Pack Info

This document specifies the PackInfo structure that describes compressed data streams in a 7z archive.

Overview

PackInfo defines:

Where compressed data starts in the archive
How many compressed streams exist
The size of each compressed stream
Optional CRC checksums for each stream

Structure

PackInfo ::= 0x06 PackPos NumPackStreams [Sizes] [Digests] 0x00

PackPos ::= NUMBER
NumPackStreams ::= NUMBER
Sizes ::= 0x09 PackSize* 0x00
Digests ::= 0x0A DigestsData
PackSize ::= NUMBER

Optional section semantics: When an optional section (e.g., [Sizes]) is absent, the entire structure including its property ID is omitted from the byte stream. For example, when Sizes is absent, the 0x09 byte does not appear. The parser proceeds directly from NumPackStreams to either Digests (if present) or the terminating 0x00.

Fields

Property ID

Value: 0x06

Identifies the start of PackInfo section.

PackPos

Type: NUMBER Description: Offset from end of signature header (byte 32) to the first pack stream.

The absolute file position of pack data is:

pack_data_start = 32 + PackPos

Typical value: 0 (pack data immediately follows signature header)

Constraints:

MUST be valid offset within file
32 + PackPos + sum(PackSizes) MUST NOT exceed NextHeaderOffset + 32

NumPackStreams

Type: NUMBER Description: Number of packed (compressed) streams.

Range: 0 to implementation limit (typically 2^31)

Value of 0: Valid for archives with no compressed data (only empty files/directories)

Sizes Section

Property ID: 0x09

Contains the size of each pack stream.

Sizes ::= 0x09 PackSize[NumPackStreams] 0x00

Presence:

MUST be present if NumPackStreams > 0
MUST be absent if NumPackStreams == 0

PackSize: Size in bytes of the corresponding pack stream.

Digests Section

Property ID: 0x0A

Contains optional CRC-32 checksums for pack streams. This follows the BooleanList pattern defined in 04-DATA-ENCODING.

Digests ::= 0x0A AllAreDefined [BitField] CRC[NumDefined]

AllAreDefined ::= BYTE  # 0x00 or 0x01
BitField ::= # Present only if AllAreDefined == 0x00
CRC ::= UINT32

Presence: Optional. If absent, no pack stream CRCs are available.

AllAreDefined:

0x01: All streams have CRCs, BitField absent
0x00: BitField indicates which streams have CRCs

CRCs: Only present for streams where defined. Count = number of true values in the defined bitmap.

Complete Example

Archive with 3 pack streams:

06                  # PackInfo property ID
00                  # PackPos = 0
03                  # NumPackStreams = 3

09                  # Sizes property ID
  80 00 01          # PackSize[0] = 256 (NUMBER encoding)
  80 00 02          # PackSize[1] = 512
  80 00 04          # PackSize[2] = 1024
00                  # End of Sizes

0A                  # Digests property ID
  01                # AllAreDefined = true
  78 56 34 12       # CRC[0] = 0x12345678
  AB CD EF 01       # CRC[1] = 0x01EFCDAB
  11 22 33 44       # CRC[2] = 0x44332211

00                  # End of PackInfo

Partial Digests Example

When only some streams have CRCs:

0A                  # Digests property ID
  00                # AllAreDefined = false
  A0                # BitField = 0xA0: bit 7 (item 0) = 1, bit 5 (item 2) = 1
  78 56 34 12       # CRC for stream 0
  11 22 33 44       # CRC for stream 2
                    # (no CRC for stream 1)

See 04-DATA-ENCODING for BitField encoding details.

Pack Stream Layout

Pack streams are stored contiguously in the archive:

Offset 32 + PackPos:
┌─────────────────────┐
│   Pack Stream 0     │  PackSize[0] bytes
├─────────────────────┤
│   Pack Stream 1     │  PackSize[1] bytes
├─────────────────────┤
│        ...          │
├─────────────────────┤
│ Pack Stream N-1     │  PackSize[N-1] bytes
└─────────────────────┘

Relationship to Folders

Pack streams are consumed by folders (see 07-UNPACK-INFO):

Each folder references one or more pack streams
Simple coders (LZMA, LZMA2) use 1 pack stream per folder
Complex coders (BCJ2) may use multiple pack streams per folder
The mapping is defined in UnpackInfo

Empty PackInfo

For archives with no compressed data:

06          # PackInfo
00          # PackPos = 0
00          # NumPackStreams = 0
00          # End (no Sizes or Digests)

Zero-Length Pack Streams

A pack stream MAY have a size of zero (PackSize = 0). This is valid but unusual.

When zero-length streams occur:

A coder that outputs no data (e.g., compressing zero-length input)
Placeholder streams in complex coder configurations

Behavior:

Zero-length pack streams consume no bytes in the pack data region
The stream still exists in the pack stream index
Reading a zero-length stream returns an empty byte sequence
CRCs for zero-length streams, if present, MUST be 0x00000000 (the CRC-32 of an empty byte sequence; see C-CRC-ALGORITHM "Empty Data" section)

Validation

Implementations MUST verify:

Pack position valid: 32 + PackPos is within file
Sizes count matches: Number of PackSize values equals NumPackStreams
Total size valid: 32 + PackPos + sum(PackSizes) <= NextHeaderOffset + 32
CRC count valid: Number of CRCs equals number of defined streams
No overlaps: Pack data region doesn't overlap header region

Implementation Notes

Reading Pack Data

To read pack stream i:

offset = 32 + PackPos + sum(PackSize[0..i])
size = PackSize[i]
data = read_bytes_at(offset, size)

Writing Pack Data

When writing:

Set PackPos = 0 (data starts right after signature header)
Write each folder's compressed output sequentially
Record sizes as you write
Calculate CRCs during compression

Memory Efficiency

For archives with many streams:

Store offsets lazily (calculate from sizes on demand)
Consider memory-mapped access for large pack data regions

Pack Info ​

Overview ​

Structure ​

Fields ​

Property ID ​

PackPos ​

NumPackStreams ​

Sizes Section ​

Digests Section ​

Complete Example ​

Partial Digests Example ​

Pack Stream Layout ​

Relationship to Folders ​

Empty PackInfo ​

Zero-Length Pack Streams ​

Validation ​

Implementation Notes ​

Reading Pack Data ​

Writing Pack Data ​

Memory Efficiency ​

See Also ​