Compound File Binary Format - AbsoluteAstronomy.com

Compound File Binary Format (CFBF), also called Compound File or Compound Document, is a file format

File format

A file format is a particular way that information is encoded for storage in a computer file.Since a disk drive, or indeed any computer storage, can store only bits, the computer must have some way of converting information to 0s and 1s and vice-versa. There are different kinds of formats for...

for storing numerous files and streams within a single file on a disk. CFBF is developed by Microsoft

Microsoft

Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

and is an implementation of Microsoft COM Structured Storage.

Microsoft has opened the format for use by others and it is now used in a variety of programs from Microsoft Word

Microsoft Word

Microsoft Word is a word processor designed by Microsoft. It was first released in 1983 under the name Multi-Tool Word for Xenix systems. Subsequent versions were later written for several other platforms including IBM PCs running DOS , the Apple Macintosh , the AT&T Unix PC , Atari ST , SCO UNIX,...

and Microsoft Access

Microsoft Access

Microsoft Office Access, previously known as Microsoft Access, is a relational database management system from Microsoft that combines the relational Microsoft Jet Database Engine with a graphical user interface and software-development tools. It is a member of the Microsoft Office suite of...

to Business Objects. It also forms the basis of the Advanced Authoring Format

Advanced Authoring Format

The Advanced Authoring Format is a professional file interchange format designed for the video post production and authoring environment. The AAF was created by the Advanced Media Workflow Association . The AMWA develops specifications and technologies to facilitate the deployment and operation...

Overview

At its simplest, the Compound File Binary Format is a container, with little restriction on what can be stored within it.

A CFBF file structure loosely resembles a FAT

File Allocation Table

File Allocation Table is a computer file system architecture now widely used on many computer systems and most memory cards, such as those used with digital cameras. FAT file systems are commonly found on floppy disks, flash memory cards, digital cameras, and many other portable devices because of...

filesystem. The file is partitioned into Sectors which are chained together with a File Allocation Table (not to be mistaken with the file system of the same name) which contains chains of sectors related to each file, a Directory holds information for contained files with a Sector ID (SID) for the starting sector of a chain and so on.

Structure

The CFBF file consists of a 512-Byte header record followed by a number of sectors whose size is defined in the header. The literature defines Sectors to be either 512 or 4096 bytes in length, although the format is potentially capable of supporting sectors ranging in size from 128-Bytes upwards in powers of 2 (128, 256, 512, 1024, etc.). The lower limit of 128 is the minimum required to fit a single directory entry in
a Directory Sector.

There are several types of sector that may be present in a CFBF:

File Allocation Table (FAT) Sector - contains chains of sector indices much as a FAT does in the FAT/FAT32 filesystems
MiniFAT Sectors - similar to the FAT but storing chains of mini-sectors within the Mini-Stream
Double-Indirect FAT (DIFAT) Sector - contains chains of FAT sector indices
Directory Sector - contains directory entries
Stream Sector - contains arbitrary file data
Range Lock Sector - contains the byte-range locking area of a large file

More detail is given below for the header and each sector type.

CFBF Header format

The CFBF Header occupies the first 512 bytes of the file and information required to interpret the rest of the file. The C-Style structure declaration below (extracted from the AAFA's Low-Level Container Specification) shows the members of the CFBF header and their purpose:

typedef unsigned long ULONG; // 4 Bytes
typedef unsigned short USHORT; // 2 Bytes
typedef short OFFSET; // 2 Bytes
typedef ULONG SECT; // 4 Bytes
typedef ULONG FSINDEX; // 4 Bytes
typedef USHORT FSOFFSET; // 2 Bytes
typedef USHORT WCHAR; // 2 Bytes
typedef ULONG DFSIGNATURE; // 4 Bytes
typedef unsigned char BYTE; // 1 Byte
typedef unsigned short WORD; // 2 Bytes
typedef unsigned long DWORD; // 4 Bytes
typedef ULONG SID; // 4 Bytes
typedef GUID CLSID; // 16 Bytes

struct StructuredStorageHeader { // [offset from start (bytes), length (bytes)]
BYTE _abSig[8]; // [00H,08] {0xd0, 0xcf, 0x11, 0xe0, 0xa1, 0xb1,
// 0x1a, 0xe1} for current version
CLSID _clsid; // [08H,16] reserved must be zero (WriteClassStg/
// GetClassFile uses root directory class id)
USHORT _uMinorVersion; // [18H,02] minor version of the format: 33 is
// written by reference implementation
USHORT _uDllVersion; // [1AH,02] major version of the dll/format: 3 for
// 512-byte sectors, 4 for 4 KB sectors
USHORT _uByteOrder; // [1CH,02] 0xFFFE: indicates Intel byte-ordering
USHORT _uSectorShift; // [1EH,02] size of sectors in power-of-two;
// typically 9 indicating 512-byte sectors
USHORT _uMiniSectorShift; // [20H,02] size of mini-sectors in power-of-two;
// typically 6 indicating 64-byte mini-sectors
USHORT _usReserved; // [22H,02] reserved, must be zero
ULONG _ulReserved1; // [24H,04] reserved, must be zero
FSINDEX _csectDir; // [28H,04] must be zero for 512-byte sectors,
// number of SECTs in directory chain for 4 KB
// sectors
FSINDEX _csectFat; // [2CH,04] number of SECTs in the FAT chain
SECT _sectDirStart; // [30H,04] first SECT in the directory chain
DFSIGNATURE _signature; // [34H,04] signature used for transactions; must
// be zero. The reference implementation
// does not support transactions
ULONG _ulMiniSectorCutoff; // [38H,04] maximum size for a mini stream;
// typically 4096 bytes
SECT _sectMiniFatStart; // [3CH,04] first SECT in the MiniFAT chain
FSINDEX _csectMiniFat; // [40H,04] number of SECTs in the MiniFAT chain
SECT _sectDifStart; // [44H,04] first SECT in the DIFAT chain
FSINDEX _csectDif; // [48H,04] number of SECTs in the DIFAT chain
SECT _sectFat[109]; // [4CH,436] the SECTs of first 109 FAT sectors
};

File Allocation Table (FAT) Sectors

When taken together as a single stream the collection of FAT sectors define the status and linkage of every sector in the file. Each entry in the FAT is 4 bytes in length and contains the sector number of the next sector in a FAT chain or one of the following special values:

FREESECT (0xFFFFFFFF) - denotes an unused sector
ENDOFCHAIN (0xFFFFFFFE) - marks the last sector in a FAT chain
FATSECT (0xFFFFFFFD) - marks a sector used to store part of the FAT
DIFSECT (0xFFFFFFFC) - marks a sector used to store part of the DIFAT

Range Lock Sector

The Range Lock Sector must exist in files greater than 2GB in size, and must not exist in files smaller than 2GB. The Range Lock Sector must contain the byte range 0x7FFFFF00 to 0x7FFFFFFF in the file. This area is reserved by Microsoft's COM implementation for storing byte-range locking information for concurrent access.

Glossary