As far as I know mutex is not a signal that data is received, which would require some kind of threading / interrupt.

Moreover, it's a bit of a secondary thing.

The API would look something like a call function with a receive function/lambda on the receiving side, with memory possibly being 'handed over' or whatever. And so the level of abstraction would be slightly higher.

And it would be a no-brainer, well established thing. Searching for how to do that today and it's always complicated.

I’m afraid I don’t understand. If you want one process to signal to another that data is available, you have the receiver block on a condition variable. When the sender writes its data it signals the condition variable. This is all handled by the kernel, no signal/interrupt is required. Or if you don’t want blocking, you can have a shared ring buffer, with the two processes interacting with it atomically.