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

To use the Zip64 extensions, you need to make sure that _ZIP_ZIP64 is defined in the _features.h file. It is enabled by default. Rebuild the ZipArchive Library and your application, if you modify this definition. 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.

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 OS X). To deal with the problem, try using the CZipArchive::SetSpecialFlags() method with the CZipArchive::sfLargeNotZip64 flag.