Introduction

The Zip64 format is an extension to the standard zip format that practically removes limits in sizes and the number of files inside of a zip archive.

The ZipArchive Library automatically uses the Zip64 extensions when the regular zip archive limits are exceeded. The ZipArchive Library will not otherwise include the extensions in the archive, even if the extensions are enabled (see the paragraph below). The extensions will be removed from the archive when due to modifications they are not needed anymore. The library also automatically detects an archive in the Zip64 format.

To detect if an archive is in Zip64 format, request the central directory information with the CZipArchive::GetCentralDirInfo() method and call the
CZipCentralDir::CInfo::IsZip64() method.

Enabling Zip64 Extensions in the ZipArchive Library

The Zip64 extensions are enabled by default in the ZipArchive Library. You should keep the extensions disabled if you don't use them, otherwise the library will use 64-bit types and perform some additional processing, which may slow down your application.

With CMake: Control Zip64 extensions using the ZIP_ENABLE_ZIP64 option:

• -DZIP_ENABLE_ZIP64=ON - Enable Zip64 extensions (default)
• -DZIP_ENABLE_ZIP64=OFF - Disable Zip64 extensions

Without CMake: Make sure _ZIP_ZIP64 is defined and set to 1 in the _features.h file (default is 1). Rebuild the ZipArchive Library and your application if you modify this definition.

Limits Compared: the Standard Format Versus Zip64 Format

The maximum values allowed in each format are summarized below:

Large Collections

The maximum number of items a collection can hold, limits the maximum number of files in an archive that the ZipArchive Library can process. This limit can be determined by examining the ZIP_ARRAY_SIZE_TYPE definition. This is defined as:

• size_t in the STL version
• INT_PTR in the MFC version

The MFC version also depends on size_t type when sorting.

These types are usually defined as unsigned 32-bit integer types. You may need to use a 64-bit compiler for a larger collections support.

Additional Considerations

• Under Windows, when your system utilizes large amount of memory while extensive file operations (especially on large files), see Modification of Archives: Replacing, Renaming, Deleting and Changing Data for a possible solution.
• If you have problems reading very large files, it may be the case that they have incorrect format and they do not conform to Zip64 specifications (it can happen with some archives under macOS). To deal with the problem, try using the CZipArchive::SetSpecialFlags() method with the CZipArchive::sfLargeNotZip64 flag.
• Under Windows, the built-in Compressed Folders shell extension (also known as Windows Zip Folders) can severely slow down opening and browsing Zip64 archives that contain a large number of files. This is because the shell extension attempts to parse the archive contents in the background whenever such a file is accessed in Windows Explorer. To avoid this performance issue, disable the built-in Compressed Folders support by unregistering the zipfldr.dll shell extension (run regsvr32 /u zipfldr.dll from an elevated command prompt). You can re-enable it later by running regsvr32 zipfldr.dll. This issue is most pronounced on older Windows versions (such as Windows XP), but can also affect later versions of Windows when dealing with archives containing a very large number of files.