aPACK / aPLib file format

.aplib · .apack · compression (single file) · introduced 1998 by Joergen Ibsen

What is a aPACK / aPLib file?

aPLib is a compact LZ77-family compressor designed for the demoscene and intro packers, where the cost of the decompressor itself matters as much as the payload size. Many 64 KB intros and self-extracting executables embed an aPLib decompressor of just a few hundred bytes.

Most readers reach this page after double-clicking a .aplib file and finding their operating system unsure what to do with it. The good news: aPACK / aPLib is a well-understood format with mature tooling on every major platform. It is supported in one click by mainstream archivers on Windows, macOS, and Linux.

How to extract a aPACK / aPLib file

Opening a .aplib file is straightforward on every modern operating system. The exact steps depend on which platform you are on:

On Windows

Locate the .aplib file in File Explorer.
Right-click the file. If you have 7-Zip installed, choose 7-Zip → Extract Here. On Windows 11 the built-in extractor handles ZIP, 7z, RAR and several other formats directly through Extract All…
If prompted, pick a destination folder and click Extract.
The extracted files appear in a folder next to the original archive.

On macOS

Find the .aplib file in Finder.
Install The Unarchiver from the Mac App Store, or Keka for both creating and extracting.
Right-click the file → Open With → The Unarchiver (or just double-click after Unarchiver is set as the default).
The extracted folder appears next to the archive.

On Linux

Use your file manager: right-click the archive → Extract Here (file-roller on GNOME, Ark on KDE).

On the command line, run one of:

7z x archive.aplib    # works for almost any format

The files appear in the current directory.

How to create a aPACK / aPLib file

To pack files into a .aplib archive, you typically need an archiver that supports writing this format. Not every tool can create every format — for example, only WinRAR can create real RAR files. The sidebar lists the software known to read and write aPACK / aPLib.

On the command line, the canonical create command is:

# Use 7-Zip, PeaZip, or the format's reference tool to create .aplib files.

If you regularly create archives at scale, a dedicated archive automation suite can wrap this command in scheduled jobs, integrity checks, and offsite replication.

Strengths

Microscopic decompressor
Good ratio for its size

Weaknesses and limitations

Closed source
Niche use case

Typical use cases

Demoscene 4K / 64 K intros
Tiny self-extracting executables

Technical details

aPACK / aPLib uses the aPLib (LZ77 family) algorithm and is identified on disk by the magic byte sequence 4D 6F 6C 65. The standard MIME type is application/x-aplib. The format is a proprietary specification and is subject to license restrictions for some uses (notably creation).

Frequently asked questions

Is aPACK / aPLib safe to open?

Archive files are containers — they are only as safe as what they contain. A .aplib from a trusted source is fine; one received from a stranger can hold malicious executables, scripts, or files engineered to exploit bugs in your archiver. Always keep your archiver updated, and never run unknown executables that come out of an archive. A safe workflow is to extract into a sandbox and scan the contents before opening anything.

What is the maximum file size?

Modern implementations support archives in the multi-terabyte range. The practical limit is your filesystem (FAT32 caps at 4 GB; ext4, NTFS, APFS, and exFAT all comfortably handle multi-TB files).

Can I password-protect a aPACK / aPLib?

Some archivers can wrap aPACK / aPLib files in an encrypted container, but the format itself does not always include built-in encryption. Check your archiver's documentation.

How does aPACK / aPLib compare to other formats?

See our format comparison pages such as ZIP vs 7z, ZIP vs RAR, and tar.gz vs tar.xz for side-by-side feature tables.