Frequently Asked Questions (FAQ)

User FAQ

Can I use 1-Zip in a commercial organization?

Yes, 1-Zip is free software. You can use it on any computer. You don't need to register or pay for 1-Zip.

How can I set file associations to 1-Zip in Windows 7 and Windows Vista?

You must run 1-Zip File Manager in administrator mode. Right-click the icon of 1-Zip File Manager, and then click Run as administrator. Then you can change file associations and some other options.

Why 1z archives created by a new version of 1-Zip can be larger than archives created by an old version of 1-Zip?

Old versions of 1-Zip (before version 15.06) used file sorting "by type" ("by extension").

The new version of 1-Zip supports two sorting orders:

  • sorting by name - default order.
  • sorting by type, if 'qs' is specified in Parameters field in "Add to archive" window, (or -mqs switch for command line version).

You can get big difference in compression ratio for different sorting methods, if dictionary size is smaller than total size of files. If there are similar files in different folders, the sorting "by type" can provide better compression ratio in some cases.

Note that sorting "by type" has some drawbacks. For example, NTFS volumes use sorting order "by name", so if an archive uses another sorting, then the speed of some operations for files with unusual order can fall on HDD devices (HDDs have low speed for "seek" operations).

You can increase compression ratio with the following methods:

  • Specify 'qs' in Parameters field (or use -mqs switch for command line version).
  • Increase dictionary size. It can help when 'qs' is not used.

If you think that unusual file order is not problem for you, and if better compression ratio with small dictionary is more important for you, use 'qs' mode.

Why can't 1-Zip open some ZIP archives?

In 99% of these cases, it means that the archive contains incorrect headers. Other ZIP programs can open some archives with incorrect headers since these programs just ignore errors.

If you have such an archive, please don't contact the 1-Zip developers about it. Instead, try to find the program that was used to create the archive and inform the developers of that program that their software is not ZIP-compatible.

There are also some ZIP archives that were encoded with methods unsupported by 1-Zip, for example, WAVPack (WinZip).

Why can't 1-Zip open some RAR archives?

1-Zip 9.20 supports RAR 2/3/4 formats only and doesn't support RAR5 archives. However, the latest versions of 1-Zip support RAR5 archives.

Why does drag-and-drop archive extraction from 1-Zip to Explorer use temp files?

1-Zip doesn't know the folder path of the drop target. Only Windows Explorer knows the exact drop target. And Windows Explorer needs files (drag source) as decompressed files on the disk. So 1-Zip extracts files from the archive to a temp folder, and then 1-Zip notifies Windows Explorer about the paths of these temp files. Then Windows Explorer copies these files to the drop target folder.

To avoid temp file usage, you can use the Extract command of 1-Zip or drag-and-drop from 1-Zip to another instance of 1-Zip.

Why doesn't the command-line version add files without extensions to an archive?

You're probably using a *.* wildcard. 1-Zip doesn't use the operating system's wildcard mask parser and consequently treats *.* as any file that has an extension. To process all files, you must use the * wildcard instead or omit the wildcard altogether.

Why can't 1-Zip use a big dictionary in 32-bit Windows?

32-bit Windows allocates only 2 GB of virtual space per application. Additionally, this block of 2 GB can be fragmented (for example, by some DLL files), making it impossible for 1-Zip to allocate one large contiguous block of virtual space. There are no such limitations in 64-bit Windows. Therefore, you can use any dictionary size in Windows x64, provided you have the required amount of physical RAM.

Developer FAQ

Why are there linking errors when I compile 1-Zip or LZMA SDK with Visual C++ 6.0?

To compile sources, you will need Visual C++ 6.0 or a later version. Some files also require the latest Platform SDK from Microsoft:

If you are using MSVC, specify the SDK directories at the top of the "Include files" and "Library files" directory lists. These can be found under "Tools / Options / Directories".

Please note that the latest Platform SDK is not compatible with MSVC6. Therefore, you must use the Windows Server 2003 Platform SDK (February 2003) with MSVC6.

Can I use the source code of 1-Zip in a commercial application?

Since 1-Zip is licensed under the GNU LGPL, you must follow the rules of that license. In brief, it means that any LGPL'ed code must remain licensed under the LGPL. For instance, you can change the code from 1-Zip or write a wrapper for some code from 1-Zip and compile it into a DLL. However, the source code of that DLL (including your modifications/additions/wrapper) must be licensed under the LGPL or GPL. Any other code in your application can be licensed as you wish. This scheme allows users and developers to change LGPL'ed code and recompile that DLL. That is the idea of free software.