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The final term of the zlib license state

  1. This notice may not be removed or altered from any source distribution.

However, the Wikipedia page lists

Copyleft No

The way I understand the above quoted term, it means that the source code must retain its license when redistributed.

There are two possible ways I can see that this would not be considered copyleft:

  1. "from any source distribution" refers to verbatim copies, and thus a modified version, or a project created using the zlib-licesned software, would not be bound by this restriction.
  2. It is not considered copyleft because, while the zlib license would remain in effect, it does not have to be the only license in effect, so, for example, you could add a term that required those who distribute binaries of modified versions of the software to also distribute the source, while not directly conflicting with any of the three requirements given.

Is my interpretation of the license correct? Why would it then not be considered a copyleft license?

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  • Why the downvote? Question too obvious? – Pixelated Fish Apr 8 at 19:34
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From the wikipedia page on copyleft:

Copyleft is the practice of granting the right to freely distribute and modify intellectual property with the requirement that the same rights be preserved in derivative works created from that property.

A license is considered copyleft if the rights granted by the license must remain in force even when modifications to the work are made. This is something else than having to preserve a license notice.

As an example, I can take a piece of software under the zlib license, make changes to it and redistribute the changed version as a binary only under a no-derivatives allowed closed-source license. This is all within the rights granted by the zlib license. In doing so, I am granting the recipients of my modified software fewer rights than I got from the authors of the zlib software and that is something a copyleft license would not allow.

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