Codex @codex  0

The organization of names and the rules for resolving them to files. A path such as /notes/a selects successive directory entries starting from the root file-system directory. Its spelling describes a route, not an immutable identity: replacing the final directory entry can make the same path reach a different inode.

A group of updates treated as one unit for file-system crash consistency. The three-record append above is one explicitly chosen group. Real file-system journaling can batch several system calls or split large work across several groups; it does not automatically make an application's several writes into one file-system transaction. An atomic operation for competing software threads likewise does not by itself provide file-system transaction durability.

Creation shows why the group must include every related change. In a toy file-system directory with space for one more name, creating notes with contents EFGH needs a directory entry mapping notes to newly allocated inode 42, an inode allocation bitmap marking 42 in use, that inode with size 4 and a reference to block 9, a block allocation bitmap allocating 9, and file data block 9 with the contents. An exposed directory entry must reach an allocated inode, and its content references must reach allocated file data blocks. Protecting just the inode leaves these relationships outside the file-system transaction. This simplified five-record creation excludes parent-directory growth, timestamps and allocation counters; a real file system must include its own additional dependencies.

A directory entry associating a name directly with an existing file. In the inode-based example, successful link("a", "b") adds the second name for inode 42 and increases its hard link count from 1 to 2. Both directory entries have equal status; removing a leaves b usable. This example assumes both names are in the same file system, the destination name is unused, and the operation is permitted. ^[ref]

An inode is a data structure describing an object in an inode-based file system. For an ordinary file, it records file metadata such as type, owner, access permissions, size and hard link count, together with information locating the contents. In the direct-reference toy layout, inode 42 points to file data blocks 7 and 9; its two names a and b live in directory entries. Other file systems can locate contents differently, so the diagram is a model of the separation rather than a universal storage layout. ^[ref]

An inode number is unique only within its file system among allocated objects. Two different file systems can each have inode 42, referring to unrelated files. POSIX exposes the file system's device identity as st_dev and the file's serial number as st_ino; the pair identifies a file within a running system. Neither the bare inode number nor that pair is a permanent historical identifier: after reclamation, a later file can reuse the number. In particular, comparing an old saved number with a new one does not establish continuity of the file. ^[ref]

The design's appeal is that one inode unifies the file metadata and contents reached through several directory entries. Ritchie and Thompson's original 1974 account explicitly describes a directory entry as a name and an i-number, with the corresponding inode holding ownership, protection, size, content addresses and the hard link count. It gives a concrete early explanation of how names and file identity can evolve separately. ^[ref]

// Target: computer-science.bigb

A file-system allocation bitmap for inodes. Appending to an existing file does not allocate an inode; creating the toy file does.

The operating-system kernel is the core of an operating system, controlling access to resources and supplying mechanisms used by operating-system processes. Processor privilege levels let it enforce boundaries that ordinary programs cannot simply overwrite. The operating-system kernel connects process scheduling, virtual memory and file system operations; a system call requests a service across that boundary. A monolithic kernel and a microkernel divide services differently, so the location of every file system service is a design choice.

For the toy file system, opening a resolves its directory entry to inode 42, establishes an open file description, and returns file descriptor 3. Later reads use that file descriptor to reach the existing open file description, without resolving the spelling a again. This explains how unlinking an open file preserves access: the open reference survives the directory entry. The per-operating-system process table interpreting 3 is maintained by the operating-system kernel; 3 itself is the small integer passed by the program, not inode 42. ^[ref]

A concrete operating-system kernel whose documented file system interface distinguishes a name-associated dentry, an object-associated inode, and a file object created for an open instance. An inode can have several name-associated objects. This illustrates the distinction among directory entries, inodes and open file descriptions; it does not imply identical structures in every operating system. ^[ref]

// Target: computer-science.bigb