lookup function return value in inode_operations

Hi, What is the return value of the lookup function under the inode_operations struct ? I see that the function gets a dentry as a parameter, which can be used to associate an inode. For example: If we have: struct dentry * somefs_lookup(struct inode *parent_inode, struct dentry *child_dentry, unsigned int flags) { d_add(child_dentry, inode); } so, what is the meaning of the return "struct dentry" in this lookup function ? In some simple filesystems that I saw, they return NULL here and it seem to work just fine. So what is that the lookup function should do if we are implementing our own filesystem ? Thanks. -- Sankar P http://psankar.blogspot.com