Is it possible to share memory between docker containers?

As suggested by @jpetazzo I looked ad the source of Docker and also with the help of devs guys on #docker-dev I successfully recompiled Docker in order to drop the IPC namespace.

To achieve this, it is necessary to comment the line "NEWIPC": true, in the file default_template.go located in the folder docker/daemon/execdriver/native/template of the Docker source code.

The old code now works perfectly.

The --ipc=host and --ipc=container:id options have since been added to the Docker create and run commands to share IPC resources.

--ipc=""  : Set the IPC mode for the container,
             'container:<name|id>': reuses another container's IPC namespace
             'host': use the host's IPC namespace inside the container

IPC with the host

docker run --ipc=host <image>

IPC with another container

docker run --ipc=container:<id> <image>

IPC with another container may need the shareable option set on the initial container (if dockerd defaults IPC to private)

docker run --ipc=shareable <image>

Technically, you can share the same IPC namespace between containers, but Docker doesn't support that (yet).

If you can use mmap() instead of IPC, then you could share a volume between both containers, and map a file on that volume; it will be the same file, and therefore be shared correctly.

If you really need to share the IPC namespace (because you can't change the existing code), then it's time to write some Go code and contribute it to Docker :-)

The easiest path would probably be to add a flag to the libcontainer binding, so that you can start a container reusing the IPC namespace of the host (or of another container). Check the implementation of the --net flag, since it achieves exactly that, but for the network namespace.

Here how it is work:

start container with shareable ipc

docker run -it --rm --ipc="shareable" --name cont1 ubuntu

then start next container and share with its ipc

docker run -it --rm --name cont2 --ipc container:cont1 ubuntu