Introduction

The Zip64 format is an extension to the standard zip format that practically removes limits in sizes and item numbers within 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 some modifications they are not needed anymore. The library also automatically detects an archive in the Zip64 format.

Enabling Zip64 Extensions in the ZipArchive Library

To use the Zip64 extensions, you need to make sure that _ZIP64 is defined in the file _features.h. It is disabled 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 more 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:


^* This is limited to 2,147,483,647 on STL implementations without Zip64 enabled and on Microsoft Visual Studio 6.0 (MFC). See the next paragraph, how to work around these limitations. This limit also exists under CodeGear C++Builder and Borland C++ but without ready workarounds.

Platforms Limitations in Processing Large Data

When processing files and collections, the ZipArchive Library relies on external libraries, such as STL and MFC. If you wish to use Zip64 extensions, you need to make sure that the implementation of STL or MFC that you use, supports large files and large collections.

Large Files

Most of the implementations support large files (> 4GB) processing. The exceptions are listed below:

• STL without Zip64 enabled is limited to handling files of maximum 2GB size due to the use of signed types in file handling functions. To remove this limit enable Zip64 functionality and the library will use large types (Zip64 extensions will be used only when needed).
• Microsoft Visual Studio 6.0 (MFC) does not support large files. The workaround for this is to use the STL implementation of the CZipFile class instead of MFC implementation under this platform. This is handled by the ZipArchive Library by automatically defining ZIP_FILE_USES_STL in the _features.h file, when Zip64 is enabled under this platform. Note that the CZipFile class does not derive from CFile in this case.
• CodeGear C++Builder & Borland C++ (STL & MFC) - Zip64 functionality is not supported under this platform. The workaround for this case would be to use a custom STL implementation.

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.