How does the Unix kernel convert file offset?

Question

As the name implies, it’s hard for me to understand how the Unix kernel converts the logical offset of the file into the logical block number, and then extracts it from the i-node.

As a reference, I ask for a more detailed explanation of the bmap algorithm, described first in Maurice J. Bach's UNIX Operating System Project.

Thanks..

Answer 1

Unfortunately, I cannot help you with the original implementation of UNIX; but the task of comparing a high-level directory hierarchy with a representation on disk and for assigning ranges on a block device to files usually refers to one or more file systems. The POSIX specification makes some high-level file system requirements, but what actually ends up on disk is completely specific to each file system.

The ext2 file system is relatively simple, but a detailed explanation still fills the entire chapter of the book — one of which is Understanding the Linux Kernel by Daniel P. Beauvais and Marco Cesati.

A shorter introductory text at a higher level comes with the Linux kernel, and can also be found here . This directory also contains documentation for other Linux file systems , as well as a VFS layer that handles many higher-level aspects that are common to all or most file systems. The ext2 Wikipedia article also covers it at a high level. It also contains some links to more detailed descriptions on the Internet.

This, unfortunately, is not a topic that will fit into the SO answer, but I hope the above pointers help you get started.