2

In GPL v3.0 paragraph 5a it says:

The work must carry prominent notices stating that you modified it, and giving a relevant date.

Is adding your copyright notice enough? (like described in this question)

/*
 * Some open source application
 * Component Foo
 * (C) 2019 by The guy who made a modification (modification-author@foo.bar)
 * (C) 2018 by Original Author(original-author@foo.bar)
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

Or should there be a separate modification notice? And how should this notice look?

In particular, what is the recommended way of creating a "prominent notice" for a bigger project that includes a modified version of another smaller project licensed under GPL v3.0?

2

The GPL makes a distinction between verbatim copies and modified copies. For that, it is relevant who made modifications. For example, this affects who has to provide source code in which form.

This measure also protects users. For example, I'm not allowed to surreptitiously insert unwanted behaviour into the software and pass it off as the original – unless I make it clear that I modified the software.

How you make these notices is secondary, as long as they are prominent. This depends on the context. My personal preference is to gather such notices in a LICENSE file, or exercise the additional terms mechanism to implement a NOTICE file as in the Apache 2 license, or to include such information in a README. In practice, where only the source form of the software is provided via a version control system, the version control information might suffice as prominent notice.

Adding a copyright notice to a source file may or may not be sufficiently prominent notice. It definitely isn't a prominent notice if you distribute binaries of that software. It may also not be sufficiently prominent if it is just one source file of many.

But if the place of the notice is prominent, then I think that using a copyright notice is fine as the notice of modification. It seems like a very reasonable approach that is not excessively obtrusive but still sufficiently prominent.

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