C / C++ / MFC

1-Jul-09 5:02

I have a program running on several PCs at the same time (VS6.0, C++, MFC). The customers data files are stored centrally on the network. He's complaining that the same data file can be open on different computers at the same time and changes made to one are being overwritten by the other.

Is it my problem to check if two PCs are accessing the same file or is it an IT function?

If it is my problem is there anyway to detect programatically whether a file is already open somewhere else?

Thanks

David Crow1-Jul-09 5:26

1-Jul-09 5:26

Caslen wrote:
If it is my problem is there anyway to detect programatically whether a file is already open somewhere else?

Open it exclusively. All other attempts will fail.

"Old age is like a bank account. You withdraw later in life what you have deposited along the way." - Unknown

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

Caslen1-Jul-09 5:33

1-Jul-09 5:33

CFile::shareExclusive <-- Just what I need, thanks for the pointer.

I think shareDenyWrite would be better though allowing subsequent users read only access is that correct? Does it display a message indicating read only?

Stuart Dootson1-Jul-09 6:12

Stuart Dootson

1-Jul-09 6:12

Caslen wrote:
Does it display a message indicating read only?

What is 'it'?

Anyway - don't do that, because you still have the problem of other users reading the file while you're writing to it (because file writes aren't atomic operations) and potentially getting an inconsistent view of the file. That's why you get 'single writer, multiple reader' locks for use with in-memory resources - the resource is locked exclusive while the writer's active, but otherwise, multiple people can read the resource concurrently.

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

Caslen1-Jul-09 6:18

1-Jul-09 6:18

Stuart Dootson wrote:
What is 'it'?

If a user tries to open an already open file is a message displayed but obviously that is up to the programmer to display the message. I've been rooting through the code (not mine) and it uses IStorage objects opened with the STGM_SHARE_EXCLUSIVE flag set. Is there any condition where this might not prevent a second copy from being opened?

Stuart Dootson1-Jul-09 7:05

Stuart Dootson

1-Jul-09 7:05

Sorry, don't know - IStorage isn't something I've used

Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p

David Crow1-Jul-09 7:18

1-Jul-09 7:18

Caslen wrote:
I've been rooting through the code (not mine) and it uses IStorage objects opened with the STGM_SHARE_EXCLUSIVE flag set. Is there any condition where this might not prevent a second copy from being opened?

Yes. It prevents others from subsequently opening the object in any mode.

Caslen1-Jul-09 21:54

1-Jul-09 21:54

I'm unsure of how the file is prevented from opening and particularly in my case where it clearly is allowed to be opened in some (but not all) cases.

On site there are several versions of software in use - it is possible that an older version does not use the STGM_SHARE_EXCLUSIVE flag. If this older version first opened the data file would the new version with the flag still be able to open it?

What about the opposite situation where the new verion opens the data file first - would the older version be 'aware' that the file is already open if it doesn't recognise the STGM_SHARE_EXCLUSIVE flag?

Thanks for your help on this.

David Crow2-Jul-09 3:55

2-Jul-09 3:55

The file system keeps track of what files are open and with what locking/sharing attributes. When you use an API to open a file, it will ask the file system if that is currently allowable. If it's not, the API will return a value indicating such.

Caslen2-Jul-09 5:15

2-Jul-09 5:15

Ok thats beginning to make more sense, the older version of software is probably opening the file, not marking it as open so the newer version can still open it, the newer version is modifying the data file and saving it before the older version then saves an unmodified data file. This ties in with the customers complaint that he is losing changes he's making to his files.

A couple more questions (sorry!) and probably drifting out of scope here but...

1. Would the file system itself be smart enough to know that a file has been changed since it was opened? (ie if another PC modifies the file between the first PC opening it and closing it)

2. On a typical file system would it be possible to automatically mark a file as already open?

Thanks

Steve

David Crow2-Jul-09 5:18

2-Jul-09 5:18

Caslen wrote:
1. Would the file system itself be smart enough to know that a file has been changed since it was opened? (ie if another PC modifies the file between the first PC opening it and closing it)

No.

Caslen wrote:
2. On a typical file system would it be possible to automatically mark a file as already open?

What do you mean by "automatically?"

Caslen2-Jul-09 5:30

2-Jul-09 5:30

When the file system receives a request to open a file it marks the file as open and refuses access to further requests to open it until the first user closes his copy. This could cause problems - what if the first user never closes the file?

David Crow2-Jul-09 5:33

Class Template in Visual C++

2-Jul-09 5:33

Caslen wrote:
what if the first user never closes the file?

Then subsequent requests will fail.

emira671-Jul-09 4:32

emira67

1-Jul-09 4:32

Hello,

I formed class template that represent two dimensional array :

template<class T>
class Array2D {
protected:  

public:
      T *data;

        const unsigned rows, cols, size;
        Array2D(unsigned r, unsigned c) : rows(r), cols(c), size(r*c) {

      data = new T[size];
        }
        ~Array2D() { delete data; }
        void setValue(unsigned row, unsigned col, T value) { 
                data[(row*cols)+col] = value;
        }
        T getValue(unsigned row, unsigned col) const {
                return data[(row*cols)+col];
        }
};

And one dimensional data array is present:

array<double>^ input={20,4,6,15,7,2,1,8,9};

I have function it's inputs are class template and one dimensional data array.I set one dimensional array values to class template :

void function(Array2d<double> &x,array< double>^ input,int width,int,height)
{


double temp;
temp1=0;

double temp2;

int i,j;

// assign input array values to two dimensional array
for(i=0;i<width;i++)
for(j=0;j<height;j++)
{
{ temp2=input[temp1];
  x.setvalue(i,j,temp2);
  temp1=temp1+1;
}
}

}

After all I declared 3x3 class template:

Array2D<double>A(3,3);

I sent it to function:

function(A,input,3,3);

And I tried to print Array2D values:

double temp;
for(i=0;i<3;i++)
for(j=0;j<3;j++)
{
{temp=A.getValue(i,j)

Console:Write("{0}",temp);}}

I executed this applicaiton in CLR Application in Visual Studio 2008 amd it worked .But I want to implement this code on Windows Form Application,and it gave error like these on Form Application:

error C2065: 'Array2D' : undeclared identifier
error C2065: 'A' : undeclared identifier

How can I overcome this error or where should I locate class template?

Best Regards...

Re: Class Template in Visual C++

led mike1-Jul-09 4:40

led mike

1-Jul-09 4:40

emira67 wrote:
error C2065: 'Array2D' : undeclared identifier
error C2065: 'A' : undeclared identifier

If you don't know how to resolve those compiler errors then you are attempting something that is beyond your current C++ programming capacity. You need to go back to basics and learn more about them before you move on to more advanced subjects.

Also I do not recommend moving on to using C++/CLI (managed or mixed mode) until you have a firm grasp of C++.

Re: Class Template in Visual C++

David Crow1-Jul-09 4:47