[Qt-interest] passing socket descriptors between processes

[Thomi Richards]

Hi,

I have a need to pass a connected TCP socket descriptor between two Qt
applications. Essentially what I'm after is to have one process that
listens on a particular port and accepts incoming connectsions, then
spawns another copy of itself running as a different user. This new
process then handles all traffic on the original socket. After a bit
of trial and error I've found that it's not as easy as calling
QAbstractSocket::setSocketDescriptor(...). There are two methods I've
found that should work:

The first is to use an AF_UNIX domain socket to pass the socket
descriptor from the server to the client process. This method has been
discussed here:
https://www.linuxquestions.org/questions/programming-9/how-to-share-socket-between-processes-289978/

The second method is to create the new process by calling fork() - in
the child process the socket descriptor should still be available.

I was hoping that someone on this list has tried to do something
similar. As I see it, neither of the possible solutions are great.
Using UNIX sockets will obviously only for for machines that support
that particular mechanism of IPC. Using fork() seems a little risky as
well. I've tried to create a test app for this scenario, but I'm
having difficulty. I *think* there's a problem whereby forking() after
QCoreApplication::exec() has been called leads to some strange
behaviour. Can anyone suggest a better way to do this? I need to
support Macs, windows, and Linux boxes, so whatever method I choose I
need to be able to get it going on multiple operating systems without
too much hacking.

I'd be very grateful for any hints you may have.


Cheers,