7z reader · onearchive docs

How SevenZipReader (src/OneArchive/SevenZip/) reads 7z archives and the choices the format document leaves open. Sources: 7zFormat.txt (header structures) and Methods.txt (method IDs) from the 7-Zip documentation; every point the documents do not settle was fixed by a black-box run of 7-Zip 26.00 (testdata/7z/generate.py writes the vectors).

What is read

Signature header (32 bytes): signature 37 7A BC AF 27 1C, version (major must be 0), the start-header CRC over bytes 12–31, then the next header's offset, size and CRC. A next-header size of 0 is an empty archive (7-Zip writes one for an empty input).
Header: plain (0x01) or encoded (0x17: a streams info whose single folder decodes to the real header; up to four levels). Archive properties are skipped. Additional-streams info is parsed and ignored; names, times or attributes that point into it (External != 0) raise ArchiveUnsupportedException.
Streams info: pack info, folders (coders, bond pairs, packed stream indices, unpack sizes, folder CRC), substreams (counts, sizes, CRCs).
Files info: empty-stream, empty-file and anti bit vectors, names (UTF-16LE), creation, access and modification times (100 ns ticks since 1601-01-01 UTC), Windows attributes. Dummy padding, start positions, comments and unknown properties are skipped by their size.

Choices where the document is silent (all checked against 7-Zip output)

Point	What the reader does
Substream sizes	A folder with n files stores n−1 sizes; the last is the folder's unpack size minus the others. With no `kSize`, only folders with one file are allowed.
Substream CRCs	A folder with exactly one file and a folder CRC takes that CRC; digests are stored only for the other files, in folder order.
Directory	An empty-stream entry that is not marked empty-file, or any entry whose attributes carry `0x10`. Its size is 0 and any data stream is ignored.
Unix mode	Attribute bit `0x8000` says the high 16 bits are `st_mode`. The permission bits (`& 0xFFF`) become `Mode`; the type bits choose symlink (`0xA000`), directory, device or FIFO. Without the bit, `Mode` is null.
Symbolic link	A file of type symlink whose content is the target; `LinkTarget` is read when the header is read (at most 4 × `MaxPathLength` bytes).
Anti-items	Listed in `Items` with `IsAnti`; `GetNextEntry` skips them, so extraction never sees a deletion marker.
Names	Kept exactly as stored, without a trailing `/`. Extraction applies the shared path rules.
Coder order	Coders are kept in header order. BCJ2 (four inputs) is parsed and its graph checked, but not decoded.
Stream directions	`NumInStreams` are the coder's packed-side inputs when decoding, `NumOutStreams` its outputs; a bond feeds a coder input from another coder's output; unbound inputs are the folder's packed streams, in the listed order; the one unbound output is the result.

Coders

Decoded: Copy (00), Deflate (04 01 08), Deflate64 (04 01 09, raw blocks, no properties, same as Deflate), BZip2 (04 02 02), LZMA (03 01 01, 5 property bytes: lc/lp/pb and the dictionary size; the unpack size comes from the header, an end marker is optional), LZMA2 (21, one property byte, the dictionary size code, at most 40), Delta (03, one property byte: distance minus one), the branch filters x86, PowerPC, IA-64, ARM, ARM Thumb, SPARC, ARM64 and RISC-V (short IDs 04–0B; the older long IDs 03 03 xx xx for the first six; an optional 4-byte start offset as property), and Zstandard under the ID F7 11 01 that Methods.txt lists for external codecs (the frame is self-describing, so the coder's properties are ignored). Any other coder, or one with several streams (BCJ2), raises ArchiveUnsupportedException naming the method ID, when the content is opened; listing works. Each coder's output is cut to the unpack size the header gives; a coder that ends early is a truncated-data error naming the folder's first packed stream offset.

Limits and safety

SevenZipReaderOptions.MaxHeaderSize (default 64 MiB) bounds the next header and any encoded header, checked on the declared size before allocating.
MaxDictionarySize (default 64 MiB) bounds an LZMA or LZMA2 dictionary, checked on the header's property bytes before the decoder allocates (a typed limit error otherwise). It is separate from ArchiveLimits.MaxWindowSize because 7-Zip's own default dictionary is 16 MiB. The zip LZMA and xz entries use MaxWindowSize.
MaxCodersPerFolder (default 32) and 64 streams per folder; every count read from the header is checked against the remaining header bytes or the limit before an array is allocated.
File count against ArchiveLimits.MaxEntryCount; each folder's unpack size against MaxTotalSize before any decoding; the declared entry sizes through the shared Admit check; the expansion ratio over the bytes read from packed streams and bytes produced.
Coder graphs: bonds and packed indices must use each stream once, and every coder must be reachable from the result (a loop is refused).
Packed stream positions are checked against the file length when the header is read.
A file's CRC-32 is checked when it has been read to its end (VerifyChecksum, default on). The header CRCs always are.
The 7z header is at the end of the file, so the source must seek. Archive.Open copies a non-seekable source into memory first, up to MaxTotalSize.

Random access and streaming

OpenEntryStream(entry) leases a decoder for the entry's folder and gives it back when the entry stream is disposed; the reader keeps the two most recently used idle decoders, so opening a solid folder's entries in archive order decodes the folder once, while going back to an earlier entry decodes from the folder's start. A decoder that threw is discarded and the next use starts the folder over. In sequential reading the reader keeps the folder's decoder open, so an archive read in order decodes each folder once. An entry stream can seek; seeking backwards re-decodes, and a seek turns off the CRC check for that stream.

Not done yet

The BCJ2 coder. PPMd (variant H) is read (ppmd.md). AES-256 is read (7z-aes-derivation.md). Writing is in 7z-writer.md.

Zip entries (methods 14 and 95)

Method 14 is read as APPNOTE 5.8 describes: a 4-byte header (LZMA SDK major and minor version, then the properties size, which must be 5), the 5 property bytes, then raw LZMA data. The uncompressed size comes from the zip headers; a streamed entry (size unknown) must end with an end marker. Method 95 is one xz stream. Both use ArchiveLimits.MaxWindowSize for the dictionary. Both are read only.