Re: shareDenyWrite is not enough
OK. You either allow writes or don't allow shared writes. If you don't allow
shared writes, only one process will be able to open the file for writing,
no other process will be able to open the file for writing AND reading. If
you allow shared writes, all processes will be able to open the file for
reading AND writing. If you want exclusive write access, use LockFileEx.
There is no way to allow writing to a file based on trusted/untrusted
process. Only on basis of account-based access control.
ANother way would be to use DuplicateHandle, when a writer opens the file in
exclusive mode, and then hands the duplicates of the handle (with read-only
access) to its brethren. The handle would need to be opened in overlapped
mode, and you cannot use stdio to work with those.
Yet another way would be to use a service to access the file. The client
processes would request reading and writing to it.
"Alexander" <the44secs@yahoo.com> wrote in message
news:03400894-db1f-4a7a-af7d-2397a61d4502@m3g2000hsc.googlegroups.com...
The file was created with modeWrite and modeCreate. However, this file
is created by another program at a different time and it has no
bearing on the issue at hand.
Consider that the flags I posted (modeRead, shareDenyNone) allow
shared reads while modeRead alone doesn't. Therefore, the problem is
simply how to allow shared reads while disallowing shared writes.
I don't want to disallow shared writes in order to block another
process. None of the process will try to write to this file. I'm just
trying to prevent accidental writing to this file by _unknown_parties_
while my processes are reading it. In short, I'm trying to forsee the
unforseeable and I thought it should be a simple issue of setting the
right flags... it doesn't seem so, though.
On Jul 22, 7:30 pm, Victor <Vic...@discussions.microsoft.com> wrote:
And how was this file created? With which flags?
The correct ones should have been:
modeRead | modeWrite | modeCreate (and modeNoTruncate as optional)
+
shareDenyWrite
In this case all other Open operations might be successful ONLY with the
flags
modeRead | shareDenyNone
Note also, that if the process created this file exits, then it must set
attributes of this file as "Read Only", otherwise any other process will
be
able to write to it.
Victor
"Alexander" wrote:
There is very little mystery, I'm afraid. Here it is. All threads use
this same code:
CStdioFile fIn;
if( fIn.Open( csPath, CFile::modeRead | CFile::shareDenyNone ) )
{
...
}
That code works but, naturally, it allows shared write which is what I
want to prevent.
If shareDenyNone is taken out, shared read is not allowed.
On Jul 22, 1:57 am, Victor <Vic...@discussions.microsoft.com> wrote:
Could you show your exact code you are using to open this file?
Please, check it in all processes you are currecntly using.
Victor.
"Alexander" wrote:
Yep. All processes had those two flags shareDenyWrite and modeRead.
I still don't understand why this is a problem for shared reads.
On Jul 21, 4:55 am, "Tom Serface" <tom.nos...@camaswood.com> wrote:
I bet you need EVERY process trying to access the file to use
shareDenyWrite
and if any of them are opening it in compatibility mode or with
any other
access type (like modeRead) you will run into these problems.
Tom
"Alexander" <the44s...@yahoo.com> wrote in message
news:3943299e-d6bb-4115-9ea3-974d04dae0eb@x36g2000prg.googlegroups.com....
That was my conclusion. No flags implies neither shared read or
write
are allowed... which is weird because it means that in order to
allow
shared read I need shareDenyNone... unless shareDenyWrite
authorizes
shared reading as a side effect which is bizarre.
Ok, guys. I'm confused. Here is the data:
1. The file is not created by these processes (it is guaranteed to
exist beforehand) and it is never updated.
2. I want all processes to be able to read it simultaneously.
3. This is a text file and all processes use the class CStdioFile
to
handle it.
4. Which are the flags (or lack of) that allow simultenous read
and
lock _everything_ else out?
Thank you all.
On Jul 20, 3:39 pm, "Anthony Wieser" <newsgroups-sanss...@wieser-
software.com> wrote:
"Alexander" <the44s...@yahoo.com> wrote in message
news:daf6776b-f790-46d2-a071-934348ec5a09@p25g2000pri.googlegroups.com....
Thank you for the reply.
The answer is no. All process use either shareDenyWrite or
nothing.
When you say nothing, do you mean ZERO?
If so, that means share exclusive, denying access to any other
open..
Anthony Wieser
Wieser Software Ltd