Appendix D: Compatibility

This appendix documents compatibility considerations with 7-Zip and other implementations.

7-Zip Versions

Version History

Version	Release	Notable Changes
9.20	2010	Stable baseline
15.06	2015	ARM64 filter
18.01	2018	LZMA2 improvements
19.00	2019	Encryption improvements
21.00	2021	Performance improvements
22.00	2022	Bug fixes
23.00	2023	RISC-V filter
24.00	2024	Current stable

Baseline Compatibility

For maximum compatibility, target 7-Zip 9.20 features:

• LZMA, LZMA2 compression
• BCJ filters (x86, ARM, PPC, IA64, SPARC)
• Delta filter
• AES-256 encryption
• Solid and non-solid modes

Implementation Differences

p7zip

Fork of 7-Zip for Unix/Linux:

• Same format support as 7-Zip
• May lag behind in new features
• Development less active since 2016

7-Zip-zstd

Fork with additional compression methods:

• Zstandard (zstd)
• Brotli
• LZ4, LZ5, Lizard

Note: Archives using these methods require compatible tools.

py7zr

Python implementation:

• No BCJ2 support
• No Deflate64 support
• UTF-8 symlink targets

libarchive

Multi-format library:

• Zstandard support
• Some edge cases handled differently

Known Quirks

File Name Encoding

7-Zip always uses UTF-16-LE for file names. Some implementations:

• May produce invalid UTF-16 for certain characters
• Handle surrogate pairs differently
• Normalize paths differently

Recommendation: Validate UTF-16 when reading, produce valid UTF-16 when writing.

Timestamp Precision

7-Zip stores full FILETIME precision (100ns). Some implementations:

• Truncate to second precision
• Lose sub-second information
• Use different epoch conversions

Recommendation: Preserve full precision when possible.

Unix Attributes

The UNIX_EXTENSION flag (bit 15) usage varies:

• Some tools always set it on Unix
• Some only when permissions differ from default
• Windows 7-Zip ignores it

Recommendation: Set UNIX_EXTENSION when storing Unix-specific attributes.

Empty Files vs Directories

Distinction relies on EmptyFile property:

• If EmptyStream but not EmptyFile → directory
• Some tools mark empty files incorrectly

Recommendation: Also check DIRECTORY attribute (0x10) for robustness.

Solid Block Sizes

Different tools use different solid block size defaults:

• 7-Zip GUI: Configurable
• Command line: Various defaults
• Other tools: Often "solid" or "non-solid" only

Recommendation: Document solid block size when relevant.

Interoperability Testing

Test Archive Sources

1. Create archives with 7-Zip
2. Create archives with p7zip
3. Download test archives from py7zr repository

Test Scenarios

Scenario	Test
Basic extraction	Extract 7-Zip created archive
Compression methods	Test all common methods
Solid archives	Extract from solid blocks
Encryption	Various passwords and settings
Unicode names	Non-ASCII characters
Long paths	Paths > 260 characters
Symbolic links	Unix and Windows styles
Empty files	Zero-byte files
Directories	Various attributes

Validation Checklist

• [ ] CRCs match
• [ ] File sizes correct
• [ ] Timestamps preserved
• [ ] Attributes correct
• [ ] Paths correct
• [ ] Symlinks point to correct targets

Writing Compatible Archives

Recommended Settings

For maximum compatibility:

Compression: LZMA2
Dictionary: 16 MiB or less
Solid: Off (or small blocks)
Header compression: LZMA2
Header encryption: Off
Filters: BCJ for executables only

Settings to Avoid

These reduce compatibility:

Setting	Compatibility Impact
Zstd, Brotli, LZ4	Requires 7-Zip-zstd
Very large dictionary	Memory issues on 32-bit
BCJ2	py7zr cannot read
Deflate64	Some tools read-only
Very long solid blocks	Memory during extraction

Error Messages

Common Errors from Other Tools

Error	Likely Cause
"Unsupported method"	Extended compression (zstd, etc.)
"Data error"	Corruption or wrong password
"CRC failed"	Corruption
"Unexpected end"	Truncated archive
"Headers error"	Header corruption

Our Errors for Their Archives

Condition	Appropriate Response
Unknown method	Skip entry with warning
Invalid UTF-16	Replace invalid chars
Future version	Warn and attempt read
Missing CRCs	Proceed without verification

Multi-Volume Compatibility

Naming

Standard: .7z.001, .7z.002, etc.

Some tools use:

• .7z.1, .7z.2 (no padding)
• .001, .002 (base name only)
• .part1.7z, .part2.7z (alternative)

Recommendation: Accept standard naming, produce standard naming.

Splitting Boundaries

7-Zip splits at configured size. When joining:

• Read volumes in order
• Verify all present before extraction
• Handle missing volumes gracefully

SFX Compatibility

Stubs

7-Zip provides Windows stubs. For other platforms:

• Linux: Community-created stubs
• macOS: Manual stub creation

Config Block

7-Zip's config format is Windows-centric. Cross-platform SFX:

• May not support all options
• May ignore GUI settings on non-Windows

Future Compatibility

Reserved Fields

• Reserved flag bits: Always write as 0
• Unknown properties: Skip by size
• Unknown methods: Report and skip

Version Handling

• Major version > 0: Reject
• Minor version > 4: Warn but attempt

Community Resources

Test Archives

• py7zr test data: Various edge cases
• 7-Zip test suite: Official tests
• libarchive tests: Multi-format tests

Implementation References

• 7-Zip source: Authoritative
• py7zr: Well-documented Python
• sevenz-rust2: Another Rust implementation

See Also

• Philosophy - Compatibility stance
• Method IDs - Method support matrix
• Error Conditions - Error handling