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1389

answers:

4

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?

A: 

Have you considered memory mapping the file in a shared memory space to do the sharing?

Another option would be to use standard IPC mechanisms like pipes.

Vinko Vrsalovic
+4  A: 

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" why 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.

Curt Sampson
I still have a question in using the sendmsg(). I thought putting domain socket as the first parameter is enough, but seems like I still have to fill the msghdr.msg_name as the sockaddr_un of the domain socket - why? anyway to prevent it?
Francis
+2  A: 

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.

Jonathan Leffler
+2  A: 

I've found (from another question) a good library: libancillary. This is simply what I need!

Francis