant.

Included with the Linux sources is scripts/checkpatch.pl - which checks for things like coding style compliance, but also warns on use of outdated interfaces.

If you have a checked out (clean) git repository of the kernel, you can simply copy your source file into there and run

linux$ git diff HEAD | scripts/checkpatch.pl -

Doing that on the above code does point out a few things not just coding-style related.

StackExchange Code Review Q#31938, answer score: 3