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POhow does open works for normal file and device drivers
 

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    <p>Currently, I am learning Linux device drivers. And got stuck over how opening a device file works ? </p> <p>What I got until now... Consider the a simple code that opens a normal file..</p> <pre><code>#incldue&lt;stdio.h&gt; int main() { FILE fp; char buffer[20]; fp = fopen(/home/yoggi/foo.txt, "r"); fread(buffer, 5, 1, fp); } </code></pre> <p>In above program, The fopen(), c-library function, is a wrapper function to the system call <strong>open()</strong>, which intern calls <strong>sys_open() or file_open() in VFS layer</strong> function. As linux supports a number of file system, virtual file system then transfer the control to actual file system handler to the opening that file. </p> <pre><code>1) How does virtual file system(VFS) get to know on which file system the underline file resides? 2) How does it then calls the file_open or open function of that particular filesystem to open file. </code></pre> <p>In case of device drivers similar things happens. Suppose a simple device driver.</p> <pre><code>#include &lt;linux/module.h&gt; // othher includes... static dev_t first; // Global variable for the first device number static struct cdev c_dev; // Global variable for the character device structure static struct class *cl; // Global variable for the device class static int my_open(struct inode *i, struct file *f) { printk(KERN_INFO "Driver: open()\n"); return 0; } static ssize_t my_read(struct file *f, char __user *buf, size_t len, loff_t *off) { printk(KERN_INFO "Driver: read()\n"); return 0; } struct file_operations pugs_fops = { .owner = THIS_MODULE, .open = my_open, .read = my_read, }; static int __init ofcd_init(void) /* Constructor */ { printk(KERN_INFO "Namaskar: ofcd registered"); if (alloc_chrdev_region(&amp;first, 0, 1, "Shweta") &lt; 0) { return -1; } if ((cl = class_create(THIS_MODULE, "chardrv")) == NULL) { unregister_chrdev_region(first, 1); return -1; } if (device_create(cl, NULL, first, NULL, "mynull") == NULL) { class_destroy(cl); unregister_chrdev_region(first, 1); return -1; } cdev_init(&amp;c_dev, &amp;pugs_fops); if (cdev_add(&amp;c_dev, first, 1) == -1) { device_destroy(cl, first); class_destroy(cl); unregister_chrdev_region(first, 1); return -1; } return 0; } static void __exit ofcd_exit(void) /* Destructor */ { cdev_del(&amp;c_dev); device_destroy(cl, first); class_destroy(cl); unregister_chrdev_region(first, 1); printk(KERN_INFO "Alvida: ofcd unregistered"); } module_init(ofcd_init); module_exit(ofcd_exit); MODULE_LICENSE("GPL"); MODULE_AUTHOR("Anil Kumar Pugalia &lt;email_at_sarika-pugs_dot_com&gt;"); MODULE_DESCRIPTION("Our First Character Driver"); </code></pre> <p>Firstly we allocate major minor numbers for the device. Register for the range of device files and Linking the device file operations to the device driver functions. </p> <p>Some of the term I didn't get are..</p> <pre><code>1) What does actually cdev_add() do? in terms of registering a device to the kernel. 2) Registering a device to the kernel means? 3) How does a open(/dev/mynull, O_RONLY); called on a device file actually calls the open function of driver which is mapped while initializing the device by calling routine cdev_init(&amp;c_dev, &amp;pugs_fops); ? </code></pre>
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