In some countries such copyright assignments are in fact invalid.

Germany had to amend the law back in the day to make Open Source legal.

Before that it wasn't possible to forgoe payment and even now it's not possible in many cases.

do you have a reference to that law change?

as far as i know only the moral rights can not be given up, but i am unaware that there was an issue with giving away work for free.

Contracts typically require consideration. It is important part of contract law.

There are lot of nuances what constitutes consideration, it varies by jurisdiction and also there is lot of common law ( i.e. based on judicial precedents ) that can be complicated and only experts could really could even a given opinion on that and only way to be really know is after that issue comes up in a case for first time in that jurisdiction.

That's not true in most of continental Europe, nor in Quebec (Canada), since their legal system has a different origin than what you're describing.

Using Quebec's terminology by way of example, a contract where an advantage is given in exchange for rendering the obligation (so roughly a contract with consideration) is called an onerous contract, and most typical contracts for services fall into this category. The kind where one obligates oneself without receiving an advantage in return is called a gratuitous contract, and they classify gifts in this category.

IANAL, but as far as I know consideration is usually a feature of common law not civil law.

Correct. As per my direct reply to the person you're responding to, the kind of concept that's analogous to a contract with consideration is called an onerous contract and the kind that lacks it is a gratuitous contract. (Quebec terminology; it's a standard continental European style civil law system with its ultimate main origin in the Custom of Paris which governed most French colonies until 1763.)

I found an article on the german Wikipedia

https://de.m.wikipedia.org/wiki/Linux-Klausel

The referenced article is §32 UrhG

thank you. so it wasn't that it was not possible to give away stuff for free but rather that there was a proposal to change the law which would have made it difficult, if not impossible, and as a result the added clause ensured that it would remain possible, at least as long as it is given to the general public.