Hi, Thanks a lot for the help Saket and Pranay :) Best regards, Lokesh On Mon, Nov 17, 2014 at 12:35 PM, Saket Sinha <saket.sinha89@gmail.com> wrote:

Hi Lokesh,

I am putting down the entire call flow from the userspace open to the driver implementation below -

1. touch program is used to create an empty file. touch is part of the fileutils package, and is implemented in src/touch.c. What it basically does is set some flags and invoke the open() syscall, then calls close().

2. sys_open() in fs/open.c - it grabs the filename from userland, if the filename seems OK - grab the next free file descriptor, actually open the file with function filp_open() and give the file descriptor to the user.

3. filp open() in fs/open.c - Create a nameidata struct (which creates the file), opens the dentry associated with the nameidata structure in function path_openat() and returns a file pointer to the file itself.

4.path_openat in fs/namei.c - Standard path init(), path walk() dance, if the inode does not yet exist, we actually create it in do_last->lookup_open() by vfs_create().

5. vfs_create() in fs/namei.c - This triggers filesystem specific create by the statement dir->i_op->create.

6. Now go to filesystem specific create in the inode_opetations structure for example for ext2- Inode operations for Ext2 are defined in fs/ext2/namei.c:

struct inode_operations ext2_dir_inode_operations = { create: ext2_create, lookup: ext2_lookup, link: ext2_link, unlink: ext2_unlink, symlink: ext2_symlink, mkdir: ext2_mkdir, rmdir: ext2_rmdir, . . . . . . . . . . . };

Hope this helps.

Regards, Saket Sinha

On Thu, Nov 13, 2014 at 11:14 AM, Pranay Srivastava <pranjas@gmail.com> wrote:

...

Hi Lokesh

On Wed, Nov 12, 2014 at 6:42 PM, lokesh kumar <lokeshkumar.222@gmail.com> wrote:

...

Hi,

I have a question regarding open systemcall

"open(dev_name, O_RDWR, 0)" -->userspace

suppose devname is /dev/my_device

Now when we look into open file operation, we have prototype "int (*open) (struct inode *, struct file *);"

could anyone tell me how inode and file structure pointer are filled.

I think logic is written in fs/open.c

but i got confused in callbacks used in it.could anyone please tell me which topics should i cover to understand open system call.

You should probably first understand how the file is located. See the role of lookup.

...

Thanks.

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