On most UNIX systems passing an open file between processes can be easily done for child/parent processes by fork(); however I need to share a fd "after" the child was already forked.
I've found some webpages telling me that sendmsg() may work for arbitary processes; but that seems very OS dependent and complex. The portlisten seems like the best example I can find, but I'd prefer a good wrapper library like libevent that hides all the magic of kqueue, pool, ....
Does anyone know if there's some library (and portable way) to do this?
Your best bet is to try sending the file descriptor over a Unix domain socket. This is described in Stephens, and in a few places on the web, but I can dig up code for you if you ask nicely.
This will be pretty portable these days; a lot of the things considered "non-portable" way back when (such as mmap!) are extremely common now. If you need to be more portable than "most systems these days," you've got a lot of interesting issues ahead of you, but possibly if you tell us more about what you're doing and what platforms you're working on (perhaps non-Unix POSIX platforms?) we might be able to help out.
There is a Unix domain socket-based mechanism for transferring file descriptors (such as sockets - which cannot be memory mapped, of course) between processes - using the sendmsg() system call.
You can find more in Stevens (as mentioned by Curt Sampson), and also at Wikipedia.
You can find a much more recent question with working code at Sending file descriptor by Linux socket.
