Technical Note TN1150

HFS Plus Volume Format

CONTENTS HFS Plus Basics Core Concepts Volume Header B-Trees Catalog File Extents Overflow File Allocation File Attributes File Startup File Hard Links Symbolic Links Journal HFSX Metadata Zone Hot Files Unicode Subtleties HFS Wrapper Volume Consistency Checks Summary Downloadables

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. [Mar 05, 2004]

HFS Plus Basics

HFS Plus is a volume format for Mac OS. HFS Plus was introduced with Mac OS 8.1. HFS Plus is architecturally very similar to 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. Mac OS versions less than 9.0 do 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 (for example, custom icons for files), the data or resource fork is appropriate. But in other cases (for example, 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.

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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 storage space on a disk is divided into units called sectors. A sector is the smallest part of a disk that the disk's driver software will read or write in a single operation (without having to read or write additional data before or after the requested data). The size of a sector is usually based on the way the data is physically laid out on the disk. For hard disks, sectors are typically 512 bytes. For optical media, sectors are typically 2048 bytes.

Most of the data structures on an HFS Plus volume do not depend on the size of a sector, with the exception of the journal. Because the journal does rely on accessing individual sectors, the sector size is stored in the jhdr_size field of the journal header (if the volume has a journal).

HFS Plus allocates space in units called allocation blocks; an allocation block is simply a group of consecutive bytes. 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. Current implementations of the file system are 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.

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 1024 bytes from the start of the volume.

A copy of the volume header, known as the alternate volume header, is stored starting at 1024 bytes before the end of the volume. The first 1024 bytes of volume (before the volume header), and the last 512 bytes of the volume (after the alternate volume header) are reserved. All of the allocation blocks containing the volume header, alternate volume header, or the reserved areas before the volume header or after the alternate volume header, are marked as used in the allocation file. The actual number of allocation blocks marked this way depends 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 (1024 bytes from the start of the volume). 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 space on the media into 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. Bits are numbered from 0 to n-1 (for types UIntn and SIntn), with bit 0 being the least significant bit.

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 must be ignored by this comparison. For more details on these subtleties, see Unicode Subtleties.

A variant of HFS Plus, called HFSX, allows volumes whose names are compared in a case-sensitive fashion. The names are fully decomposed and in canonical order, but no Unicode characters are ignored during the comparison.

Text Encodings

Traditional 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.

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 HFSPlusBSDInfo structure.

struct HFSPlusBSDInfo {
    UInt32  ownerID;
    UInt32  groupID;
    UInt8   adminFlags;
    UInt8   ownerFlags;
    UInt16  fileMode;
    union {
        UInt32  iNodeNum;
        UInt32  linkCount;
        UInt32  rawDevice;
    } special;
};
typedef struct HFSPlusBSDInfo HFSPlusBSDInfo;

The fields have the following meaning:

ownerID

The Mac OS X user ID of the owner of the file or folder. Mac OS X versions prior to 10.3 treats user ID 99 as if it was the user ID of the user currently logged in to the console. If no user is logged in to the console, user ID 99 is treated as user ID 0 (root). Mac OS X version 10.3 treats user ID 99 as if it was the user ID of the process making the call (in effect, making it owned by everyone simultaneously). These substitutions happen at run time. The actual user ID on disk is not changed.

groupID

The Mac OS X group ID of the group associated with the file or folder. Mac OS X typically maps group ID 99 to the group named "unknown." There is no run time substitution of group IDs in Mac OS X.

adminFlags

BSD flags settable by the super-user only. This field corresponds to bits 16 through 23 of the st_flags field of struct stat in Mac OS X. See the manual page for chflags(2) for more information. The following table gives the bit position in the adminFlags field and the name of the corresponding mask used in the st_flags field.

Bit	st_flags mask	Meaning
0	SF_ARCHIVED	File has been archived
1	SF_IMMUTABLE	File may not be changed
2	SF_APPEND	Writes to file may only append

ownerFlags

BSD flags settable by the owner of the file or directory, or by the super-user. This field corresponds to bits 0 through 7 of the st_flags field of struct stat in Mac OS X. See the manual page for chflags(2) for more information. The following table gives the bit position in the ownerFlags field and the name of the corresponding mask used in the st_flags field.

Bit	st_flags mask	Meaning
0	UF_NODUMP	Do not dump (back up or archive) this file
1	UF_IMMUTABLE	File may not be changed
2	UF_APPEND	Writes to file may only append
3	UF_OPAQUE	Directory is opaque (see below)

fileMode

BSD file type and mode bits. Note that the constants from the header shown below are in octal (base eight), not hexadecimal.

#define S_ISUID 0004000 /* set user id on execution */ #define S_ISGID 0002000 /* set group id on execution */ #define S_ISTXT 0001000 /* sticky bit */ #define S_IRWXU 0000700 /* RWX mask for owner */ #define S_IRUSR 0000400 /* R for owner */ #define S_IWUSR 0000200 /* W for owner */ #define S_IXUSR 0000100 /* X for owner */ #define S_IRWXG 0000070 /* RWX mask for group */ #define S_IRGRP 0000040 /* R for group */ #define S_IWGRP 0000020 /* W for group */ #define S_IXGRP 0000010 /* X for group */ #define S_IRWXO 0000007 /* RWX mask for other */ #define S_IROTH 0000004 /* R for other */ #define S_IWOTH 0000002 /* W for other */ #define S_IXOTH 0000001 /* X for other */ #define S_IFMT 0170000 /* type of file mask */ #define S_IFIFO 0010000 /* named pipe (fifo) */ #define S_IFCHR 0020000 /* character special */ #define S_IFDIR 0040000 /* directory */ #define S_IFBLK 0060000 /* block special */ #define S_IFREG 0100000 /* regular */ #define S_IFLNK 0120000 /* symbolic link */ #define S_IFSOCK 0140000 /* socket */ #define S_IFWHT 0160000 /* whiteout */

In some versions of Unix, the sticky bit, S_ISTXT, is used to indicate that an executable file's code should remain in memory after the executable finishes; this can help performance if the same executable is used again soon. Mac OS X does not use this optimization. If the sticky bit is set for a directory, then Mac OS X restricts movement, deletion, and renaming of files in that directory. Files may be removed or renamed only if the user has write access to the directory; and is the owner of the file or the directory, or is the super-user.

special

This field is used only for certain special kinds of files. For directories, and most files, this field is unused and reserved. When used, this field is used as one of the following:

iNodeNum

For hard link files, this field contains the link reference number. See the Hard Links section for more information.

linkCount

For indirect node files, this field contains the number of hard links that point at this indirect node file. See the Hard Links section for more information.

rawDevice

For block and character special devices files (when the S_IFMT field contains S_IFCHR or S_IFBLK), this field contains the device number.

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

For HFSPlusForkData structures in the volume header, this is the fork's clump size, which is used in preference to the default clump size in the volume header.

For HFSPlusForkData structures in a catalog record, this field was intended to store a per-fork clump size to override the default clump size in the volume header. However, Apple implementations prior to Mac OS X version 10.3 ignored this field. As of Mac OS X version 10.3, this field is used to keep track of the number of blocks actually read from the fork. See the Hot Files section for more information.

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.

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Volume Header

Each HFS Plus volume contains a volume header 1024 bytes from the start of the volume. 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 starting 1024 bytes before the end of the volume. The implementation should only update this copy when the length or location of one of the special files changes. The alternate volume header is intended for use solely by disk repair utilities.

The first 1024 bytes and the last 512 bytes of the volume are reserved.

The allocation block (or blocks) containing the first 1536 bytes (reserved space plus volume header) are marked as used in the allocation file (see the Allocation File section). Also, in order to accommodate the alternate volume header and the reserved space following it, 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.

The volume header is described by the HFSPlusVolumeHeader type.

struct HFSPlusVolumeHeader {
    UInt16              signature;
    UInt16              version;
    UInt32              attributes;
    UInt32              lastMountedVersion;
    UInt32              journalInfoBlock;
 
    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;
 
    UInt32              finderInfo[8];
 
    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, or kHFSXSigWord ('HX') for an HFSX volume.

version

The version of the volume format, which is currently 4 (kHFSPlusVersion) for HFS Plus volumes, or 5 (kHFSXVersion) for HFSX volumes.

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. Any code which modifies the on disk structures must also set this field to a unique value which identifies that code. Third-party implementations of HFS Plus should place a registered creator code in this field. The value used by Mac OS 8.1 to 9.2.2 is '8.10'. The value used by Mac OS X is '10.0'. The value used by a journaled volume (including HFSX) in Mac OS X is 'HFSJ'. The value used by fsck_hfs on Mac OS X is 'fsck'.

journalInfoBlock

The allocation block number of the allocation block which contains the JournalInfoBlock for this volume's journal. This field is valid only if bit kHFSVolumeJournaledBit is set in the attribute field; otherwise, this field is reserved.

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 areas 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 space at the end of the disk is not used 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: traditional Mac OS implementations typically set 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. All Apple implementations to date ignore the rsrcClumpSize and use dataClumpSize for both data and resource forks.

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. All Apple implementations to date ignore the rsrcClumpSize and use dataClumpSize for both data and resource forks.

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 array of 32-bit items contains information used by the Mac OS Finder, and the system software boot process.

finderInfo[0] contains the directory ID of the directory containing the bootable system (for example, the System Folder in Mac OS 8 or 9, or /System/Library/CoreServices in Mac OS X). It is zero if there is no bootable system on the volume. This value is typically equal to either finderInfo[3] or finderInfo[5].

finderInfo[1] contains the parent directory ID of the startup application (for example, Finder), or zero if the volume is not bootable.

finderInfo[2] contains the directory ID of a directory whose window should be displayed in the Finder when the volume is mounted, or zero if no directory window should be opened. In traditional Mac OS, this is the first in a linked list of windows to open; the frOpenChain field of the directory's Finder Info contains the next directory ID in the list. The open window list is deprecated. The Mac OS X Finder will open this directory's window, but ignores the rest of the open window list. The Mac OS X Finder does not modify this field.

finderInfo[3] contains the directory ID of a bootable Mac OS 8 or 9 System Folder, or zero if there isn't one.

finderInfo[4] is reserved.

finderInfo[5] contains the directory ID of a bootable Mac OS X system (the /System/Library/CoreServices directory), or zero if there is no bootable Mac OS X system on the volume.

finderInfo[6] and finderInfo[7] are used by Mac OS X to contain a 64-bit unique volume identifier. One use of this identifier is for tracking whether a given volume's ownership (user ID) information should be honored. These elements may be zero if no such identifier has been created for the volume.

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.

enum {
    /* Bits 0-6 are reserved */
    kHFSVolumeHardwareLockBit       =  7,
    kHFSVolumeUnmountedBit          =  8,
    kHFSVolumeSparedBlocksBit       =  9,
    kHFSVolumeNoCacheRequiredBit    = 10,
    kHFSBootVolumeInconsistentBit   = 11,
    kHFSCatalogNodeIDsReusedBit     = 12,
    kHFSVolumeJournaledBit          = 13,
    /* Bit 14 is reserved */
    kHFSVolumeSoftwareLockBit       = 15
    /* Bits 16-31 are reserved */
};

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.

kHFSCatalogNodeIDsReusedBit (bit 12)

This bit is set when the nextCatalogID field overflows 32 bits, forcing smaller catalog node IDs to be reused. When this bit is set, it is common (and not an error) for catalog records to exist with IDs greater than or equal to nextCatalogID. If this bit is set, you must ensure that IDs assigned to newly created catalog records do not conflict with IDs used by existing records.

kHFSVolumeJournaledBit (bit 13)

If this bit is set, the volume has a journal, which can be located using the journalInfoBlock field of the Volume Header.

bit 14

An implementation must treat this bit as reserved.

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.

bits 16-31

An implementation must treat these bits as reserved.

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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 record is the user data record and is always 128 bytes long. The last record is the B-tree map record; it occupies all of the remaining space between the user data 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     keyCompareType;
    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

The value stored in this field is of type BTreeTypes:

enum BTreeTypes{ kHFSBTreeType = 0, // control file kUserBTreeType = 128, // user btree type starts from 128 kReservedBTreeType = 255 };

This field must be equal to kHFSBTreeType for the catalog, extents, and attributes B-trees. This field must be equal to kUserBTreeType for the hot file B-tree. Historically, values of 1 to 127 and kReservedBTreeType were used in B-trees used by system software in Mac OS 9 and earlier.

keyCompareType

For HFSX volumes, this field in the catalog B-tree header defines the ordering of the keys (whether the volume is case-sensitive or case-insensitive). In all other cases, an implementation must treat this as a reserved field.

Constant name	Value	Meaning
kHFSCaseFolding	0xCF	Case folding (case-insensitive)
kHFSBinaryCompare	0xBC	Binary compare (case-sensitive)

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.

User Data Record

The second record in a header node is always 128 bytes long. It provides a small space to store information associated with a B-tree.

In the HFS Plus catalog, extents, and attributes B-trees, this record is unused and reserved. In the HFS Plus hot file B-tree, this record contains general information about the hot file recording process.

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 (8KB in Mac OS X) for the catalog file
• 1 KB (4KB in Mac OS X) 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.

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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 defin