[swift-users] How much memory does withMemoryRebound bind

[Etan Kissling]

Thanks once more!

It all depends on how withMemoryRebound works.

If there is any way where it actually copies the bound memory to a separate place, then passes that copy to the closure,
it would not be a good idea to write past the end of the bound memory.

If withMemoryRebound is a simple cast like in C, everything would be fine (as we know there is an underlying sockaddr_storage big enough),
but since we only pass the smaller size to withMemoryRebound, I'm not sure of the semantics.

In the end, if it would be a simple cast, I wonder what the "capacity" parameter of withMemoryReboundis actually used for...

I also "believe" it is correct but I want to know for sure :)


> On 29 Dec 2016, at 16:17, Rien <Rien at Balancingrock.nl> wrote:
> 
> Ah, ok.
> 
> In that case, I believe it is correct because sockaddr_storage is in fact big enough to hold either the IPv4 or IPv6 structure.
> When bits go unused, that causes no harm. And when “addr” goes out of scope, it will be completely deallocated, so no memory leak either.
> 
> Regards,
> Rien
> 
> Site: http://balancingrock.nl
> Blog: http://swiftrien.blogspot.com
> Github: http://github.com/Swiftrien
> Project: http://swiftfire.nl
> 
> 
> 
> 
>> On 29 Dec 2016, at 15:52, Etan Kissling <etan at scriptreactor.com> wrote:
>> 
>> I meant the question in a more generalized sense. 
>> The sockaddr example is just one that is easily understandable :-)
>> 
>> Thanks for the link though, interesting read!
>> 
>>> On 29 Dec 2016, at 14:47, Rien <Rien at Balancingrock.nl> wrote:
>>> 
>>> I used the code from http://blog.obdev.at/representing-socket-addresses-in-swift-using-enums/ in my package SwifterSockets (see github link below)
>>> 
>>> It does not answer your question exactly, but I think it is a rather better approach to sockaddr usage.
>>> 
>>> Regards,
>>> Rien
>>> 
>>> Site: http://balancingrock.nl
>>> Blog: http://swiftrien.blogspot.com
>>> Github: http://github.com/Swiftrien
>>> Project: http://swiftfire.nl
>>> 
>>> 
>>> 
>>> 
>>>> On 29 Dec 2016, at 14:27, Etan Kissling via swift-users <swift-users at swift.org> wrote:
>>>> 
>>>> Hi,
>>>> 
>>>> When calling POSIX accept, the common way is to
>>>> sockaddr_storage addr = {0};
>>>> sockaddr_len addrlen = 0;
>>>> int clientFd = accept(serverFd, (sockaddr *) &addr, &addrlen);
>>>> 
>>>> In Swift, this translates to
>>>> var addr = sockaddr_storage()
>>>> var addrlen = sockaddr_len(0)
>>>> int clientFd = withUnsafeMutablePointer(to: addr) {
>>>> $0.withMemoryRebound(to: sockaddr.self, capacity: 1) { addr in
>>>>     Foundation.accept(socket, addr, &addrlen)
>>>> }
>>>> }
>>>> 
>>>> Since sockaddr is smaller than sockaddr_storage, I wonder if this is correct.
>>>> 
>>>> If withMemoryRebound would be the same as the simple C cast, it would be okay.
>>>> However, since it also requires passing the capacity, I wonder if there may be cases
>>>> where it actually copies out the memory region, which could lead to memory corruption.
>>>> 
>>>> ==> How can I guarantee that withMemoryRebound binds the complete sockaddr_storage,
>>>>    and prevent cases where only the first MemoryLayout<sockaddr>.size bytes are bound?
>>>> 
>>>> Thanks
>>>> 
>>>> Etan
>>>> _______________________________________________
>>>> swift-users mailing list
>>>> swift-users at swift.org
>>>> https://lists.swift.org/mailman/listinfo/swift-users
>>> 
>> 
>