Bundling 7z.dll with Proprietary Software

Bundling the 7z.dll library with proprietary software is legally permissible, but it requires strict adherence to open-source licensing conditions. Because 7-Zip and its dynamic link libraries are primarily distributed under the GNU Lesser General Public License (LGPL), developers can link against the binary without being forced to open-source their own proprietary code. However, commercial vendors must still fulfill specific obligations regarding source code availability, attribution, dynamic linking mechanics, and sub-module restrictions.

The Governing License: GNU LGPL

The core of 7-Zip, including the compiled 7z.dll, is distributed under the GNU LGPL (typically LGPL version 2.1 or later), with some portions covered by the BSD 3-Clause license or public domain dedications.

Unlike the standard GNU General Public License (GPL), the LGPL allows a library to be utilized by non-GPL and proprietary programs. However, it explicitly protects the freedom of the library itself.

Requirements for Bundling 7z.dll

To remain compliant while packaging 7z.dll alongside closed-source software, you must meet the following obligations:

  1. Maintain Dynamic Linking Keep 7z.dll as a distinct, dynamically linked file. Do not statically link the 7-Zip source code directly into your proprietary executable. The LGPL requires that end users have the technical ability to replace or upgrade 7z.dll with a custom or modified version of their own without breaking compatibility or being blocked by anti-tamper mechanisms.

  2. Provide Source Code for the Library You are not required to disclose your application's proprietary source code, but you must provide access to the source code of 7z.dll.

    • If you made any modifications to 7z.dll, you must distribute those modified source files under the LGPL.
    • If you use the unmodified binary, you must either include the corresponding 7-Zip source archive with your distribution or provide a written offer valid for at least three years (or a direct, stable download link) directing users to the exact source code corresponding to the version you shipped.

  3. Include Copyright and License Notices You must include proper attribution in your software documentation, about box, or legal notices. This includes:

    • A clear statement that your application utilizes the 7-Zip library.
    • A reference to Igor Pavlov as the author/copyright holder.
    • A prominent notice stating that 7z.dll is licensed under the GNU LGPL.
    • A full copy of the LGPL license text included with your installer or software package.

Beware of the unRAR Restriction

Certain builds of 7-Zip include code to decompress RAR archives. The unRAR portion is governed by a distinct license from Eugene Roshal, which contains an explicit restriction: the code may not be used or reverse-engineered to re-create the RAR compression algorithm. If your version of 7z.dll incorporates unRAR capabilities, ensure that your application does not violate this specific restriction, or build a custom version of 7z.dll with RAR support explicitly disabled.