Version

Version: 0.5.0
Date: 2023-Jan-06

License

Note: the scope of the copyrighted material does, of course, not extend onto any source or binary code derived from the specification.

What is SOZip ?

A Seek-Optimized ZIP file (SOZip) is a ZIP file that contains one or several Deflate-compressed files that are organized and annotated such that a SOZip-aware reader can perform very fast random access (seek) within a compressed file.

SOZip makes it possible to access large compressed files directly from a .zip file without prior decompression. It is not a new file format, but a profile of the existing ZIP format, done in a fully backward compatible way. ZIP readers that are non-SOZip aware can read a SOZip-enabled file normally and ignore the extended features that support efficient seek capability.

Use cases

This specification is intended to be general purpose / not domain specific.

SOZip was first developed to serve geospatial use cases, which commonly have large compressed files inside of ZIP archives. In particular, it makes it possible for users to read large Geographic Information Systems (GIS) files using the Shapefile, GeoPackage or FlatGeobuf formats (which have no native provision for compression) compressed in .zip files without prior decompression.

Efficient random access and selective decompression are a requirement to provide acceptable performance in many usage scenarios: spatial index filtering, access to a feature by its identifier, etc.

High-level specification

The SOZip optimization relies on two independent and combined mechanisms:

The first mechanism is the generation of a Deflate compressed stream that is structured in such a way that it contains data chunks that can be compressed and uncompressed independently from preceding and proceeding chunks in the compression stream. Conceptually, such a compressed file could be split into multiple independently compressed files, but from the point of view of a non-SOZip-aware ZIP reader, it will be a fully single legit compressed stream for the whole file. The chunking relies on the block flush mechanisms of the ZLib library, an example of which is the pigz utility with its --independent option. Block flushes are done at a regular interval of the input uncompressed stream, with the consequence of slight degradation of the compression rate. In the rest of this document, this interval is called chunk size. A typical value for it is 32 kilobytes.
The second mechanism is the creation of a hidden index file containing an array that maps file offsets of the uncompressed file, at every chunk size interval, to the corresponding offset in the Deflate compressed stream. This index is the structure that allows SOZip-aware readers to skip about throughout the file.

The below diagram shows the organization in high level records of a SOZip-enabled ZIP consisting of a SOZIP-optimized file my.gpkg:

my.gpkg file preceded by its local header
.my.gpkg.sozip.idx index file preceded by its local header
Central directory with an entry corresponding to my.gpkg (and none for the index file)
End of Central directory

If we zoom on the content of those 2 files, we can see that:

the Deflate stream of my.gpkg consists in many concatenated independent chunks.
the .my.gpkg.sozip.idx index file contains the offsets to the beginning of each chunk.

Detailed specification

A ZIP file is said to be SOZip-enabled if it contains one or several Deflate compressed files meeting the following requirements, in additions to the requirements of the .ZIP File Format Specification.

A SOZip-enabled file may contain a mix of SOZip-compressed and regular compressed or uncompressed files.

A file may be SOZIP-compressed only if its uncompressed size is strictly greater than the chunk size (otherwise there is no point in doing the SOZip optimization).

Chunked Deflate-compressed stream

A SOZip-compressed file MUST be created with compression_method = 8 (Deflate).
A SOZip-compressed file MUST have a corresponding local file header and a central directory file header.
Those headers MAY use extended fields. Typically for the ZIP64 extension if the compressed and/or uncompressed size of a file exceeds 4 GB. And/or the "Info-ZIP Unicode Path Extra Field" file name extension.
A SOZip writer MUST issue a call to deflate() ZLib method with the Z_SYNC_FLUSH mode, followed by a call with the Z_FULL_FLUSH flag, at a fixed interval (called chunk size) of the data read from the input uncompressed stream.
Z_SYNC_FLUSH and Z_FULL_FLUSH are not required (but may be used) for the final chunk, whose size may be smaller or equal to the chunk size. However the last call to deflate() to encode the last chunk MUST be done with the Z_FINISH flag, to finalize a valid Deflate stream.

Note: an explanation of the Z_SYNC_FLUSH and Z_FULL_FLUSH mode can be found at https://www.bolet.org/~pornin/deflate-flush-fr.html
The writer MUST collect the offset of each chunk, except for the initial chunk size whose offset is zero. Offsets MUST be relative to the start of the compressed data.

Note: a pseudo-code (among many possible variations) written in C++, using zlib, can be found in Annex E

Hidden index file

Storage of the index file

The index file MUST be stored as a uncompressed file.
The index file name MUST be :
- ${path_to_filename}/.${filename}.sozip.idx where ${path_to_filename} is the name of the directory if ${filename} contains directory paths. For example my_dir/.rivers.gpkg.sozip.idx if the filename stored in the archive is my_dir/rivers.gpkg.sozip.idx
- or .${filename}.sozip.idx if there is no directory path in the filename. For example .rivers.gpkg.sozip.idx
Note the leading dot ('.') character preceding the index filename, to indicate its hidden status.
The index file MUST be preceded by a ZIP local file header (cf paragraph 4.3.7 of the .ZIP File Format Specification)
That local file header MAY use extended fields. Typically for the "Info-ZIP Unicode Path Extra Field" file name extension. If the local file header of the indexed file uses the "Info-ZIP Unicode Path Extra Field" extension, the local file header of the index file MUST also use the "Info-ZIP Unicode Path Extra Field" extension.
That local file header MUST be immediately placed after the compressed file.
That file MUST NOT be listed as a central directory file header, to remain invisible.

Content of the index file

The hidden index file is made of a 32-byte header followed by a section of varying size (called offset section) which contains the values of the offsets collected during the generation of the Deflate-compressed data.

In the following, uint32 is a 32-bit unsigned integer, encoded in little-endian order (least significant byte first). uint64 is a 64-bit unsigned integer, encoded in little-endian order.

Header

Offset	Type	Name	Comment
0	uint32	version	Version number.
4	uint32	skip_bytes	Number of bytes to skip after header.
8	uint32	chunk_size	Chunk size in bytes.
12	uint32	offset_size	Size in bytes of an entry in offset section
16	uint64	uncompress_size	Size in bytes of the uncompressed file.
24	uint64	compress_size	Size in bytes of the compressed file.

Specification of fields:

version: MUST be set to 1 for this specification
skip_bytes: number of bytes between the end of the header and the beginning of the offset section. Generally set to 0. This could be set to a non-zero value to store extra content, unspecified currently. SOZip readers SHOULD skip over such extra content.
chunk_size: Interval, in uncompressed stream, at which Z_SYNC_FLUSH + Z_FULL_FLUSH are performed. It MUST not be zero (a value of 4096 or bigger is strongly recommended). A value lower than 100 MB is strongly recommended for performance and compatibility with SOZip readers. 32 KB is a generally safe default value.
offset_size: Number of bytes to encode each entry of the offset section. This MUST be 8 (uint64 values).
uncompress_size: Size in bytes of the uncompressed file (not the index, but the file subject to SOZip compression). This field is redundant with other information found in the local and central directory file headers of the compressed file, and is provides a reader a consistency check of the SOZip index with the compressed file. uncompress_size must be strictly greater than chunk_size
compress_size: Size in bytes of the compressed file (not the index, but the file subject to SOZip compression). This field is redundant with other information found in the local and central directory file headers of the compressed file, and is here so that a reader can check the consistency of the SOZip index with the compressed file.

Offset section

The offset section MUST contain exactly (uncompress_size - 1) / chunk_size (floor rounding) entries, each of size offset_size bytes.
Each entry MUST be a uint64 expressing the offset at which a compressed chunk starts. That offset is a relative offset, with respect to the start of the compressed stream (consequently, the indexed file and its index could be potentially relocated within the .zip file without requiring to regenerate the index). The offset of the first compressed chunk MUST be omitted, as always 0.

The first offset value is thus the offset in the compressed stream where uncompressed bytes in the range [chunk_size, min(2 * chunk_size, uncompressed_size)[ are encoded.

The second offset value is the offset in the compressed stream where uncompressed bytes in the range [2 * chunk_size, min(3 * chunk_size, uncompressed_size)[ are encoded. And so on.
As a consequence of the generation of the index, values in the offset section MUST be in strictly ascending order, and MUST be strictly lower than compress_size.

Normative references

.ZIP File Format Specification
DEFLATE Compressed Data Format Specification version 1.3

Annex A: Software implementations

GDAL (C/C++ library)

The sozip development branch of contains:

a CPLAddFileInZip() C function that can compress a file and add it to an new or existing ZIP file, and enable the SOZip optimization when relevant.
an implementation of the VSIGetFileMetadata() method that can be used on a filename of the form "/vsizip//path/to/my.zip/filename/inside" and with domain = "ZIP" to get information if a SOZip index is available for that file.
a modified /vsizip/ virtual file system that can use the SOZip optimization to perform fast random access to a compressed file within a ZIP.
a new command line utility, sozip, that can be used to create a seek-optimized ZIP file, to append files to an existing ZIP file, list the contents of a ZIP file and display the SOZip optimization status or validate a SOZip file.
Updated Shapefile and GeoPackage drivers that can directly generate SOZip-enabled .shz/.shp.zip or .gpkg.zip files.

This development branch is available in the rouault/sozip Docker image.

Examples:

Create a new ZIP file with an input file called in.gpkg:

docker run --rm -it -v $PWD:$PWD rouault/sozip sozip -j $PWD/out.zip $PWD/in.gpkg

Create a SOZip-optimized zip file called out.zip from an existing ZIP file called in.zip.

docker run --rm -it -v $PWD:$PWD rouault/sozip sozip --convert-from=$PWD/in.zip $PWD/out.zip

List the content of a ZIP file and check if files in it are SOZip-optimized:
```
docker run --rm -it -v $PWD:$PWD rouault/sozip sozip -l $PWD/in.zip
```

Validate a SOZip file:

docker run --rm -it -v $PWD:$PWD rouault/sozip sozip --validate $PWD/my.zip

Extracting the polygons extracting a georeferenced window of interest from a remote SOZip-optimized GeoPackage file:

docker run --rm -it -v $PWD:$PWD rouault/sozip \
    ogr2ogr $PWD/out.geojson /vsizip//vsicurl/http://even.rouault.free.fr/sozip/nz-building-outlines.gpkg.zip \
    -spat 1749900 5946100 1741000 5946200

sozipfile (Python module)

sozipfile is a fork of Python zipfile module, which implements the SOZip implementation in the ZIP writer, and can be used to check if a file within a ZIP is SOZip-optimized.

MapServer (Web mapping server written in C/C++, using GDAL)

The sozip development branch of MapServer, when built against a SOZip-capable GDAL, can generate SOZip-enabled output files if the mapfile has a ZIP output format, such as:

    OUTPUTFORMAT
      NAME "OGRGPKGZIP"
      DRIVER "OGR/GPKG"
      MIMETYPE "application/zip; driver=ogr/gpkg"
      FORMATOPTION "STORAGE=memory"
      FORMATOPTION "FORM=zip"
      FORMATOPTION "FILENAME=result.gpkg.zip"
    END

QGIS (Geographic Information System desktop and server application, using GDAL)

QGIS can read efficiently SOZip files when built against a SOZip-capable GDAL, through the use of GDAL /vsizip/ virtual file system.

Annex B: Advantages and limitations

Advantages

SOZip allows multithreaded compression of independent chunks. This is for example used in the GDAL implementation.
SOZip allows multithreaded decompression of independent chunks.
For decompression, faster alternatives to zlib can be used, such as libdeflate. This is for example used in the GDAL implementation.
SOZip has excellent backward compatibility. A data producer may deliver a SOZip enabled file with good confidence that nearly all existing ZIP readers can decompress it (at time of writing, we are not aware of ZIP readers that reject a SOZip enabled file.)

Limitations

Compression efficiency is reduced by the flushes done to isolate chunks. The larger the chunk size, the more efficient the compression, but random seeking will be less efficient due to more data being decompressed.
SOZip inherits all the limitations of the base ZIP format: in particular update in place of a SOZip optimized file requires rewriting the entire ZIP, or appending the updated version of the modified file at the end of the ZIP (with rewriting of the central header records and end of central directory record).

Annex C: Discussion about design choices

Why use the Deflate compression and not an alternative compression method ?

Deflate has been chosen as it is supported by all existing ZIP implementations. Other compression methods (LZMA, BZIP2, etc.) are supported more sparsely. Furthermore, given that the SOZip optimization results in non-optimal compression rate, it is likely that compression schemes that offer higher compression than Deflate would perform in a suboptimal way, due to the chunking mechanism resetting the dictionary at each chunk boundary.
Why encoding the hidden index as hidden and not visible ?

This design decision has pros and cons.

Pros:
- End-users will not see those indexes which are not directly useful for them.
- Non SOZip-aware software will not be disturbed by a file they don't expect (some readers could for example expect a precise list of files to be in a .zip)
- If the index file was visible, and the SOZip archive was regenerated by a non SOZip-aware writer that does an edit operation to an existing archive by recreating a new file, it would preserve the index file, but could potentially recompress the compressed file without using the chunked Deflate techniques, which could confuse SOZip readers (although a SOZip reader must use information from the header of the SOZip index to check its consistence with the compressed file).
Cons:
- Appending new files to a SOZip-enabled file may cause the SOZip index to be lost when using some non SOZip-aware ZIP writer. However, ZIP implementations that have an append-in-place strategy will generally preserve the hidden index. Refer to Annex D for a list of known implementations that can append-in-place.
Why encoding the hidden index as a file preceded by a local header ?

We have found at least one reader, Java's java.util.zip.ZipInputStream, which operates in a streaming way, and stops its enumeration of the content of a ZIP file at the first encountered content that is not a local header.
Why placing the hidden index after the compressed file and not before ?

Both can make sense. It has been observed though that if a hidden local header (that is not listed in the central directory entries) is located immediately at the beginning of a zip file, the 7zip utility will expose the hidden file. And, combined with the question at the previous paragraph, if the content is not preceded by a local header, 7zip will emit a warning ("The archive is open with offset"). It is also slightly easier to generate the index after the compressed file, given that the content of the index depends on information collected during creation of the chunked Deflate compressed stream. This also makes it potentially possible for a streaming writer to write a SOZip optimized file.
Why not compressing the index file ?

Having it uncompressed makes it easier for implementations. And for very large files, having it uncompressed makes it possible to seek at a random location in a truly constant time. A 64 GB compressed file, with a chunk size of 32 KB, requires a 15.6 MB index. For scenarios where a SOZip file is read in a on-demand piece-wise way from network storage, it would be costly in bandwith to have to download and decompress those 15 MB to read the last chunk of the 64 GB compressed file.

Annex D: Compatibility with existing ZIP implementations

SOZip-enabled files have been tested with the following ZIP capable utilities to check the backward compatibility (non exhaustive list!):

Compatible readers:

Info-ZIP unzip command line utility.
libzip: C library for reading, creating, and modifying zip archives
7zip command line utility or graphical interface.
WinZip graphical interface.
WinRAR graphical interface.
Windows Explorer default ZIP reader.
MacOSX default ZIP extractor.
MacOSX zipinfo command line utility
ark KDE (Linux/Unix typically) graphical interface
file-roller GNOME (Linux/Unix typically) graphical interface
Java's java.util.zip.ZipFile class.
Java's java.util.zip.ZipInputStream class, with the caveat that it sees the hidden index files (being a streaming reader, it only takes into account local file records)
Python zipfile module
GDAL /vsizip/ virtual file system, in all existing GDAL versions, can read SOZip-enabled files. Versions >= 3.7 will be able to take advantage of the SOZip index (earlier verions will ignore it.)

Partially compatible readers:

zipdetails requires the use of its main branch, or a version greater than 2.108. Currently released versions (2.108 or earlier) will error out on SOZip files, rejecting them because the Local header record of a .sozip.idx file has no matching Central header record.

Compatible writers:

Info-ZIP zip command line utility, used to create / edit zip files, can append new files to a SOZip-enabled file, if using the -g/--grow option, while preserving their existing SOZip-optimization
Python zipfile module can append new files to a SOZip-enabled file, while preserving their existing SOZip-optimization.
GDAL sozip command line utility can create SOZip-enabled files, and append new files to a SOZip-enabled file, while preserving their existing SOZip-optimization.
GDAL /vsizip/ virtual file system, in all existing GDAL versions, can append new files to a SOZip-enabled file, while preserving their existing SOZip-optimization.

However a number of writers, while attempting to "append" to a SOZip-enabled file, actually create a new file from scratch, and will loose the hidden index.

Annex E: Pseudo-code for SOZip Deflate stream generation

Licensed under CC0 or MIT at the choice of the user (that is feel free to reuse and adapt without any constaint).

    // Setup zlib
    z_stream zStream;
    memset(&zStream, 0, sizeof(zStream));

    int ret = deflateInit2(&zStream, Z_DEFAULT_COMPRESSION,
                           Z_DEFLATED, -MAX_WBITS, 8,
                           Z_DEFAULT_STRATEGY);
    // TODO: add error checking

    std::vector<uint64_t> offsets;
    const uint64_t start_offset = tell_position(compressed_file_handle);

    bool first_block = true;
    uint32_t crc32Val = 0;
    uint64_t uncompress_size = 0;
    while (true)
    {
        // Acquire input data, up to chunk_size bytes
        std::vector<uint8_t> uncompressed_data = read(uncompressed_stream, chunk_size);
        uncompress_size += uncompressed_data.size();

        if (!first_block && !uncompressed_data.empty())
        {
            // Track the offset in the compressed stream of the beginning of
            // the new chunk (except for first chunk, and also if the uncompressed
            // file size was exactly a multiple of chunk_size)
            offsets.push_back(uint64_little_endian(
                tell_position(compressed_file_handle) - start_offset));
        }
        first_block = false;

        // Prepare output buffer (with security margin for very small chunks,
        // or chunks that compresses poorly)
        std::vector<uint8_t> compressed_data(uncompressed_data.size() +
                                             uncompressed_data.size() / 10 + 16);

        // Compress data
        zStream.avail_in = static_cast<uInt>(uncompressed_data.size());
        zStream.next_in = &uncompressed_data[0];
        zStream.avail_out = static_cast<uInt>(compressed_data.size());
        zStream.next_out = &compressed_data[0];

        if (uncompressed_data.size() < chunk_size)
        {
            ret = deflate(&zStream, Z_FINISH);
            // TODO: add error checking
        }
        else
        {
            ret = deflate(&zStream, Z_SYNC_FLUSH);
            // TODO: add error checking
            ret = deflate(&zStream, Z_FULL_FLUSH);
            // TODO: add error checking
        }

        // Write output buffer
        compressed_data.resize(compressed_data.size() - zStream.avail_out);
        write(compressed_file_handle, compressed_data);

        // Update crc32 of uncompressed data
        crc32Val = crc32(crc32Val, &uncompressed_data[0],
                         static_cast<uInt>(uncompressed_data.size()));

        // Exit loop if no more input data
        if (uncompressed_data.size() < chunk_size)
        {
            break;
        }
    }

    if (uncompress_size > 0 )
        assert (offsets.size() == (uncompress_size - 1) / chunk_size);

    // TODO: store crc32Val in local and central header records

    // Cleanup
    deflateEnd(&zStream);

Annex F: Notes to independently decode a SOZip chunk

The chunking process, involving Z_FULL_FLUSH, used during SOZip generation makes it possible to decompress chunks in an independent way.

Given uncompressed_offset being an offset of the uncompressed file, multiple of chunk_size, the reader should retrieve in the offset section of the .sozip.idx (stored in an array compressed_offset_table[]), the start and end offsets of the chunk, like below:

    assert (uncompressed_offset % chunk_size) == 0);
    size_t entry_idx = uncompressed_offset / chunk_size;
    uint64_t relative_start_offset;
    if (entry_idx == 0)
        relative_start_offset = 0;
    else
        relative_start_offset = compressed_offset_table[entry_idx - 1];

    uint64_t relative_end_offset;
    if (entry_idx < compressed_offset_table.size() )
        relative_end_offset = compressed_offset_table[entry];
    else
        relative_end_offset = compress_size;

    size_t compressed_chunk_size = relative_end_offset - relative_start_offset;

    seek(compressed_stream, absolute_offset_of_start_of_deflate_stream + relative_start_offset);
    std::vector<uint8_t> compressed_chunk_data = read(compressed_stream, compressed_chunk_size);

Given that each chunk, except the last one, is not a standalone Deflate stream, reading code must be careful to present it in a way that will not confuse the Deflate decoding library. The only precaution to take is to dynamically patch the last flush Deflate block at the end of the compressed chunk to be advertized as the final Deflate block of the chunk.

Given the use of Z_FULL_FLUSH, the last 5 bytes of a chunk (that is not the last one) should be \x00, \x00, \x00, \xFF, \xFF. The patching operation consists in changing the first byte of this 5 byte sequence from \x00 to \x01 as below:

    if (compressed_chunk_size >= 5 &&
        memcmp(compressed_chunk_data.data() + compressed_chunk_size - 5, "\x00\x00\x00\xFF\xFF", 5) == 0)
    {
        // Tag this flush Deflate block as the last one of the chunk.
        compressed_chunk_data[compressed_chunk_size - 5] = 0x01;
    }

With the above, if using zlib, a chunk can then be decompressed with:

    z_stream zStream;
    memset(&zStream, 0, sizeof(zStream));

    int err = inflateInit2(&zStream, -MAX_WBITS);
    // TODO: add error checking

    size_t decompressed_size = chunk_size_for_all_chunks_except_last_one;
    std::vector<uint8_t> decompressed_data(decompressed_size);
    zStream.avail_in = compressed_chunk_size;
    zStream.next_in = compressed_chunk_data;
    zStream.avail_out = decompressed_size;
    zStream.next_out = decompressed_data;

    err = inflate(&zStream, Z_FINISH);
    if (err == Z_STREAM_END && zStream.avail_in == 0 && zStream.avail_out == 0 )
    {
        // success
    }
    inflateEnd(&zStream);

Or if using libdeflate:

    struct libdeflate_decompressor *decompressor = libdeflate_alloc_decompressor();
    // TODO: add error checking

    size_t decompressed_size = chunk_size_for_all_chunks_except_last_one;
    std::vector<uint8_t> decompressed_data(decompressed_size);
    size_t actual_decompressed_size = 0;
    if (libdeflate_deflate_decompress(
            decompressor,
            compressed_chunk_data, compressed_chunk_size,
            decompressed_data.data(), decompressed_size,
            &actual_decompressed_size) == LIBDEFLATE_SUCCESS &&
        actual_decompressed_size == decompressed_size)
    {
        // success
    }
    libdeflate_free_decompressor(decompressor);

All pseudo-code in this annex is licensed under CC0 or MIT at the choice of the user (that is feel free to reuse and adapt without any constaint).

Annex G: Examples

Examples of SOZip-enabled files can be found in the sozip-examples repository.

Annex H: commented dump of a dummy SOZip file

The following invokation of GDAL's sozip utility generates a dummy SOZip enabled file that contains a tiny file "foo" with "foo" as content, and using a chunk size of 2 bytes.

printf "foo" > foo
sozip --overwrite --enable-sozip=yes --sozip-chunk-size=2 foo.zip foo

Dump of the local file record of the SOZip compressed "foo" file:

Offset	Type	Values	Comment
0	uint32	0x04034B50	Local file header signature (for the compressed file)
4	uint16	20	version needed to extract
6	uint16	0	general purpose bit flag
8	uint16	8	compression method (8=deflate)
10	uint16	32168	last mod file time
12	uint16	22053	last mod file date
14	uint32	0x8C736521	CRC32
18	uint32	16	compressed size
22	uint32	3	uncompressed size
26	uint16	3	filename length
28	uint16	0	extra field length
30	byte[]	'f', 'o', 'o'	filename
33	byte[]	0x4A 0xCB 0x07	deflate block for first chunk (corresponding to 'f', 'o')
36	byte[]	0x00 0x00 0x00 0xFF 0xFF	uncompressed block of size 0 encoding a Z_SYNC_FLUSH
41	byte[]	0x00 0x00 0x00 0xFF 0xFF	uncompressed block of size 0 encoding a Z_FULL_FLUSH
46	byte[]	0xCB 0x07 0x00	deflate block for second chunk (corresponding to 'o')

Dump of the local file record of the uncompressed ".foo.sozip.idx" index file:

Offset	Type	Values	Comment
49	uint32	0x04034B50	Local file header signature (for the index file)
53	uint16	20	version needed to extract
55	uint16	0	general purpose bit flag
57	uint16	0	compression method (0=uncompressed)
59	uint16	32168	last mod file time
61	uint16	22053	last mod file date
63	uint32	0x56FEC86C	CRC32
67	uint32	36	compressed size
71	uint32	36	uncompressed size
75	uint16	14	filename length
77	uint16	0	extra field length
79	byte[]	'.', 'f', 'o', 'o', '.', 's', 'o', 'z', 'i', 'p', '.', 'i', 'd', 'x'	filename
93	uint32	1	SOZip version
97	uint32	0	Bytes to skip
101	uint32	2	chunk_size
105	uint32	8	offset_size
109	uint64	3	uncompress_size
117	uint64	16	compress_size
125	uint64	13	(relative) offset to the start of the 2nd compressed chunk (absolute offset is 33 + 13 = 46)

Dump of the central header record describing the "foo" file:

Offset	Type	Values	Comment
133	uint32	0x02014B50	Central file header signature (compressed file)
137	uint16	0	version made by
139	uint16	20	version needed to extract
141	uint16	0	general purpose bit flag
143	uint16	8	compression method (8=deflate)
145	uint16	32168	last mod file time
147	uint16	22053	last mod file date
151	uint32	0x8C736521	CRC32
155	uint32	16	compressed size
159	uint32	3	uncompressed size
161	uint16	3	file name length
163	uint16	0	extra field length
165	uint16	0	file comment length
167	uint16	0	disk number start
169	uint16	0	internal file attributes
171	uint32	0	external file attributes
175	uint32	0	relative offset of local header (0 here because this is the first file, and there's no leading content)
179	byte[]	'f', 'o', 'o'	filename

Dump of the end of central directory record:

Offset	Type	Values	Comment
182	uint32	0x06054B50	end of central dir signature
186	uint16	0	number of this disk
188	uint16	0	number of the disk with the start of the central directory
190	uint16	1	total number of entries in the central directory on this disk
192	uint16	1	total number of entries in the central directory
194	uint32	49	size of the central directory (= 182 - 133)
198	uint32	133	offset of start of central directory with respect to the starting disk number
202	uint16	0	.ZIP file comment length

Output of "zipdetails foo.zip" (using main branch of https://github.com/pmqs/zipdetails):

0000 LOCAL HEADER #1       04034B50
0004 Extract Zip Spec      14 '2.0'
0005 Extract OS            00 'MS-DOS'
0006 General Purpose Flag  0000
     [Bits 1-2]            0 'Normal Compression'
0008 Compression Method    0008 'Deflated'
000A Last Mod Time         56257DA8 'Thu Jan  5 16:45:16 2023'
000E CRC                   8C736521
0012 Compressed Length     00000010
0016 Uncompressed Length   00000003
001A Filename Length       0003
001C Extra Length          0000
001E Filename              'foo'
0021 PAYLOAD               J...............

0031 LOCAL HEADER #2       04034B50
     Orphan Entry: No
     matching central
     directory
0035 Extract Zip Spec      14 '2.0'
0036 Extract OS            00 'MS-DOS'
0037 General Purpose Flag  0000
0039 Compression Method    0000 'Stored'
003B Last Mod Time         56257DA8 'Thu Jan  5 16:45:16 2023'
003F CRC                   56FEC86C
0043 Compressed Length     00000028
0047 Uncompressed Length   00000028
004B Filename Length       000E
004D Extra Length          0000
004F Filename              '.foo.sozip.idx'

WARNING!
Expected Zip header not found at offset 0x5D
Skipping 0x24 bytes to Central Directory...

0085 CENTRAL HEADER #1     02014B50
0089 Created Zip Spec      00 '0.0'
008A Created OS            00 'MS-DOS'
008B Extract Zip Spec      14 '2.0'
008C Extract OS            00 'MS-DOS'
008D General Purpose Flag  0000
     [Bits 1-2]            0 'Normal Compression'
008F Compression Method    0008 'Deflated'
0091 Last Mod Time         56257DA8 'Thu Jan  5 16:45:16 2023'
0095 CRC                   8C736521
0099 Compressed Length     00000010
009D Uncompressed Length   00000003
00A1 Filename Length       0003
00A3 Extra Length          0000
00A5 Comment Length        0000
00A7 Disk Start            0000
00A9 Int File Attributes   0000
     [Bit 0]               0 'Binary Data'
00AB Ext File Attributes   00000000
00AF Local Header Offset   00000000
00B3 Filename              'foo'

00B6 END CENTRAL HEADER    06054B50
00BA Number of this disk   0000
00BC Central Dir Disk no   0000
00BE Entries in this disk  0001
00C0 Total Entries         0001
00C2 Size of Central Dir   00000031
00C6 Offset to Central Dir 00000085
00CA Comment Length        0000

WARNINGS

* Expected Zip header not found at offset 0x61, skipped 0x24 bytes

Done

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sozip_specification.md

sozip_specification.md

Version

License

What is SOZip ?

Use cases

High-level specification

Detailed specification

Chunked Deflate-compressed stream

Hidden index file

Storage of the index file

Content of the index file

Header

Offset section

Normative references

Annex A: Software implementations

GDAL (C/C++ library)

sozipfile (Python module)

MapServer (Web mapping server written in C/C++, using GDAL)

QGIS (Geographic Information System desktop and server application, using GDAL)

Annex B: Advantages and limitations

Advantages

Limitations

Annex C: Discussion about design choices

Annex D: Compatibility with existing ZIP implementations

Annex E: Pseudo-code for SOZip Deflate stream generation

Annex F: Notes to independently decode a SOZip chunk

Annex G: Examples

Annex H: commented dump of a dummy SOZip file

Files

sozip_specification.md

Latest commit

History

sozip_specification.md

File metadata and controls

Version

License

What is SOZip ?

Use cases

High-level specification

Detailed specification

Chunked Deflate-compressed stream

Hidden index file

Storage of the index file

Content of the index file

Header

Offset section

Normative references

Annex A: Software implementations

GDAL (C/C++ library)

sozipfile (Python module)

MapServer (Web mapping server written in C/C++, using GDAL)

QGIS (Geographic Information System desktop and server application, using GDAL)

Annex B: Advantages and limitations

Advantages

Limitations

Annex C: Discussion about design choices

Annex D: Compatibility with existing ZIP implementations

Annex E: Pseudo-code for SOZip Deflate stream generation

Annex F: Notes to independently decode a SOZip chunk

Annex G: Examples

Annex H: commented dump of a dummy SOZip file