CONTENTS

HFS Plus Basics

Core Concepts

Volume Header

B-Trees

Catalog File

Extents Overflow File

Allocation File

Attributes File

Startup File

Unicode Subtleties

HFS Wrapper

Volume Consistency Checks

Summary

This Technote describes the on-disk format for an HFS Plus volume. It does not describe any programming interfaces for HFS Plus volumes.

This technote is directed at developers who need to work with HFS Plus at a very low level, below the abstraction provided by the File Manager programming interface. This includes developers of disk recovery utilities and programmers implementing HFS Plus support on other platforms.

This technote assumes that you have a conceptual understanding of the HFS volume format, as described in Inside Macintosh: Files.

HFS Plus Basics

HFS Plus is a new volume format for Mac OS. HFS Plus was introduced with Mac OS 8.1. HFS Plus is architecturally very similar to the HFS, although there have been a number of changes. The following table summarizes the important differences.

Table 1 HFS and HFS Plus Compared

The extent to which these HFS Plus features are available through a programming interface is OS dependent. Currently (version 8.5), Mac OS does not provide programming interfaces for any HFS Plus-specific features.

To summarize, the key goals that guided the design of the HFS Plus volume format were:

• efficient use of disk space,
• international-friendly file names,
• future support for named forks, and
• ease booting on non-Mac OS operating systems.

The following sections describes these goals, and the differences between HFS and HFS Plus required to meet these goals.

Efficient Use of Disk Space

HFS divides the total space on a volume into equal-sized pieces called allocation blocks. It uses 16-bit fields to identify a particular allocation block, so there must be less than 2¹⁶ (65,536) allocation blocks on an HFS volume. The size of an allocation block is typically the smallest multiple of 512 such that there are less than 65,536 allocation blocks on the volume (i.e., the volume size divided by 65,535, rounded up to a multiple of 512). Any non-empty fork must occupy an integral number of allocation blocks. This means that the amount of space occupied by a fork is rounded up to a multiple of the allocation block size. As volumes (and therefore allocation blocks) get bigger, the amount of allocated but unused space increases.

HFS Plus uses 32-bit values to identify allocation blocks. This allows up to 2 ³² (4,294,967,296) allocation blocks on a volume. More allocation blocks means a smaller allocation block size, especially on volumes of 1 GB or larger, which in turn means less average wasted space (the fraction of an allocation block at the end of a fork, where the entire allocation block is not actually used). It also means you can have more files, since the available space can be more finely distributed among a larger number of files. This change is especially beneficial if the volume contains a large number of small files.

International-Friendly File Names

HFS uses 31-byte strings to store file names. HFS does not store any kind of script information with the file name to indicate how it should be interpreted. File names are compared and sorted using a routine that assumes a Roman script, wreaking havoc for names that use some other script (such as Japanese). Worse, this algorithm is buggy, even for Roman scripts. The Finder and other applications interpret the file name based on the script system in use at runtime.

HFS Plus uses up to 255 Unicode characters to store file names. Allowing up to 255 characters makes it easier to have very descriptive names. Long names are especially useful when the name is computer-generated (such as Java class names).

The HFS catalog B-tree uses 512-byte nodes. An HFS Plus file name can occupy up to 512 bytes (including the length field). Since a B-tree index node must store at least two keys (plus pointers and node descriptor), the HFS Plus catalog must use a larger node size. The typical node size for an HFS Plus catalog B-tree is 4 KB.

In the HFS catalog B-tree, the keys stored in an index node always occupy a fixed amount of space, the maximum key size. In HFS Plus, the keys in an index node may occupy a variable amount of space determined by the actual size of the key. This allows for less wasted space in index nodes and creates, on typical disks, a substantially larger branching factor in the tree (requiring fewer node accesses to find any given record).

Future Support for Named Forks

Files on an HFS volume have two forks: a data fork and a resource fork, either of which may be empty (zero length). Files and directories also contain a small amount of additional information (known as catalog information or metadata) such as the modification date or Finder info.

Apple software teams and third-party developers often need to store information associated with particular files and directories. In some cases (e.g., custom icons for files), the data or resource fork is appropriate. But in other cases (e.g., custom icons for directories, or File Sharing access privileges), using the data or resource fork is not appropriate or not possible.

A number of products have implemented special-purpose solutions for storing their file- and directory-related data. But because these are not managed by the file system, they can become inconsistent with the file and directory structure.

HFS Plus has an attribute file, another B-tree, that can be used to store additional information for a file or directory. Since it is part of the volume format, this information can be kept with the file or directory as is it moved or renamed, and can be deleted when the file or directory is deleted. The contents of the attribute file's records have not been fully defined yet, but the goal is to provide an arbitrary number of forks, identified by Unicode names, for any file or directory.

Easy Startup of Alternative Operating Systems

HFS Plus defines a special startup file, an unstructured fork that can be found easily during system startup. The location and size of the startup file is described in the volume header. The startup file is especially useful on systems that don't have HFS or HFS Plus support in ROM. In many respects, the startup file is a generalization of the HFS boot blocks, one that provides a much larger, variable-sized amount of storage.

Core Concepts

HFS Plus uses a number of interrelated structures to manage the organization of data on the volume. These structures include:

• the volume header
• the catalog file
• the extents overflow file
• the attributes file
• the allocation file (bitmap)
• the startup file

Each of these complex structures is described in its own section. The purpose of this section is to give an overview of the volume format, describe how the structures fit together, and define the primitive data types used by HFS Plus.

Terminology

HFS Plus is a specification of how a volume (files that contain user data, along with the structure to retrieve that data) exists on a disk (the medium on which user data is stored). The disk is divided into 512 byte logical blocks of data, known as sectors. Sectors are identified by a sector number, starting at 0 and continuing to the last sector on the disk (whose number is the disk size divided by 512 minus 1).

HFS Plus allocates sectors in groups called allocation blocks; an allocation block is simply a group of consecutive sectors. The size (in bytes) of an allocation block is a power of two, greater than or equal to 512, which is set when the volume is initialized. This value cannot be easily changed without reinitializing the volume. Allocation blocks are identified by a 32-bit allocation block number, so there can be at most 2³² allocation blocks on a volume. Future implementations of the file system will be optimized for 4K allocation blocks.

All of the volume's structures, including the volume header, are part of one or more allocation blocks (with the possible exception of the alternate volume header, discussed below). This differs from HFS, which has several structures (including the boot blocks, master directory block, and bitmap) which are not part of any allocation block.

You can find the first sector of an allocation block by simply multiplying the allocation block number by the allocation block size and dividing by the sector size (512).

To promote file contiguity and avoid fragmentation, disk space is typically allocated to files in groups of allocation blocks, or clumps. The clump size is always a multiple of the allocation block size. The default clump size is specified in the volume header.

Every HFS Plus volume must have a volume header. The volume header contains sundry information about the volume, such as the date and time of the volume's creation and the number of files on the volume, as well as the location of the other key structures on the volume. The volume header is always located at sector 2.

A copy of the volume header, known as the alternate volume header, is stored in the second to last sector of the volume (starting 1024 bytes before the end of the volume). Sectors 0 and 1, and the last sector on the volume, are reserved. The actual number of allocation blocks occupied by the volume header and the alternate volume header varies depending on the allocation block size.

An HFS Plus volume contains five special files, which store the file system structures required to access the file system payload: folders, user files, and attributes. The special files are the catalog file, the extents overflow file, the allocation file, the attributes file and the startup file. Special files only have a single fork (the data fork) and the extents of that fork are described in the volume header.

The catalog file is a special file that describes the folder and file hierarchy on a volume. The catalog file contains vital information about all the files and folders on a volume, as well as the catalog information, for the files and folders that are stored in the catalog file. The catalog file is organized as a B-tree (or "balanced tree") to allow quick and efficient searches through a large folder hierarchy.

The catalog file stores the file and folder names, which consist of up to 255 Unicode characters, as described below.

The attributes file is another special file which contains additional data for a file or folder. Like the catalog file, the attributes file is organized as a B-tree. In the future, it will be used to store information about additional forks. (This is similar to the way the catalog file stores information about the data and resource forks of a file.)

HFS Plus tracks which allocation blocks belong to a fork by maintaining a list of the fork's extents. An extent is a contiguous range of allocation blocks allocated to some fork, represented by a pair of numbers: the first allocation block number and the number of allocation blocks. For a user file, the first eight extents of each fork are stored in the volume's catalog file. Any additional extents are stored in the extents overflow file, which is also organized as a B-tree.

The extents overflow file also stores additional extents for the special files except for the extents overflow file itself. However, if the startup file requires more than the eight extents in the Volume Header (and thus requires additional extents in the extents overflow file), it would be much harder to access, and defeat the purpose of the startup file. So, in practice, a startup file should be allocated such that it doesn't need additional extents in the extents overflow file.

The allocation file is a special file which specifies whether an allocation block is used or free. This performs the same role as the HFS volume bitmap, although making it a file adds flexibility to the volume format.

The startup file is another special file which facilitates booting of non-Mac OS computers from HFS Plus volumes.

Finally, the bad block file prevents the volume from using certain allocation blocks because the portion of the media that stores those blocks is defective. The bad block file is neither a special file nor a user file; this is merely convention used in the extents overflow file. See Bad Block File for more details.

Broad Structure

The bulk of an HFS Plus volume consists of seven types of information or areas:

1. user file forks,
2. the allocation file (bitmap),
3. the catalog file,
4. the extents overflow file,
5. the attributes file,
6. the startup file, and
7. unused space.

The general structure of an HFS Plus volume is illustrated in Figure 1.

Figure 1 Organization of an HFS Plus Volumes

The volume header is always at a fixed location (sector 2). However, the special files can appear anywhere between the volume header block and the alternate volume header block. These files can appear in any order and are not necessarily contiguous.

The information on HFS Plus volumes (with the possible exception of the alternate volume header, as discussed below) is organized solely in allocation blocks. Allocation blocks are simply a means of grouping sectors into more convenient parcels. The size of an allocation block is a power of two, and at least 512. The allocation block size is a volume header parameter whose value is set when the volume is initialized; it cannot be changed easily without reinitializing the volume.

Primitive Data Types

This section describes the primitive data types used on an HFS Plus volume. All data structures in this volume are defined in the C language. The specification assumes that the compiler will not insert any padding fields. Any necessary padding fields are explicitly declared.

Reserved and Pad Fields

In many places this specification describes a field, or bit within a field, as reserved. This has a definite meaning, namely:

• When creating a structure with a reserved field, an implementation must set the field to zero.
• When reading existing structures, an implementation must ignore any value in the field.
• When modifying a structure with a reserved field, an implementation must preserve the value of the reserved field.

This definition allows for backward-compatible enhancements to the volume format.

Pad fields have exactly the same semantics as a reserved field. The different name merely reflects the designer's goals when including the field, not the behavior of the implementation.

Integer Types

All integer values are defined by one of the following primitive types: UInt8, SInt8, UInt16, SInt16, UInt32, SInt32, UInt64, and SInt64. These represent unsigned and signed (2's complement) 8-bit, 16-bit, 32-bit, and 64-bit numbers.

All multi-byte integer values are stored in big-endian format. That is, the bytes are stored in order from most significant byte through least significant byte, in consecutive bytes, at increasing offset from the start of a block.

HFS Plus Names

File and folder names on HFS Plus consist of up to 255 Unicode characters with a preceding 16-bit length, defined by the type HFSUniStr255.

struct HFSUniStr255 {
    UInt16  length;
    UniChar unicode[255];
};
typedef struct HFSUniStr255 HFSUniStr255;
typedef const  HFSUniStr255 *ConstHFSUniStr255Param;

UniChar is a UInt16 that represents a character as defined in the Unicode character set defined by The Unicode Standard, Version 2.0 [Unicode, Inc. ISBN 0-201-48345-9].

HFS Plus stores strings fully decomposed and in canonical order. HFS Plus compares strings in a case-insensitive fashion. Strings may contain Unicode characters that should be ignored by this comparison. For more details on these subtleties, see Unicode Subtleties.

Text Encodings

Current Mac OS programming interfaces pass filenames as Pascal strings (either as a StringPtr or as a Str63 embedded in an FSSpec). The characters in those strings are not Unicode; the encoding varies depending on how the system software was localized and what language kits are installed. Identical sequences of bytes can represent vastly different Unicode character sequences. Similarly, many Unicode characters belong to more than one Mac OS text encoding.

HFS Plus includes two features specifically designed to help Mac OS handle the conversion between Mac OS-encoded Pascal strings and Unicode. The first feature is the textEncoding field of the file and folder catalog records. This field is defined as a hint to be used when converting the record's Unicode name back to a Mac OS- encoded Pascal string.

The valid values for the textEncoding field are defined in Table 2.

Table 2 Text Encodings

The second use of text encodings in HFS Plus is the encodingsBitmap field of the volume header. For each encoding used by a catalog node on the volume, the corresponding bit in the encodingsBitmap field must be set. Bit 0 is the least significant bit; bit 63 is the most significant bit.

It is acceptable for a bit in this bitmap to be set even though no names on the volume use that encoding. This means that when an implementation deletes or renames an object, it does not have to clear the encoding bit if that was the last name to use the given encoding.

HFS Plus Dates

HFS Plus stores dates in several data structures, including the volume header and catalog records. These dates are stored in unsigned 32-bit integers (UInt32) containing the number of seconds since midnight, January 1, 1904, GMT. This is slightly different from HFS, where the value represents local time.

The maximum representable date is February 6, 2040 at 06:28:15 GMT.

The date values do not account for leap seconds. They do include a leap day in every year that is evenly divisible by four. This is sufficient given that the range of representable dates does not contain 1900 or 2100, neither of which have leap days.

The implementation is responsible for converting these times to the format expected by client software. For example, the Mac OS File Manager passes dates in local time; the Mac OS HFS Plus implementation converts dates between local time and GMT as appropriate.

HFS Plus Permissions

For each file and folder, HFS Plus maintains a record containing access permissions, defined by the HFSPlusPermissions structure. This structure is intended as a placeholder. Future specifications may define a way to layer POSIX or AFP permissions onto the HFS Plus volume format.

struct HFSPlusPermissions {
    UInt32  ownerID;
    UInt32  groupID;
    UInt32  permissions;
    UInt32  specialDevice;
};
typedef struct HFSPlusPermissions HFSPlusPermissions;

The fields have the following meaning:

ownerID

An implementation must treat this as a reserved field. This field is intended to hold a number uniquely identifying the owner of the file or folder.

groupID

An implementation must treat this as a reserved field. This field is intended to hold a number uniquely identifying the group of the file or folder.

permissions

An implementation must treat this as a reserved field. This field is intended to hold the access rights for the owner, group, and all other users.

specialDevice

An implementation must treat this as a reserved field. This field is intended for use by POSIX-based systems to hold extra information that would typically be held in a POSIX file system.

Fork Data Structure

HFS Plus maintains information about the contents of a file using the HFSPlusForkData structure. Two such structures -- one for the resource and one for the data fork -- are stored in the catalog record for each user file. In addition, the volume header contains a fork data structure for each special file.

An unused extent descriptor in an extent record would have both startBlock and blockCount set to zero. For example, if a given fork occupied three extents, then the last five extent descriptors would be all zeroes.

struct HFSPlusForkData {
    UInt64                  logicalSize;
    UInt32                  clumpSize;
    UInt32                  totalBlocks;
    HFSPlusExtentRecord     extents;
};
typedef struct HFSPlusForkData HFSPlusForkData;
 
typedef HFSPlusExtentDescriptor HFSPlusExtentRecord[8];

The fields have the following meaning:

logicalSize

The size, in bytes, of the valid data in the fork.

clumpSize

The clump size for the fork in bytes. This is a hint to the implementation as to the size by which a growing file should be extended.

totalBlocks

The total number of allocation blocks used by all the extents in this fork.

extents

An array of extent descriptors for the fork. This array holds the first eight extent descriptors. If more extent descriptors are required, they are stored in the extents overflow file.

The HFSPlusExtentDescriptor structure is used to hold information about a specific extent.

struct HFSPlusExtentDescriptor {
    UInt32                  startBlock;
    UInt32                  blockCount;
};
typedef struct HFSPlusExtentDescriptor HFSPlusExtentDescriptor;

The fields have the following meaning:

startBlock

The first allocation block in the extent.

blockCount

The length, in allocation blocks, of the extent.

Volume Header

Each HFS Plus volume contains a volume header at sector 2. The volume header -- analogous to the master directory block (MDB) for HFS -- contains information about the volume as a whole, including the location of other key structures in the volume. The implementation is responsible for ensuring that this structure is updated before the volume is unmounted.

A copy of the volume header, the alternate volume header, is stored in the second to last sector of the volume (starting 1024 bytes before the end of the volume). The implementation should only update this copy when the length of one of the special files changes. The alternate volume header is intended for use solely by disk repair utilities.

The first two sectors (before the volume header) and the last sector (after the alternate volume header) are reserved.

The allocation block (or blocks) containing sectors 0 through 2 are marked as used in the allocation file (see the Allocation File section). Also, in order to accommodate the alternate volume header and last (reserved) sector, the last allocation block (or two allocation blocks, if the volume is formatted with 512-byte allocation blocks) is also marked as used in the allocation file,.

IMPORTANT: The alternate volume header is always stored in the second to last sector. If the disk size is not an even multiple of the allocation block size, this sector may lie beyond the last allocation block. However, the last allocation block (or two allocation blocks for a volume formatted with 512-byte allocation blocks) is still reserved even if the alternate volume header is not stored there.

The volume header is described by the HFSPlusVolumeHeader type.

struct HFSPlusVolumeHeader {
    UInt16              signature;
    UInt16              version;
    UInt32              attributes;
    UInt32              lastMountedVersion;
    UInt32              reserved;
 
    UInt32              createDate;
    UInt32              modifyDate;
    UInt32              backupDate;
    UInt32              checkedDate;
 
    UInt32              fileCount;
    UInt32              folderCount;
 
    UInt32              blockSize;
    UInt32              totalBlocks;
    UInt32              freeBlocks;
 
    UInt32              nextAllocation;
    UInt32              rsrcClumpSize;
    UInt32              dataClumpSize;
    HFSCatalogNodeID    nextCatalogID;
 
    UInt32              writeCount;
    UInt64              encodingsBitmap;
 
    UInt8               finderInfo[32];
 
    HFSPlusForkData     allocationFile;
    HFSPlusForkData     extentsFile;
    HFSPlusForkData     catalogFile;
    HFSPlusForkData     attributesFile;
    HFSPlusForkData     startupFile;
};
typedef struct HFSPlusVolumeHeader HFSPlusVolumeHeader;

The fields have the following meaning:

signature

The volume signature, which must be kHFSPlusSigWord ('H+') for an HFS Plus volume.

version

The version of the volume format, which is currently 4 (kHFSPlusVersion).

attributes

Volume attributes, as described below.

lastMountedVersion

A value which uniquely identifies the implementation that last mounted this volume for writing. This value can be used by future implementations to detect volumes that were last mounted by older implementations and check them for deficiencies. When a volume has been mounted for writing, the implementation should update this field with its own unique value. Third-party implementations of HFS Plus should place a registered creator code in this field. The value used by Mac OS 8.1 is '8.10' (kHFSPlusMountVersion).

reserved

An implementation must treat this as a reserved field.

createDate

The date and time when the volume was created. See HFS Plus Dates for a description of the format.

modifyDate

The date and time when the volume was last modified. See HFS Plus Dates for a description of the format.

backupDate

The date and time when the volume was last backed up. The volume format requires no special action on this field; it simply defines the field for the benefit of user programs. See HFS Plus Dates for a description of the format.

checkedDate

The date and time when the volume was last checked for consistency. Disk checking tools, such as Disk First Aid, must set this when they perform a disk check. A disk checking tool may use this date to perform periodic checking of a volume.

fileCount

The total number of files on the volume. The fileCount field does not include the special files. It should equal the number of file records found in the catalog file.

folderCount

The total number of folders on the volume. ThefolderCount field does not include the root folder. It should equal the number of folder records in the catalog file, minus one (since the root folder has a folder record in the catalog file).

blockSize

The allocation block size, in bytes.

totalBlocks

The total number of allocation blocks on the disk. For a disk whose size is an even multiple of the allocation block size, all sectors on the disk are included in an allocation block, including the volume header and alternate volume header. For a disk whose size is not an even multiple of the allocation block size, only the allocation blocks that will fit entirely on the disk are counted here. The remaining sectors at the end of the disk are not use by the volume format (except for storing the alternate volume header, as described above).

freeBlocks

The total number of unused allocation blocks on the disk.

nextAllocation

Start of next allocation search. The nextAllocation field is used by Mac OS as a hint for where to start searching for free allocation blocks when allocating space for a file. It contains the allocation block number where the search should begin. An implementation that doesn't want to use this kind of hint can just treat the field as reserved. [Implementation details: the current implementation for Mac OS 8 typically sets it to the first allocation block of the extent most recently allocated. It is not set to the allocation block immediately following the most recently allocated extent because of the likelihood of that extent being shortened when the file is closed (since a whole clump may have been allocated but not actually used).] See Allocation File section for details.

rsrcClumpSize

The default clump size for resource forks, in bytes. This is a hint to the implementation as to the size by which a growing file should be extended.

dataClumpSize

The default clump size for data forks, in bytes. This is a hint to the implementation as to the size by which a growing file should be extended.

nextCatalogID

The next unused catalog ID. See Catalog File for a description of catalog IDs.

writeCount

This field is incremented every time a volume is mounted. This allows an implementation to keep the volume mounted even when the media is ejected (or otherwise inaccessible). When the media is re-inserted, the implementation can check this field to determine when the media has been changed while it was ejected. It is very important that an implementation or utility change the writeCount field if it modifies the volume's structures directly. This is particularly important if it adds or deletes items on the volume.

encodingsBitmap

This field keeps track of the text encodings used in the file and folder names on the volume. This bitmap enables some performance optimizations for implementations that don't use Unicode names directly. See the Text Encoding sections for details.

finderInfo

This field contains information used by the Mac OS Finder. It's format is not part of the HFS Plus specification.

allocationFile

Information about the location and size of the allocation file. See Fork Data Structure for a description of the HFSPlusForkData type.

extentsFile

Information about the location and size of the extents file. See Fork Data Structure for a description of the HFSPlusForkData type.

catalogFile

Information about the location and size of the catalog file. See Fork Data Structure for a description of the HFSPlusForkData type.

attributesFile

Information about the location and size of the attributes file. See Fork Data Structure for a description of the HFSPlusForkData type.

startupFile

Information about the location and size of the startup file. See Fork Data Structure for a description of the HFSPlusForkData type.

Volume Attributes

The attributes field of a volume header is treated as a set of one-bit flags. The definition of the bits is given by the constants listed below. Bit 0 is the least significant bit; bit 31 is the most significant bit.

enum {
    /* Bits 0-6 are reserved */
    kHFSVolumeHardwareLockBit       =  7,
    kHFSVolumeUnmountedBit          =  8,
    kHFSVolumeSparedBlocksBit       =  9,
    kHFSVolumeNoCacheRequiredBit    = 10,
    kHFSBootVolumeInconsistentBit   = 11,
    /* Bits 12-14 are reserved */
    kHFSVolumeSoftwareLockBit       = 15
};

The bits have the following meaning:

bits 0-7

An implementation must treat these as reserved fields.

kHFSVolumeUnmountedBit (bit 8)

This bit is set if the volume was correctly flushed before being unmounted or ejected. An implementation must clear this bit on the media when it mounts a volume for writing. An implementation must set this bit on the media as the last step of unmounting a writable volume, after all other volume information has been flushed. If an implementation is asked to mount a volume where this bit is clear, it must assume the volume is inconsistent, and do appropriate consistency checking before using the volume.

kHFSVolumeSparedBlocksBit (bit 9)

This bit is set if there are any records in the extents overflow file for bad blocks (belonging to file ID kHFSBadBlockFileID). See Bad Block File for details.

kHFSVolumeNoCacheRequiredBit (bit 10)

This bit is set if the blocks from this volume should not be cached. For example, a RAM or ROM disk is actually stored in memory, so using additional memory to cache the volume's contents would be wasteful.

kHFSBootVolumeInconsistentBit (bit 11)

This bit is similar to kHFSVolumeUnmountedBit, but inverted in meaning. An implementation must set this bit on the media when it mounts a volume for writing. An implementation must clear this bit on the media as the last step of unmounting a writable volume, after all other volume information has been flushed. If an implementation is asked to mount a volume where this bit is set, it must assume the volume is inconsistent, and do appropriate consistency checking before using the volume.

bits 12 through 14

An implementation must treat these as reserved fields.

kHFSVolumeSoftwareLockBit (bit 15)

This bit is set if the volume is write-protected due to a software setting. Any implementations must refuse to write to a volume with this bit set. This flag is especially useful for write-protecting a volume on a media that cannot be write-protected otherwise, or for protecting an individual partition on a partitioned device.

B-Trees

This section describes the B-tree structure used for the catalog, extents overflow, and attributes files. A B-tree is stored in file data fork. Each B-tree has a HFSPlusForkData structure in the volume header that describes the size and initial extents of that data fork.

A B-tree file is divided up into fixed-size nodes, each of which contains records, which consist of a key and some data. The purpose of the B-tree is to efficiently map a key into its corresponding data. To achieve this, keys must be ordered, that is, there must be a well-defined way to decide whether one key is smaller than, equal to, or larger than another key.

The node size (which is expressed in bytes) must be power of two, from 512 through 32,768, inclusive. The node size of a B-tree is determined when the B-tree is created. The logical length of a B-tree file is just the number of nodes times the node size.

There are four kinds of nodes.

• Each B-tree contains a single header node. The header node is always the first node in the B-tree. It contains the information needed to find other any other node in the tree.
• Map nodes contain map records, which hold any allocation data (a bitmap that describes the free nodes in the B-tree) that overflows the map record in the header node.
• Index nodes hold pointer records that determine the structure of the B-tree.
• Leaf nodes hold data records that contain the data associated with a given key. The key for each data record must be unique.

All nodes share a common structure, described in the next section.

Node Structure

Nodes are indicated by number. The node's number can be calculated by dividing its offset into the file by the node size. Each node has the same general structure, consisting of three main parts: a node descriptor at the beginning of the node, a list of record offsets at the end of the node, and a list of records. This structure is depicted in Figure 2.

Figure 2 The structure of a node

The node descriptor contains basic information about the node as well as forward and backward links to other nodes. The BTNodeDescriptor data type describes this structure.

struct BTNodeDescriptor {
    UInt32    fLink;
    UInt32    bLink;
    SInt8     kind;
    UInt8     height;
    UInt16    numRecords;
    UInt16    reserved;
};
typedef struct BTNodeDescriptor BTNodeDescriptor;

The fields have the following meaning:

fLink

The node number of the next node of this type, or 0 if this is the last node.

bLink

The node number of the previous node of this type, or 0 if this is the first node.

kind

The type of this node. There are four node kinds, defined by the constants listed below.

height

The level, or depth, of this node in the B-tree hierarchy. For the header node, this field must be zero. For leaf nodes, this field must be one. For index nodes, this field is one greater than the height of the child nodes it points to. The height of a map node is zero, just like for a header node. (Think of map nodes as extensions of the map record in the header node.)

numRecords

The number of records contained in this node.

reserved

An implementation must treat this as a reserved field.

A node descriptor is always 14 (which is sizeof(BTNodeDescriptor)) bytes long, so the list of records contained in a node always starts 14 bytes from the start of the node. The size of each record can vary, depending on the record's type and the amount of information it contains.

The records are accessed using the list of record offsets at the end of the node. Each entry in this list is a UInt16 which contains the offset, in bytes, from the start of the node to the start of the record. The offsets are stored in reverse order, with the offset for the first record in the last two bytes of the node, the offset for the second record is in the previous two bytes, and so on. Since the first record is always at offset 14, the last two bytes of the node contain the value 14.

The kind field of the node descriptor describes the type of a node, which indicates what kinds of records it contains and, therefore, its purpose in the B-tree hierarchy. There are four kinds of node types given by the following constants:

enum {
    kBTLeafNode       = -1,
    kBTIndexNode      =  0,
    kBTHeaderNode     =  1,
    kBTMapNode        =  2
};

It's important to realise that the B-tree node type determines the type of records found in the node. Leaf nodes always contain data records. Index nodes always contain pointer records. Map nodes always contain map records. The header node always contains a header record, a reserved record, and a map record. The four node types and their corresponding records are described in the subsequent sections.

Header Nodes

The first node (node 0) in every B-tree file is a header node, which contains essential information about the entire B-tree file. There are three records in the header node. The first record is the B-tree header record. The second is a reserved record that is always 128 bytes long. The last record is the B-tree map record; it occupies all of the remaining space between the reserved record and the record offsets. The header node is shown in Figure 3.

Figure 3 Header node structure

The fLink field of the header node's node descriptor contains the node number of the first map node, or 0 if there are no map nodes. The bLink field of the header node's node descriptor must be set to zero.

Header Record

The B-tree header record contains general information about the B-tree such as its size, maximum key length, and the location of the first and last leaf nodes. The data type BTHeaderRec describes the structure of a header record.

struct BTHeaderRec {
    UInt16    treeDepth;
    UInt32    rootNode;
    UInt32    leafRecords;
    UInt32    firstLeafNode;
    UInt32    lastLeafNode;
    UInt16    nodeSize;
    UInt16    maxKeyLength;
    UInt32    totalNodes;
    UInt32    freeNodes;
    UInt16    reserved1;
    UInt32    clumpSize;      // misaligned
    UInt8     btreeType;
    UInt8     reserved2;
    UInt32    attributes;     // long aligned again
    UInt32    reserved3[16];
};
typedef struct BTHeaderRec BTHeaderRec;

The fields have the following meaning:

treeDepth

The current depth of the B-tree. Always equal to the height field of the root node.

rootNode

The node number of the root node, the index node that acts as the root of the B-tree. See Index Nodes for details. There is a possibility that the rootNode is a leaf node. See Inside Macintosh: Files, pp. 2-69 for details.

leafRecords

The total number of records contained in all of the leaf nodes.

firstLeafNode

The node number of the first leaf node. This may be zero if there are no leaf nodes.

lastLeafNode

The node number of the last leaf node. This may be zero if there are no leaf nodes.

nodeSize

The size, in bytes, of a node. This is a power of two, from 512 through 32,768, inclusive.

maxKeyLength

The maximum length of a key in an index or leaf node. HFSVolumes.h has the maxKeyLength values for the catalog and extents files for both HFS and HFS Plus (kHFSPlusExtentKeyMaximumLength, kHFSExtentKeyMaximumLength, kHFSPlusCatalogKeyMaximumLength, kHFSCatalogKeyMaximumLength). The maximum key length for the attributes B-tree will probably be a little larger than for the catalog file. In general, maxKeyLength has to be small enough (compared to nodeSize) so that a single node can fit two keys of maximum size plus the node descriptor and offsets.

totalNodes

The total number of nodes (be they free or used) in the B-tree. The length of the B-tree file is this value times the nodeSize.

freeNodes

The number of unused nodes in the B-tree.

reserved1

An implementation must treat this as a reserved field.

clumpSize

Ignored for HFS Plus B-trees. The clumpSize field of the HFSPlusForkData record is used instead. For maximum compatibility, an implementation should probably set the clumpSize in the node descriptor to the same value as the clumpSize in the HFSPlusForkData when initializing a volume. Otherwise, it should treat the header records's clumpSize as reserved.

btreeType

Must be 0 for HFS Plus B-trees.

reserved2

An implementation must treat this as a reserved field.

attributes

A set of bits used to describe various attributes of the B-tree. The meaning of these bits is given below.

reserved3

An implementation must treat this as a reserved field.

The following constants define the various bits that may be set in the attributes field of the header record.

enum {
    kBTBadCloseMask           = 0x00000001,
    kBTBigKeysMask            = 0x00000002,
    kBTVariableIndexKeysMask  = 0x00000004
};

The bits have the following meaning:

kBTBadCloseMask

This bit indicates that the B-tree was not closed properly and should be checked for consistency. This bit is not used for HFS Plus B-trees. An implementation must treat this as reserved.

kBTBigKeysMask

If this bit is set, the keyLength field of the keys in index and leaf nodes is UInt16; otherwise, it is a UInt8. This bit must be set for all HFS Plus B-trees.

kBTVariableIndexKeysMask

If this bit is set, the keys in index nodes occupy the number of bytes indicated by their keyLength field; otherwise, the keys in index nodes always occupy maxKeyLength bytes. This bit must be set for the HFS Plus Catalog B-tree, and cleared for the HFS Plus Extents B-tree.

Bits not specified here must be treated as reserved.

Reserved Record

The second record in a header node is always 128 bytes long. It is not used by HFS Plus B-trees. An implementation must treat the contents of this record as a reserved field.

Map Record

The remaining space in the header node is occupied by a third record, the map record. It is a bitmap that indicates which nodes in the B-tree are used and which are free. The bits are interpreted in the same way as the bits in the allocation file.

All tolled, the node descriptor, header record, reserved record, and record offsets occupy 256 bytes of the header node. So the size of the map record (in bytes) is nodeSize minus 256. If there are more nodes in the B-tree than can be represented by the map record in the header node, map nodes are used to store additional allocation data.

Map Nodes

If the map record of the header node is not large enough to represent all of the nodes in the B-tree, map nodes are used to store the remaining allocation data. In this case, the fLink field of the header node's node descriptor contains the node number of the first map node.

A map node consists of the node descriptor and a single map record. The map record is a continuation of the map record contained in the header node. The size of the map record is the size of the node, minus the size of the node descriptor (14 bytes), minus the size of two offsets (4 bytes), minus two bytes of free space. That is, the size of the map record is the size of the node minus 20 bytes; this keeps the length of the map record an even multiple of 4 bytes. Note that the start of the map record is not aligned to a 4-byte boundary: it starts immediately after the node descriptor (at an offset of 14 bytes).

The B-tree uses as many map nodes as needed to provide allocation data for all of the nodes in the B-tree. The map nodes are chained through the fLink fields of their node descriptors, starting with the header node. The fLink field of the last map node's node descriptor is zero. The bLink field is not used for map nodes and must be set to zero for all map nodes.

Keyed Records

The records in index and leaf nodes share a common structure. They contain a keyLength, followed by the key itself, followed by the record data.

The first part of the record, keyLength, is either a UInt8 or a UInt16, depending on the attributes field in the B-tree's header record. If the kBTBigKeysMask bit is set in attributes, the keyLength is a UInt16; otherwise, it's a UInt8. The length of the key, as stored in this field, does not include the size of the keyLength field itself.

Immediately following the keyLength is the key itself. The length of the key is determined by the node type and the B-tree attributes. In leaf nodes, the length is always determined by keyLength. In index nodes, the length depends on the value of the kBTVariableIndexKeysMask bit in the B-tree attributes in the header record. If the bit is clear, the key occupies a constant number of bytes, determined by the maxKeyLength field of the B-tree header record. If the bit is set, the key length is determined by the keyLength field of the keyed record.

Following the key is the record's data. The format of this data depends on the node type, as explained in the next two sections. However, the data is always aligned on a two-byte boundary and occupies an even number of bytes. To meet the first alignment requirement, a pad byte must be inserted between the key and the data if the size of the keyLength field plus the size of the key is odd. To meet the second alignment requirement, a pad byte must be added after the data if the data size is odd.

Index Nodes

The records in an index node are called pointer records. They contain a keyLength, a key, and a node number, expressed a UInt32. The node whose number is in a pointer record is called a child node of the index node. An index node has two or more children, depending on the size of the node and the size of the keys in the node.

Leaf Nodes

The bottom level of a B-tree is occupied exclusively by leaf nodes, which contain data records instead of pointer records. The data records contain a keyLength, a key, and the data associated with that key. The data may be of variable length.

In an HFS Plus B-tree, the data in the data record is the HFS Plus volume structure (such as a CatalogRecord, ExtentRecord, or AttributeRecord) associated with the key.

Searching for Keyed Records

A B-tree is highly structured to allow for efficient searching, insertion, and removal. This structure primarily affects the keyed records (pointer records and data records) and the nodes in which they are stored (index nodes and leaf nodes). The following are the ordering requirements for index and leaf nodes:

• Keyed records must be placed in a node such that their keys are in ascending order.
• All the nodes in a given level (whose height field is the same) must be chained via their fLink and bLink field. The node with the smallest keys must be first in the chain and its bLink field must be zero. The node with the largest keys must be last in the chain and its fLink field must be zero.
• For any given node, all the keys in the node must be less than all the keys in the next node in the chain (pointed to by fLink). Similarly, all the keys in the node must be greater than all the keys in the previous node in the chain (pointed to by bLink).

Keeping the keys ordered in this way makes it possible to quickly search the B-tree to find the data associated with a given key. Figure 4 shows a sample B-tree containing hypothetical keys (in this case, the keys are simply integers).

When an implementation needs to find the data associated with a particular search key, it begins searching at the root node. Starting with the first record, it searches for the record with the greatest key that is less than or equal to the search key. In then moves to the child node (typically an index node) and repeats the same process.

This process continues until a leaf node is reached. If the key found in the leaf node is equal to the search key, the found record contains the desired data associated with the search key. If the found key is not equal to the search key, the search key is not present in the B-tree.

Figure 4 A sample B-Tree

HFS and HFS Plus B-Trees Compared

The structure of the B-trees on an HFS Plus volume is a closely related to the B-tree structure used on an HFS volume. There are three principal differences: the size of nodes, the size of keys within index nodes, and the size of a key length (UInt8 vs. UInt16).

Node Sizes

In an HFS B-tree, nodes always have a fixed size of 512 bytes.

In an HFS Plus B-tree, the node size is determined by a field (nodeSize) in the header node. The node size must be a power from 512 through 32,768. An implementation must use the nodeSize field to determine the actual node size.

HFS Plus uses the following default node sizes:

• 4 KB for the catalog file
• 1 KB for the extents overflow file
• 4 KB for the attributes file

These sizes are set when the volume is initialized and cannot be easily changed. It is legal to initialize an HFS Plus volume with different node sizes, but the node sizes must be large enough for an index node to contain two keys of maximum size (plus the other overhead such as a node descriptor, record offsets, and pointers to children).

Key Size in an Index Node

In an HFS B-tree, all of the keys in an index node occupy a fixed amount of space: the maximum key length for that B-tree. This simplifies the algorithms for inserting and deleting records because, within an index node, one key can be replaced by another key without worrying whether there is adequate room for the new key. However, it is also somewhat wasteful when the keys are variable length (such as in the catalog file, where the key length varies with the length of the file name).

In an HFS Plus B-tree, the keys in an index node are allowed to vary in size. This complicates the algorithms for inserting and deleting records, but reduces wasted space when the length of a key can vary (such as in the catalog file). It also means that the number of keys in an index node will vary with the actual size of the keys.

Catalog File

HFS Plus uses a catalog file to maintain information about the hierarchy of files and folders on a volume. A catalog file is organized as a B-tree file, and hence consists of a header node, index nodes, leaf nodes, and (if necessary) map nodes. The location of the first extent of the catalog file (and hence of the file's header node) is stored in the volume header. From the catalog file's header node, an implementation can obtain the node number of the root node of the B-tree. From the root node, an implementation can search the B-tree for keys, as described in the previous section.

The B-Trees chapter defined a standard rule for the node size of HFS Plus B-trees. As the catalog file is a B-tree, it inherits the requirements of this rule. In addition, the node size of the catalog file must be at least 4 KB (kHFSPlusCatalogMinNodeSize).

Each file or folder in the catalog file is assigned a unique catalog node ID (CNID). For folders, the CNID is the folder ID, sometimes called a directory ID, or dirID; for files, it's the file ID. For any given file or folder, the parent ID is the CNID of the folder containing the file or folder, known as the parent folder.

The catalog node ID is defined by the CatalogNodeID data type.

typedef UInt32 HFSCatalogNodeID;

The first 16 CNIDs are reserved for use by Apple Computer, Inc., and include the following standard assignments:

enum {
    kHFSRootParentID            = 1,
    kHFSRootFolderID            = 2,
    kHFSExtentsFileID           = 3,
    kHFSCatalogFileID           = 4,
    kHFSBadBlockFileID          = 5,
    kHFSAllocationFileID        = 6,
    kHFSStartupFileID           = 7,
    kHFSAttributesFileID        = 8,
    kHFSBogusExtentFileID       = 15,
    kHFSFirstUserCatalogNodeID  = 16
};

These constants have the following meaning:

kHFSRootParentID

Parent ID of the root folder.

kHFSRootFolderID

Folder ID of the root folder.

kHFSExtentsFileID

File ID of the extents overflow file.

kHFSCatalogFileID

File ID of the catalog file.

kHFSBadBlockFileID

File ID of the bad block file. The bad block file is not a file in the same sense as a special file and a user file. See Bad Block File for details.

kHFSAllocationFileID

File ID of the allocation file (introduced with HFS Plus).

kHFSStartupFileID

File ID of the startup file (introduced with HFS Plus).

kHFSAttributesFileID

File ID of the attributes file (introduced with HFS Plus).

kHFSBogusExtentFileID

Used temporarily during ExchangeFiles operations.

kHFSFirstUserCatalogNodeID

First CNID available for use by user files and folders.

In addition, the CNID of zero is never used and serves as a nil value.

The implementation must store a number greater than the largest CNID used by any file or folder on the volume in the nextCatalogID field of the volume header. Typically CNIDs are allocated sequentially, starting at kHFSFirstUserCatalogNodeID. When the CNID reaches its maximum ($FFFFFFFF), the disk is out of CNIDs and no new files or folders can be created.

As the catalog file is a B-tree file, it inherits its basic structure from the definition in B-Trees. Beyond that, you need to know only two things about an HFS Plus catalog file to interpret its data:

1. the format of the key used both in index and leaf nodes, and
2. the format of the leaf node data records (file, folder, and thread records).

Catalog File Key

For a given file, folder, or thread record, the catalog file key consists of the parent folder's CNID and the name of the file or folder. This structure is described using the HFSPlusCatalogKey type.

struct HFSPlusCatalogKey {
    UInt16              keyLength;
    HFSCatalogNodeID    parentID;
    HFSUniStr255        nodeName;
};
typedef struct HFSPlusCatalogKey HFSPlusCatalogKey;

The fields have the following meaning:

keyLength

The keyLength field is required by all keyed records in a B-tree. The catalog file, in common with all HFS Plus B-trees, uses a large key length (UInt16).

parentID

For file and folder records, this is the folder containing the file or folder represented by the record. For thread records, this is the CNID of the file or folder itself.

nodeName

This field contains Unicode characters, fully decomposed and in canonical order. For file or folder records, this is the name of the file or folder inside the parentID folder. For thread records, this is the empty string.

Catalog file keys are compared first by parentID and then by nodeName. The parentID is compared as an unsigned 32-bit integer. The nodeName should be compared in a case-insensitive way, as described in the String Comparison Algorithm section.

For more information about how catalog keys are used to find file, folder, and thread records within the catalog tree, see Catalog Tree Usage.

Catalog File Data

A catalog file leaf node can contain four different types of data records:

1. A folder record contains information about a single folder.
2. A file record contains information about a single file.
3. A folder thread record provides a link between a folder and its parent folder, and lets you find a folder record given just the folder ID.
4. A file thread record provides a link between a file and its parent folder, and lets you find a file record given just the file ID. (In both the folder thread and the file thread record, the thread record is used to map the file or folder ID to the actual parent directory ID and name.)

Each record starts with a recordType field, which describes the type of catalog data record. The recordType field contains one of the following values:

enum {
    kHFSPlusFolderRecord        = 0x0001,
    kHFSPlusFileRecord          = 0x0002,
    kHFSPlusFolderThreadRecord  = 0x0003,
    kHFSPlusFileThreadRecord    = 0x0004
};

The values have the following meaning:

kHFSPlusFolderRecord

This record is a folder record. You can use the HFSPlusCatalogFolder type to interpret the data.

kHFSPlusFileRecord

This record is a file record. You can use the HFSPlusCatalogFile type to interpret the data.

kHFSPlusFolderThreadRecord

This record is a folder thread record. You can use the HFSPlusCatalogThread type to interpret the data.

kHFSPlusFileThreadRecord

This record is a file thread record. You can use the HFSPlusCatalogThread type to interpret the data.

The next three sections describe the folder, file, and thread records in detail.

enum {
    kHFSFolderRecord            = 0x0100,
    kHFSFileRecord              = 0x0200,
    kHFSFolderThreadRecord      = 0x0300,
    kHFSFileThreadRecord        = 0x0400
};

Catalog Folder Records

The catalog folder record is used in the catalog B-tree file to hold information about a particular folder on the volume. The data of the record is described by the HFSPlusCatalogFolder type.

struct HFSPlusCatalogFolder {
    SInt16              recordType;
    UInt16              flags;
    UInt32              valence;
    HFSCatalogNodeID    folderID;
    UInt32              createDate;
    UInt32              contentModDate;
    UInt32              attributeModDate;
    UInt32              accessDate;
    UInt32              backupDate;
    HFSPlusPermissions  permissions;
    DInfo               userInfo;
    DXInfo              finderInfo;
    UInt32              textEncoding;
    UInt32              reserved;
};
typedef struct HFSPlusCatalogFolder HFSPlusCatalogFolder;
 

The fields have the following meaning:

recordType

The catalog data record type. For folder records, this is always kHFSPlusFolderRecord.

flags

This field contains bit flags about the folder. No bits are currently defined for folder records. An implementation must treat this as a reserved field.

valence

The number of files and folders directly contained by this folder. This is equal to the number of file and folder records whose key's parentID is equal to this folder's folderID.

folderID

The CNID of this folder. Remember that the key for a folder record contains the CNID of the folders parent, not the CNID of the folder itself.

createDate

The date and time the folder was created. See HFS Plus Dates for a description of the format. Again, the createDate of the Volume Header is NOT stored in GMT; it is local time. (Further, if the volume has an HFS wrapper, the creation date in the MDB should be the same as the createDate in the Volume Header).

contentModDate

The date and time the folder's contents were last changed. This is the time when a file or folder was created or deleted inside this folder, or when a file or folder was moved in or out of this folder. See HFS Plus Dates for a description of the format.

attributeModDate

The attributeModDate is set when a named fork (attribute) -- other than the data and resource fork -- is written to. The contentModDate is used for the data and resource forks, and when adding/removing items in a directory. An implementation which doesn't use named forks should treat the attributeModDate as reserved.

accessDate

The date and time the folder's contents were last read. This field has no analog in the HFS catalog record. It represents the last time the folder's contents were read. This field exists to support POSIX semantics when the volume is mounted on non-Mac OS platforms. See HFS Plus Dates for a description of the format.

backupDate

The date and time the folder was last backed up. The volume format requires no special action on this field; it simply defines the field for the benefit of user programs. See HFS Plus Dates for a description of the format.

permissions

This field contains folder permissions, similar to those defined by POSIX or AFP. See HFS Plus Permissions for a description of the format.

userInfo

This field contains information used by the Mac OS Finder. Its format is not part of the HFS Plus specification.

finderInfo

This field contains information used by the Mac OS Finder. Its format is not part of the HFS Plus specification.

textEncoding

A hint as to text encoding from which the folder name was derived. This hint can be used to improve the quality of the conversion of the name to a Mac OS-encoded Pascal string. See Text Encodings for details.

reserved

An implementation must treat this as a reserved field.

Catalog File Records

The catalog file record is used in the catalog B-tree file to hold information about a particular file on the volume. The data of the record is described by the HFSPlusCatalogFile type.

struct HFSPlusCatalogFile {
    SInt16              recordType;
    UInt16              flags;
    UInt32              reserved1;
    HFSCatalogNodeID    fileID;
    UInt32              createDate;
    UInt32              contentModDate;
    UInt32              attributeModDate;
    UInt32              accessDate;
    UInt32              backupDate;
    HFSPlusPermissions  permissions;
    FInfo               userInfo;
    FXInfo              finderInfo;
    UInt32              textEncoding;
    UInt32              reserved2;
 
    HFSPlusForkData     dataFork;
    HFSPlusForkData     resourceFork;
};
typedef struct HFSPlusCatalogFile HFSPlusCatalogFile;
 

The fields have the following meaning:

recordType

The catalog data record type. For files records, this is always kHFSPlusFileRecord.

flags

This field contains bit flags about the file. The currently defined bits are described below. An implementation must treat undefined bits as reserved.

reserved1

An implementation must treat this as a reserved field.

fileID

The CNID of this file.

createDate

The date and time the file was created. See HFS Plus Dates for a description of the format.

contentModDate

The date and time the file's contents were last changed by extending, truncating, or writing either of the forks. See HFS Plus Dates for a description of the format. The contentModDate is set when the data or resource forks are extended, truncated, or written to. The attributeModDate is set when any other named fork (attribute) is extended, truncated, or written to. Since the current Mac OS APIs don't support the other named forks, it treats attributeModDate as reserved.

attributeModDate

The date and time the file's attributes were last changed. The attributeModDate is set when any other named fork (attribute) is extended, truncated, or written to. Since the current Mac OS APIs don't support the other named forks, it treats attributeModDate as reserved (see the Attributes File section). See HFS Plus Dates for a description of the format.

accessDate

The date and time the file's contents were last read. This field has no analog in the HFS catalog record. It represents the last time either of a file's forks was read. This field exists to support POSIX semantics when the volume is mounted on non-Mac OS platforms. See HFS Plus Dates for a description of the format.

backupDate

The date and time the file was last backed up. The volume format requires no special action on this field; it simply defines the field for the benefit of user programs. See HFS Plus Dates for a description of the format.

permissions

This field contains file permissions, similar to those defined by POSIX. See HFS Plus Permissions for a description of the format.

userInfo

This field contains information used by the Mac OS Finder. Its format is not part of the HFS Plus specification.

finderInfo

This field contains information used by the Mac OS Finder. Its format is not part of the HFS Plus specification.

textEncoding

A hint as to text encoding from which the file name was derived. This hint can be used to improved the quality of the conversion of the name to a Mac OS encoded Pascal string. See Text Encodings for details.

reserved2

An implementation must treat this as a reserved field.

dataFork

Information about the location and size of the data fork. See Fork Data Structure for a description of the HFSPlusForkData type.

resourceFork

Information about the location and size of the resource fork. See Fork Data Structure for a description of the HFSPlusForkData type.

For each fork, the first eight extents are described by the HFSPlusForkData field in the catalog file record. If a fork is sufficiently fragmented to require more than eight extents, the remaining extents are described by extent records in the extents overflow file.

The following constants define bit flags in the file record's flags field:

enum {
    kHFSFileLockedBit       = 0x0000,
    kHFSFileLockedMask      = 0x0001,
    kHFSThreadExistsBit     = 0x0001,
    kHFSThreadExistsMask    = 0x0002
};

The values have the following meaning:

kHFSFileLockedBit, kHFSFileLockedMask

If kHFSFileLockedBit is set, then none of the forks may be extended, truncated, or written to. They may only be opened for reading (not for writing). The catalog information (like finderInfo and userInfo) may still be changed.

kHFSThreadExistsBit, kHFSThreadExistsMask

This bit incidates that the file has a thread record. As all files in HFS Plus have thread records, this bit must be set.

Catalog Thread Records

The catalog thread record is used in the catalog B-tree file to link a CNID to the file or folder record using that CNID. The data of the record is described by the HFSPlusCatalogThread type.

struct HFSPlusCatalogThread {
    SInt16              recordType;
    SInt16              reserved;
    HFSCatalogNodeID    parentID;
    HFSUniStr255        nodeName;
};
typedef struct HFSPlusCatalogThread HFSPlusCatalogThread;

The fields have the following meaning:

recordType

The catalog data record type. For thread records, this is kHFSPlusFileRecord or kHFSPlusFolderRecord, depending on whether the thread record refers to a file or a folder. Both types of thread record contain the same data.

reserved1

An implementation must treat this as a reserved field.

parentID

The CNID of the parent of the file or folder referenced by this thread record.

nodeName

The name of the file or folder referenced by this thread record.

The next section explains how thread records can be used to find a file or folder using just its CNID.

Catalog Tree Usage

File and folder records always have a key that contains a non-empty nodeName. The file and folder records for the children are all consecutive in the catalog, since they all have the same parentID in the key, and vary only by nodeName.

The key for a thread record is the file's or folder's CNID (not the CNID of the parent folder) and an empty (zero length) nodeName. This allows a file or folder to by found using just the CNID. The thread record contains the parentID and nodeName field of the file or folder itself.

Finding a file or folder by its CNID is a two-step process. The first step is to use the CNID to look up the thread record for the file or folder. This yields the file or folder's parent folder ID and name. The second step is to use that information to look up the real file or folder record.

Since files do not contain other files or folders, there are no catalog records whose key has a parentID equal to a file's CNID and nodeName with non-zero length.These unused key values are reserved.

Extents Overflow File

HFS Plus tracks which allocation blocks belong to a file's forks by maintaining a list of extents (contiguous allocation blocks) that belong to that file, in the appropriate order. Each extent is represented by a pair of numbers: the first allocation block number of the extent and the number of allocation blocks in the extent. The file record in the catalog B-tree contains a record of the first eight extents of each fork. If there are more than eight extents in a fork, the remaining extents are stored in the extents overflow file.

Like the catalog file, the extents overflow file is B-tree. However, the structure of the extents overflow file is relatively simple compared to that of a catalog file. The extents overflow file has a simple, fixed length key and a single type of data record.

Extents Overflow File Key

The structure of the key for the extents overflow file is described by the HFSPlusExtentKey type.

struct HFSPlusExtentKey {
    UInt16              keyLength;
    UInt8               forkType;
    UInt8               pad;
    HFSCatalogNodeID    fileID;
    UInt32              startBlock;
};
typedef struct HFSPlusExtentKey HFSPlusExtentKey;

The fields have the following meaning:

keyLength

The keyLength field is required by all keyed records in a B-tree. The extents overflow file, in common with all HFS Plus B-trees, uses a large key length (UInt16). Keys in the extents overflow file always have the same length, kHFSPlusExtentKeyMaximumLength (10).

forkType

The type of fork for which this extent record applies. This must be either 0 for the data fork or $FF for the resource fork.

pad

An implementation must treat this as a pad field.

fileID

The CNID of the file for which this extent record applies.

startBlock

The offset, in allocation blocks, into the fork of the first extent described by this extent record. The startBlock field lets you directly find the particular extents for a given offset into a fork.

Two HFSPlusExtentKey structures are compared by comparing their fields in the following order: fileID, forkType, startBlock. Thus, all the extent records for a particular fork are grouped together in the B-tree, right next to all the extent records for the other fork of the file.

Extents Overflow File Data

The data records for an extents overflow file (the extent records) are described by the HFSPlusExtentRecord type, which is described in detail in Fork Data Structure.

Extents Overflow File Usage

The most important thing to remember about extents overflow file is that it is only used for forks with more than eight extents. In most cases, forks have fewer extents, and all the extents information for the fork is held in its catalog file record. However, for more fragmented forks, the extra extents information is stored in the extents overflow file.

When an implementation needs to map a fork offset into a sector on disk, it first looks through the extent records in the catalog file record. If the fork offset is within one these extents, the implementation can find the corresponding sector without consulting the extents overflow file.

If, on the other hand, the fork offset is beyond the last extent recorded in the catalog file record, the implementation must look in the next extent record, which is stored in the extents overflow file. To find this record, the implementation must form a key, which consists of information about the fork (the fork type and the file ID) and the offset info the fork (the start block).

Because extent records are partially keyed off the fork offset of the first extent in the record, the implementation must have all the preceding extent records in order to know the fork offset to form the key of the next extent record. For example, if the fork has two extent records in the extents overflow file, the implementation must read the first extent record to calculate the fork offset for the key for the second extent record.

However, you can use the startBlock in the extent key to go directly to the record you need. Here's a complicated example:

We've got a fork with a total of 23 extents (very fragmented!). The blockCounts for the extents, in order, are as follows: one extent of 6 allocation blocks, 14 extents of one allocation block each, two extents of two allocation blocks each, one extent of 7 allocation blocks, and five more extents of one allocation block each. The fork contains a total of 36 allocation blocks.

The block counts for the catalog's fork data are: 6, 1, 1, 1, 1, 1, 1, 1. There is an extent overflow record whose startBlock is 13 (0+6+1+1+1+1+1+1+1), and has the following block counts: 1, 1, 1, 1, 1, 1, 1, 2. There is a second extent overflow record whose startBlock is 22 (13+1+1+1+1+1+1+1+2), and has the following block counts: 2, 7, 1, 1, 1, 1, 1, 0. Note this last record only contains seven extents.

Suppose the allocation block size for the volume is 4K. Suppose we want to start reading from the file at an offset of 108K. We want to know where the data is on the volume, and how much contiguous data is there.

First, we divide 108K (the fork offset) by 4K (the allocation block size) to get 27, which is the number of allocation blocks from the start of the fork. So, we want to know where fork allocation block #27 is. We notice that 27 is greater than or equal to 13 (the number of allocation blocks in the catalog's fork data), so we're going to have to look in the extents B-tree.

We construct a search key with the appropriate fileID and forkType, and set startBlock to 27 (the desired fork allocation block number). We then search the extents B-tree for the record whose key is less than or equal to our search key. We find the second extent overflow record (the one with startBlock=22). It has the same fileID and forkType, so things are good. Now we just need to figure out which extent within that record to use.

We compute 27 (the desired fork allocation block) minus 22 (the startBlock) and get 5. So, we want the extent that is 5 allocation blocks "into" the record. We try the first extent. It's only two allocation blocks long, so the desired extent is 3 allocation blocks after that first extent in the record. The next extent is 7 allocation blocks long. Since 7 is greater than 3, we know the desired fork position is within this extent (the second extent in the second overflow record). Further, we know that there are 7-3=4 contiguous allocation blocks (i.e., 16K).

We grab the startBlock for that second extent (i.e., the one whose blockCount is 7); suppose this number is 444. We add 3 (since the desired position was 3 allocation blocks into the extent we found). So, the desired position is in allocation block 444+3=447 on the volume. That is 447*4K=1788K from the start of the HFS Plus volume. (Since the Volume Header always starts 1K after the start of the HFS Plus volume, the desired fork position is 1787K after the start of the Volume Header.)

Bad Block File

The extent overflow file is also used to hold information about the bad block file. The bad block file is used to mark areas on the disk as bad, unable to be used for storing data. This is typically used to map out bad sectors on the disk.

When an HFS Plus volume is embedded within an HFS wrapper (the way Mac OS normally initializes a hard disk), the space used by the HFS Plus volume is marked as part of the bad block file within the HFS wrapper itself. (This sounds confusing because you have a volume within another volume.)

The bad block file is not a file in the same sense as a user file (it doesn't have a file record in the catalog) or one of the special files (it's not referenced by the volume header). Instead, the bad block file uses a special CNID (kHFSBadBlockFileID) as the key for extent records in the extents overflow file. When a block is marked as bad, an extent with this CNID and encompassing the bad block is added to the extents overflow file. The block is marked as used in the allocation file. These steps prevent the block from being used for data by the file system.

Bad block extent records are always assumed to reference the data fork. The forkType field of the key must be 0.

HFS uses a similar mechanism to store information about bad blocks. This facility is used by the HFS Wrapper to hold an entire HFS Plus volume as bad blocks on an HFS disk.

Allocation File

HFS Plus uses an allocation file to keep track of whether each allocation block in a volume is currently allocated to some file system structure or not. The contents of the allocation file is a bitmap. The bitmap contains one bit for each allocation block in the volume. If a bit is set, the corresponding allocation block is currently in use by some file system structure. If a bit is clear, the corresponding allocation block is not currently in use, and is available for allocation.

Each byte in the allocation file holds the state of eight allocation blocks. The byte at offset X into the file contains the allocation state of allocations blocks (X * 8) through (X * 8 + 7). Within each byte, the most significant bit holds information about the allocation block with the lowest number, the least significant bit holds information about the allocation block with the highest number. Listing 1 shows how you would test whether an allocation block is in use, assuming that you've read the entire allocation file into memory.

Listing 1 Determining whether an allocation block is in use

static Boolean IsAllocationBlockUsed(UInt32 thisAllocationBlock, 
                                     UInt8 *allocationFileContents)
{
    UInt8 thisByte;
    
    thisByte = allocationFileContents[thisAllocationBlock / 8];
    return (thisByte & (1 << (7 - (thisAllocationBlock % 8)))) != 0;
}

The size of the allocation file depends on the number of allocation blocks in the volume, which in turn depends both on the number of sectors on the disk and on the size of the volume's allocation blocks (the number of sectors per allocation block). For example, a volume on a 1 GB disk and having an allocation block size of 4 KB needs an allocation file size of 256 Kbits (32 KB, or 8 allocation blocks). Since the allocation file itself is allocated using allocation blocks, it always occupies an integral number of allocation blocks (its size may be rounded up).

The allocation file may be larger than the minimum number of bits required for the given volume size. Any unused bits in the bitmap must be set to zero.

Attributes File

The HFS Plus attributes file is reserved for implementing named forks in the future. An attributes file is organized as a B-tree file. It a special file, described by an HFSPlusForkData record in the volume header, with no entry in the catalog file. An attributes files has a variable length key and three data record types, which makes it roughly as complex as the catalog file.

It is possible for a volume to have no attributes file. If the first extent of the attributes file (stored in the volume header) has zero allocation blocks, the attributes file does not exist.

The B-Trees chapter defined a standard rule for the node size of HFS Plus B-trees. As the attributes file is a B-tree, it inherits the requirements of this rule. In addition, the node size of the attributes file must be at least 4 KB (kHFSPlusAttrMinNodeSize).

Attributes File Data

The leaf nodes of an attributes file contain data records, known as attributes. There are two types of attributes:

1. Fork data attributes are used for attributes whose data is large. The attribute's data is stored in extents on the volume and the attribute merely contains a reference to those extents.
2. Extension attributes augment fork data attributes, allowing an fork data attribute to have more than eight extents.

Each record starts with a recordType field, which describes the type of attribute data record. The recordType field contains one of the following values.

enum {
    kHFSPlusAttrInlineData  = 0x10,
    kHFSPlusAttrForkData    = 0x20,
    kHFSPlusAttrExtents     = 0x30
};

The values have the following meaning:

kHFSPlusAttrInlineData

Reserved for future use.

kHFSPlusAttrForkData

This record is a fork data attribute. You can use the HFSPlusAttrForkData type to interpret the data.

kHFSPlusAttrExtents

This record is an extension attribute. You can use the HFSPlusAttrExtents type to interpret the data. A record of type kHFSPlusAttrExtents is really just overflow extents for a corresponding record of type kHFSPlusAttrForkData. (Think of kHFSPlusAttrForkData as being like a catalog record and kHFSPlusAttrExtents as being like an extents overflow record.)

The next two sections describe the fork data and extension attributes in detail.

Fork Data Attributes

A fork data attribute is defined by the HFSPlusAttrForkData data type.

struct HFSPlusAttrForkData {
    UInt32          recordType;
    UInt32          reserved;
    HFSPlusForkData theFork;
};
typedef struct HFSPlusAttrForkData HFSPlusAttrForkData;

The fields have the following meaning:

recordType

The attribute data record type. For fork data attributes, this is always kHFSPlusAttrForkData.

reserved

An implementation must treat this as a reserved field.

theFork

Information about the location and size of the attribute data. See Fork Data Structure for a description of the HFSPlusForkData type.

Extension Attributes

A extension attribute is defined by the HFSPlusAttrExtents data type.

struct HFSPlusAttrExtents {
    UInt32                  recordType;
    UInt32                  reserved;
    HFSPlusExtentRecord     extents;
};
typedef struct HFSPlusAttrExtents HFSPlusAttrExtents;

The fields have the following meaning:

recordType

The attribute data record type. For extension attributes, this is always kHFSPlusAttrExtents.

reserved

An implementation must treat this as a reserved field.

extents

The eight extents of the attribute data described by this record. See Fork Data Structure for a description of the HFSPlusExtentRecord type.

Attributes and the Allocation File Consistency Check

While the key structure for the attributes file is not fully specified, it is still possible for an implementation to use attribute file information in its allocation file consistency check. The leaf records of the attribute file are fully defined, so the implementation can simply iterate over them to determine which allocation blocks on the disk are being used by fork data attributes.

See Allocation File Consistency Check for details.

Startup File

The startup file is a special file intended to hold information needed when booting a system that does not have built-in (ROM) support for HFS Plus. A boot loader can find the startup file without full knowledge of the HFS Plus volume format (B-trees, catalog file, and so on). Instead, the volume header contains the location of the first eight extents of the startup file.

IMPORTANT:
It is legal for the startup file to contain more than eight extents, and for the remaining extents to be placed in the extents overflow file. However, doing so defeats the purpose of the startup file.

Note:
Mac OS does not use the startup file to boot from HFS Plus disks. Instead, it uses the HFS wrapper, as described later in this document.

Unicode Subtleties

HFS Plus makes heavy use of Unicode strings to store file and folder names. However, Unicode is still evolving, and its use within a file system presents a number of challenges. This section describes some of the challenges, along with the solutions used by HFS Plus.

IMPORTANT:
Before reading this section, you should read HFS Plus Names.

IMPORTANT:
An implementation must not use the Unicode utilities implemented by its native platform (for decomposition and comparison), unless those algorithms are equivalent to the HFS Plus algorithms defined here, and are guaranteed to be so forever. This is rarely the case. Platform algorithms tend to evolve with the Unicode standard. The HFS Plus algorithms cannot evolve because such evolution would invalidate existing HFS Plus volumes.

Note:
The Mac OS Text Encoding Converter provides several constants that let you convert to and from the canonical, decomposed form stored on HFS Plus volumes. When using CreateTextEncoding to create a text encoding, you should set the TextEncodingBase to kTextEncodingUnicodeV2_0, set the TextEncodingVariant to kUnicodeCanonicalDecompVariant, and set the TextEncodingFormat to kUnicode16BitFormat. Using these values ensures that the Unicode will be in the same form as on an HFS Plus volume, even as the Unicode standard evolves.

Unicode allows some sequences of characters to be represented by multiple, equivalent forms. For example, the character "" can be represented as the single Unicode character u+00E9 (latin small letter e with acute), or as the two Unicode characters u+0065 and u+0301 (the letter "e" plus a combining acute symbol).

To reduce complexity in the B-tree key comparison routines (which have to compare Unicode strings), HFS Plus defines that Unicode strings will be stored in fully decomposed form, with composing characters stored in canonical order. The other equivalent forms are illegal in HFS Plus strings. An implementation must convert these equivalent forms to the fully decomposed form before storing the string on disk.

The Unicode Decomposition table contains a list of characters that are illegal as part of an HFS Plus string, and the equivalent character(s) that should be used instead. Any character appearing in a column titled "Illegal", must be replaced by the character(s) in the column immediately to the right (titled "Replace With").

In addition, the Korean Hangul characters with codes in the range u+AC00 through u+D7A3 are illegal and must be replaced with the equivalent sequence of conjoining jamos, as described in the Unicode 2.0 book, section 3.10.

So, for the example given earlier, "" must be stored as the two Unicode characters u+0065 and u+0301 (in that order). The Unicode character u+00E9 may not appear in a Unicode string used as part of an HFS Plus B-tree key.

String Comparison Algorithm

In HFS Plus, strings must be compared in a case- insensitive fashion. The Unicode standard does not strictly define upper and lower case equivalence, although it does suggest some equivalences. The HFS Plus string comparison algorithm (defined below) include a concrete case equivalence definition. An implementation must use the equivalence expressed by this algorithm.

Furthermore, Unicode requires that certain formatting characters be ignored (skipped over) during string comparisons. The algorithm and tables used for case equivalence also arrange to ignore these characters. An implementations must ignore the characters that are ignored by this algorithm.

The HFS Plus string comparison algorithm is defined by the FastUnicodeCompare routine, shown below. This routine returns a value that tells the caller how the strings should be ordered relative to each other: whether the first string is less than, equal to, or greater than the second string. An HFS Plus implementation may use this routine directly, or use another routine that produces the same relative ordering.

Note:
The FastUnicodeCompare routine does not handle composed Unicode characters since they are illegal in HFS Plus strings. As described in Canonical Decomposition, all HFS Plus strings must be fully decomposed, with composing characters in canonical order.

//
//  FastUnicodeCompare - Compare two Unicode strings; produce a relative ordering
//
//      IF              RESULT
//  --------------------------
//  str1 < str2     =>  -1
//  str1 = str2     =>   0
//  str1 > str2     =>  +1
//
//  The lower case table starts with 256 entries (one for each of the upper bytes
//  of the original Unicode char).  If that entry is zero, then all characters with
//  that upper byte are already case folded.  If the entry is non-zero, then it is
//  the _index_ (not byte offset) of the start of the sub-table for the characters
//  with that upper byte.  All ignorable characters are folded to the value zero.
//
//  In pseudocode:
//
//      Let c = source Unicode character
//      Let table[] = lower case table 
//
//      lower = table[highbyte(c)]
//      if (lower == 0)
//          lower = c
//      else
//          lower = table[lower+lowbyte(c)]
//
//      if (lower == 0)
//          ignore this character
//
//  To handle ignorable characters, we now need a loop to find the next valid
//  character.  Also, we can't pre-compute the number of characters to compare;
//  the string length might be larger than the number of non-ignorable characters.
//  Further, we must be able to handle ignorable characters at any point in the
//  string, including as the first or last characters.  We use a zero value as a
//  sentinel to detect both end-of-string and ignorable characters.  Since the File
//  Manager doesn't prevent the NULL character (value zero) as part of a filename,
//  the case mapping table is assumed to map u+0000 to some non-zero value (like
//  0xFFFF, which is an invalid Unicode character).
//
//  Pseudocode:
//
//      while (1) {
//          c1 = GetNextValidChar(str1)         //  returns zero if at end of string
//          c2 = GetNextValidChar(str2)
//
//          if (c1 != c2) break                 //  found a difference
//
//          if (c1 == 0)                        //  reached end of string on both
//                                              //  strings at once?
//              return 0;                       //  yes, so strings are equal
//      }
//
//      // When we get here, c1 != c2.  So, we just need to determine which one is
//      // less.
//      if (c1 < c2)
//          return -1;
//      else
//          return 1;
//
 
SInt32 FastUnicodeCompare ( register ConstUniCharArrayPtr str1, register ItemCount length1,
                            register ConstUniCharArrayPtr str2, register ItemCount length2)
{
    register UInt16     c1,c2;
    register UInt16     temp;
    register UInt16*    lowerCaseTable;
 
    lowerCaseTable = gLowerCaseTable;
 
    while (1) {
        //  Set default values for c1, c2 in case there are no more valid chars
        c1 = 0;
        c2 = 0;
        //  Find next non-ignorable char from str1, or zero if no more
        while (length1 && c1 == 0) {
            c1 = *(str1++);
            --length1;
            if ((temp = lowerCaseTable[c1>>8]) != 0)        //  is there a subtable
                                                            //  for this upper byte?
                c1 = lowerCaseTable[temp + (c1 & 0x00FF)];  //  yes, so fold the char
        }
        //  Find next non-ignorable char from str2, or zero if no more
        while (length2 && c2 == 0) {
            c2 = *(str2++);
            --length2;
            if ((temp = lowerCaseTable[c2>>8]) != 0)        //  is there a subtable
                                                            //  for this upper byte?
                c2 = lowerCaseTable[temp + (c2 & 0x00FF)];  //  yes, so fold the char
        }
        if (c1 != c2)       //  found a difference, so stop looping
            break;
        if (c1 == 0)        //  did we reach the end of both strings at the same time?
            return 0;       //  yes, so strings are equal
    }
    if (c1 < c2)
        return -1;
    else
        return 1;
}
 
 
/*  The lower case table consists of a 256-entry high-byte table followed by
    some number of 256-entry subtables. The high-byte table contains either an
    offset to the subtable for characters with that high byte or zero, which
    means that there are no case mappings or ignored characters in that block.
    Ignored characters are mapped to zero.
 */
 
UInt16 gLowerCaseTable[] = {
 
    // High-byte indices ( == 0 if no case mapping and no ignorables )
 
    // Full data tables omitted for brevity.
    // See the Downloadables section for URL to download the code.
};
 

HFS Wrapper

An HFS Plus volume may be contained within an HFS volume in a way that makes the volume look like an HFS volume to systems without HFS Plus support. This has a two important advantages:

1. It allows a computer with HFS (but no HFS Plus) support in ROM to start up from an HFS Plus volume. When creating the wrapper, Mac OS includes a System file containing the minimum code to locate and mount the embedded HFS Plus volume and continue booting from its System file.
2. It improves the user experience when an HFS Plus volume is inserted in a computer that has HFS support but no HFS Plus support. On such a computer, the HFS wrapper will be mounted as a volume, which prevents error dialogs that might confuse the user into thinking the volume is empty, damaged, or unreadable. The HFS wrapper may also contain a Read Me document to explain the steps the user should take to access their files.

The rest of this section describes how the HFS wrapper is laid out and how the HFS Plus volume is embedded within the wrapper.

IMPORTANT:
This section does not describe the HFS Plus volume format; instead, it describes additions to the HFS volume format that allow an HFS Plus volume (or some other volume) to be embedded in an HFS volume. However, as all Mac OS volumes are formatted with an HFS wrapper, all implementations should be able to parse the wrapper to find the embedded HFS Plus volume.

Note:
An HFS Plus volume is not required to have an HFS wrapper. In that case, the volume will start at the first sector of the disk, and the volume header will be at sector 2. However, Apple software currently initializes all HFS Plus volumes with an HFS wrapper.

HFS Master Directory Block

An HFS volume always contains a Master Directory Block (MDB), in sector 2. The MDB is similar to an HFS Plus volume header. In order to support volumes embedded within an HFS volume, several unused fields of the MDB have been changed, and are now used to indicate the type, location, and size of the embedded volume.

What was formerly the drVCSize field (at offset $7C) is now named drEmbedSigWord. This two-byte field contains a unique value that identifies the type of embedded volume. When an HFS Plus volume is embedded, drEmbedSigWord must be kHFSPlusSigWord ('H+'), the same value stored in the signature field of an HFS Plus volume header.

What were formerly the drVBMCSize and drCtlCSize fields (at offset $7E) have been combined into a single field occupying four bytes. The new structure is named drEmbedExtent and is of type HFSExtentDescriptor. It contains the starting allocation block number (startBlock) where the embedded volume begins and number of allocation blocks (blockCount ) the embedded volume occupies. The embedded volume must be contiguous. Both of these values are in terms of the HFS wrapper's allocation blocks, not HFS Plus allocation blocks.

Note:
The description of the HFS volume format in Inside Macintosh: Files describes these fields as being used to store the size of various caches, and labels each one as "used internally".

To actually find the embedded volume's sectors on disk, an implementation must use the drAlBlkSiz and drAlBlSt fields of the MDB. The drAlBlkSiz field contains the size (in bytes) of the HFS allocation blocks. The drAlBlSt field contains the offset, in 512-byte sectors, of the wrapper's allocation block 0 relative to the start of the volume.

IMPORTANT:
This embedding introduces a transform between HFS Plus sectors and sectors on the disk. The HFS Plus volume exists on a virtual disk embedded within the real disk. When accessing an HFS Plus sector on an embedded disk, an implementation must add the offset of the embedded disk to the sector number. Listing 2 shows how one might do this.

Listing 2 Sector transform for embedded volumes

static UInt32 HFSPlusSectorToDiskSector(UInt32 hfsPlusSector)
{
    UInt32 embeddedDiskOffset;
    
    embeddedDiskOffset = gMDB.drAlBlSt +
                         gMDB.drEmbedExtent.startBlock * (drAlBlkSiz / 512)
    return embeddedDiskOffset + hfsPlusSector;
}

In order to prevent accidentally changing the files in the HFS wrapper, the wrapper volume must be marked as software-write-protected by setting kHFSVolumeSoftwareLockBit in the drAtrb (volume attributes) field of the MDB. All correct HFS implementations will prevent any changes to the wrapper volume.

To improve performance of HFS Plus volumes, the size of the wrapper's allocation blocks should be a multiple of the size of the HFS Plus volume's allocation blocks. In addition, the wrapper's allocation block start (drAlBlSt) should be a multiple of the HFS Plus volume's allocation block size (or perhaps 4 KB, if the HFS Plus allocation blocks are larger). If these recommendations are followed, the HFS Plus allocation blocks will be properly aligned on the disk. And, if the HFS Plus allocation block size is a multiple of the size of the device's physical blocks, then blocking and deblocking at the device driver level will be minimized.

Allocating Space for the Embedded Volume

The space occupied by the embedded volume must be marked as allocated in the HFS wrapper's volume bitmap (similar to the HFS Plus allocation file) and placed in the HFS wrapper's bad block file (similar to the HFS Plus bad block file). This doesn't mean the blocks are actually bad; it merely prevents the HFS Plus volume from being overwritten by newly created files in the HFS wrapper, being deleted accidentally, or being marked as free, usable space by HFS disk repair utilities.

The kHFSVolumeSparedBlocksMask bit of the drAtrb (volume attributes) field of the MDB must be set to indicate that the volume has a bad blocks file.

IMPORTANT:
This section is not part of the HFS Plus volume format. It describes how the existing Mac OS implementation of HFS Plus creates HFS wrappers. It is provided for your information only.

As initialized by the Mac OS Disk Initialization Package, the HFS wrapper volume contains five files in the root folder.

• Read Me -- The Read Me file, whose name is actually "Where_have_all_my_files_gone?", contains text explaining that this volume is really an HFS Plus volume but the contents cannot be accessed because HFS Plus is not currently installed on the computer. It also describes the steps needed to install HFS Plus support. Localized system software will also create a localized version of the file with localized filename and text content.
• System and Finder (invisible) -- The System file contains the minimum code to locate and mount the embedded HFS Plus volume, and to continue booting from the System file in the embedded volume. The Finder file is empty; it is there to prevent older versions of the Finder from de-blessing the wrapper's root directory, which would prevent booting from the volume.
• Desktop DB and Desktop DF (invisible) -- The Desktop DB and Desktop DF files are an artifact of the way the files on the wrapper volume are created.

In addition, the root folder is set as the blessed folder by placing its folder ID in the first SInt32 of the drFndrInfo (Finder information) field of the MDB.

Volume Consistency Checks

An HFS Plus volume is a complex data structure, consisting of many different inter-related data structures. Inconsistencies between these data structures could cause serious data loss. When an HFS Plus implementation mounts a volume, it must perform basic consistency checks to ensure that the volume is consistent. In addition, the implementation may choose to implement other, more advanced, consistency checks.

Many of these consistency checks take a significant amount of time to run. While a safe implementation might run these checks every time a volume is mounted, most implementations will want to rely on the correctness of the previous implementation that modified the disk. The implementation may avoid unnecessary checking by determining whether the volume was last unmounted cleanly. If it was, the implementation may choose to skip a consistency check.

An implementation can determine whether a volume was unmounted cleanly by looking at various flag bits in the volume header. See Volume Attributes for details.

Next Catalog Node ID Consistency Check

For an HFS Plus volume to work correctly, it's vital that the nextCatalogID field of the volume header be greater than all CNIDs currently used in the catalog file. The algorithm to ensure this is as follows.

• The implementation must iterate through all the leaf nodes of the catalog file, looking for file and folder records, determining the maximum CNID of any file or folder in the catalog.
• Once it knows the maximum CNID value, the implementation must set nextCatalogID to a value greater than it.

WARNING:
To prevent the loss of user data, an implementation must perform this check every time it mounts a volume that wasn't unmounted cleanly. It is most important that an allocation block that is in use be marked in the allocation file. It is less important that an allocation block that is not in use be cleared in the allocation file. If an allocation block is marked as in-use by the allocation file, but not actually in use by any extent, then that allocation block is really just wasting space; it isn't otherwise dangerous.

For an HFS Plus volume to work correctly, it's vital that any allocation block in use by file system structures be marked as allocated in the allocation file. The algorithm to ensure this is as follows:

• The implementation must first walk the allocation file, marking every allocation block as free. (This step can be skipped to improve the performance of the consistency check. All that will happen is that some allocation blocks may have been marked as in-use, though they are not really in use by any extent.)
• The implementation must then mark the allocation blocks containing the first three sectors and the last two sectors as allocated. These sectors are either reserved or used by the volume header.
• The implementation must then mark the allocation blocks used by all extents in all special files (the catalog file, the extents overflow file, the allocation file, the attributes file, and the startup file) as allocated. These extents are all described in the volume header.
• The implementation must then walk the leaf nodes of the catalog file, marking all allocation blocks used by extents in file records (i.e., in the HFSPlusForkData structures for the data and resource forks).
• The implementation must then walk the leaf nodes of the extents overflow file, marking all allocation blocks used by all extents in all extent records as allocated.
• The implementation must then walk the leaf nodes of the attributes file, marking all allocation blocks used by all extents described in fork data attributes and extension attributes as allocated.

WARNING:
To prevent the loss of user data, an implementation must perform this check every time it mounts a volume that wasn't unmounted cleanly.

Summary

Volume format specifications are fun.

• Inside Macintosh: Files, especially the Data Organization on Volumes section.
• Algorithms in C, Robert Sedgewick, Addison-Wesley, 1992, especially the section on B-trees.

Acrobat version of this Note (147K).

FastUnicodeCompare.c (43 KB)

• First published on February 1999.
• Updated in March 1999 to include a warning about initializing disks with a small allocation block size.
• Updated in January 2000 to clarify the allocation block usage and placement of the alternate volume header on volumes where the disk size is not an even multiple of the allocation block size.