sharing sockets

I read that
1. It is possible for two processes to share/own the same socket
2. For one process to handover a socket it owns to other process.

Is this really possible, if so how ?

For 1. I can think of forked processes sharing sockets created before
forking, Is there some other way to do this ? I cannot think of anyway
of doing 2.

thanks,
v

Hi!

> I read that
> 1. It is possible for two processes to share/own the same socket

Correct.

> 2. For one process to handover a socket it owns to other process.
>
> Is this really possible, if so how ?

Yes it is. In fact, you might even pass any descriptor (which might
refer to a file, a pipe, a FIFO, a socket...) between processes.

The technic is described in Stevens, Unix Network Programming, Vol 1,
2nd ed, §14.7 (pp381-389)

If you want, we might give you an example how to do this.


> For 1. I can think of forked processes sharing sockets created before
> forking, Is there some other way to do this ?

On linux, yes. You might created a process using clone() with the flag
CLONE_FILES. Doing so, parent and child share the same descriptors, and
operation on a descriptor is visible in both processes. That is if you
close a descriptor in the child, it will be also closed in the parent
(this differs from a forked child: in a forked child, the descriptor
would be only closed in the child).


Cheers,
Loic.

Loic Domaigne wrote:
> Hi!
>
>> I read that
>> 1. It is possible for two processes to share/own the same socket
>
>
> Correct.
>
>> 2. For one process to handover a socket it owns to other process.
>>
>> Is this really possible, if so how ?
>
>
> Yes it is. In fact, you might even pass any descriptor (which might
> refer to a file, a pipe, a FIFO, a socket...) between processes.
>
> The technic is described in Stevens, Unix Network Programming, Vol 1,
> 2nd ed, §14.7 (pp381-389)
>
> If you want, we might give you an example how to do this.
>
>
>> For 1. I can think of forked processes sharing sockets created before
>> forking, Is there some other way to do this ?
>
>
> On linux, yes. You might created a process using clone() with the flag
> CLONE_FILES. Doing so, parent and child share the same descriptors, and
> operation on a descriptor is visible in both processes. That is if you
> close a descriptor in the child, it will be also closed in the parent
> (this differs from a forked child: in a forked child, the descriptor
> would be only closed in the child).
>
>
> Cheers,
> Loic.

What functions calls are used to pass a file descriptor to another
process without fork()'ing or clone()'ing the process?

You send it via a Unix domain socket(AF_UNIX) and sendmsg() call. There
are specific things you need to set in struct msghdr. The stevens book
contains sample code
thanks,
V