Re: CreateFile/mailslot

From:

"Ben Voigt [C++ MVP]" <bvoigt@newsgroup.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 4 Aug 2009 11:01:59 -0500

Message-ID:

<A9C1DD3C-07C0-457A-9B08-B68A2933C554@microsoft.com>

"Vincent Fatica" <vince@blackholespam.net> wrote in message
news:4a77b478$1@news.vefatica.net...

On Mon, 3 Aug 2009 22:08:43 -0400, "Igor Tandetnik" <itandetnik@mvps.org>
wrote:

|Vincent Fatica wrote:
|> (VC9/XPSP3) What's going on here? There are no mail slots, certainly
|> not one with the name below. Yet CreateFile() succeeds. This is a
|> simplified (and tested) version; originally I inadvertently created
|> the slot name with wsprintf() and a quoted host name ("\".\"").
|>
|> HANDLE hSlot = CreateFile(L"\\\\\".\"\\mailslot\\my_slot",
|> GENERIC_WRITE, FILE_SHARE_READ, NULL, OPEN_EXISTING, 0, NULL);
|> wprintf(L"hSlot = %X\r\n", hSlot);
|>
|> I see the likes of:
|>
|> hSlot = E88
|>
|> Why does CreateFile() succeed and what do I have a handle to? Note
|> that a subsequent WriteFile() returns after about 5 seconds.
|
|If I had to guess, I'd imagine CreateFile opened a handle to a mailslot
|on a remote host named "." (including quotes), and WriteFile actually
|tried to access said host.

That was my guess too. Indeed, after WriteFile(), GetLastError() returns
53
(network path not found).

I guess I misunderstood. Apparently CreateFile() on a mailslot is purely
local,
giving no indication that a remote mailslot doesn't exist until you try to
use
it ... right?

WinNT Mailslots are datagrams, connection-less, so the success or failure
can actually be different for each and every write attempt, just as it would
be with a write to a datagram socket.

For more detail, see
http://technet.microsoft.com/en-us/library/cc958776.aspx

I'm doing better with a named pipe. I have one question about named
pipes:

The server reads from the named pipe and then writes L"OK" to it. If the
server
then immediately calls DisconnectNamedPipe() a **remote** client does not
get
the OK (a local client does). The pipe was created in blocking mode. How
do I
ensure that the client has read the response before calling
DisconnectNamedPipe(). Thanks.

--
- Vince