Added comments about our code.

[cascardo/kernel/samples/02.char/.git] / helloc.c

diff --git a/helloc.c b/helloc.c
index 1a2c238..0db07e3 100644 (file)
--- a/helloc.c
+++ b/helloc.c
@@ -1,6 +1,9 @@
 #include <linux/module.h>
+/* Needed for struct file_operations and others */
 #include <linux/fs.h>
+/* Needed for struct cdev */
 #include <linux/cdev.h>
+/* Needed for copying to/from user space */
 #include <asm/uaccess.h>
 
 MODULE_LICENSE("GPL");
@@ -8,51 +11,67 @@ MODULE_AUTHOR("Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>");
 MODULE_DESCRIPTION("A hello world char device");
 MODULE_VERSION("1.0.0");
 
+/* Message buffer we send to upstream */
 static char helloc_message[] = "hello, world\n";
 
+/* our read function writes our message to the user buffer */
 static ssize_t helloc_read(struct file *filp, char __user *buf, size_t len,
                           loff_t *pos)
 {
        int r;
+       /* do not read pass through the size of the message */
        if (*pos >= sizeof(helloc_message))
+       /* return end of file */
                return 0;
+       /* if len is bigger than the rest of the message, clamp it */
        if (len > sizeof(helloc_message) - *pos)
                len = sizeof(helloc_message) - *pos;
+       /* copy message to user space and return error if it fails */
        r = copy_to_user(buf, helloc_message + *pos, len);
        if (r)
                return -EFAULT;
+       /* update the file position */
        *pos += len;
        return len;
 }
 
+/* we only implement read */
 static struct file_operations helloc_fops = {
        .owner = THIS_MODULE,
        .read = helloc_read,
 };
 
+/* the device number and the char device struct */
 static dev_t dev;
 static struct cdev *cdev;
 
 static int __init helloc_init(void)
 {
        int r;
+       /* allocate any major number with only one minor */
        r = alloc_chrdev_region(&dev, 0, 1, "helloc");
        if (r)
                goto out_region;
        r = -ENOMEM;
+       /* print the major number allocated so we can create our device node */
        printk(KERN_INFO "Allocated major number %d\n", MAJOR(dev));
+       /* allocate the character device struct */
        cdev = cdev_alloc();
        if (!cdev)
                goto out_alloc;
+       /* set the module owner and the file operations of our chardev */
        cdev->owner = THIS_MODULE;
        cdev->ops = &helloc_fops;
+       /* register the chardev to the system */
        r = cdev_add(cdev, dev, 1);
        if (r)
                goto out_add;
        return 0;
 out_add:
-       kfree(&cdev->kobj);
+       /* release memory allocated to the cdev device */
+       kfree(cdev);
 out_alloc:
+       /* release the device number allocated */
        unregister_chrdev_region(dev, 1);
 out_region:
        return r;
@@ -60,7 +79,9 @@ out_region:
 
 static void __exit helloc_exit(void)
 {
+       /* remove the chardev from the system */
        cdev_del(cdev);
+       /* release the device number allocated */
        unregister_chrdev_region(dev, 1);
 }