[SOLVED] How does a linux kernel module know when its file has been opened?

Issue

This is a newbie kernel module question... I have mymodule.c with a function:

static int mymodule_open(struct inode *inode, struct file *filp)
{
    //printk(KERN_INFO "open called\n");
    /* Success */
    return 0;
}

and a user level program where the first line after variable initializations is:

FILE *pFile = fopen("/dev/mymodule", "r+");

When I run the user level program this fopen somehow calls the mymodule_open command in mymodule.c (compiled to mymodule.ko). How does it know to do this? I can't connect the dots as to how mymodule_open() knows when fopen opens up /dev/mymodule.

Solution

There is a module registration mechanism in kernel for device driver or kernel modules.

/dev/module will be linked with your module.

The Device operations and file operations structure is mapped with the device file.

something like

struct file_operations fops = {
open    : my_module_open,
release : my_module_release,
ioctl   : my_module_ioctl,
};

Device file will identify and open the module with help of major and minor number.First with device file and then file operations structure.

Also look into device registration and device file operations

Answered By - Dilip Kumar