I checked the error; "Invalid argument".
> But I've not yet heard about a buggy accept4() implementation...
This is a first, then :-) :
I blocked accept4() in favour of accept(), and now it works. Don't ask why.
Anyway, this is the software environment:
kernel 2.6.36, glibc 2.10.1. (as I said already, old stuff). Processor ARM 926 EJS, arm-linux-gnueabi.
Thanks everyone for your comments. I learned something from all of you.
@Silvio: Thanks for looking up the code, I'll save it for reference, it may be very useful in the future.