Portable way to pass file descriptor between different processes

Question

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?

Answer 1

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.

Answer 2

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.