It is possible to delete SharedMemory from another Application??

Question

I have created DLL to open and manage shared memory.The shared memory is created using CreateFileMapping(), MapViewOfFile(). Then I loaded the DLL using MFC  application and Read/Write performed to Shared memory using shared pointer returned by DLL( via MapViewOfFile()). The read/write operations performed  with the MFC app is working fine. But I got an exception when tried to delete the shared pointer directly from the MFC app. Could you please help me to find the reason why exception thrown for shared pointer deletion from the application?It is supposed to release using UnMapViewOfFile() only?I can delete if the DLL returns a normal pointer.

Thanks in advance

Answer 1

Jaydhar wrote:

I have created DLL to open and manage shared memory.The shared memory  is created using CreateFileMapping(), MapViewOfFile(). Then
I loaded the DLL using MFC application and Read/Write performed to  Shared memory using shared pointer returned by DLL( via
MapViewOfFile()). The read/write operations performed with the MFC app  is working fine. But I got an exception when tried to
delete the shared pointer directly from the MFC app.

What do you mean, delete? Do you mean this literally, as in "delete  myPointer;" ? Of course that won't work - you can only deallocate with  delete what you allocated with new (and deallocate with free() what you  allocated with malloc(); and deallocate with HeapFree what you allocated  with HeapAlloc; and so on and so forth). Allocation and deallocation  functions come in pairs, you can't mix and match them.

It is supposed to release using UnMapViewOfFile() only?

Yes.

I can delete if the DLL returns a normal pointer.

What's a "normal pointer"? I'm not familiar with the term. Are there  abnormal ones?

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Igor Tandetnik

Answer 2

Hi Igor,

Thank you for the reply.

What I meant by delete pointer is to free the memory which is allocated either by malloc or heapalloc. Previously I gt confused with the pointer to Shared memory and now I understood the difference.

The normal pointer which points somewhere in memory.It is simple and nothing abnormal:). Hopefully it'll  cleared by owner at some point.Other kinds are smart pointer and shared pointer.

Jay

Answer 3

Jaydhar wrote:

The normal pointer which points somewhere in memory.

The pointer returned by MapViewOfFile also points somewhere in memory,  and thus, by your definition, is normal.

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Igor Tandetnik

Answer 4

check this link this may help you out

http://social.msdn.microsoft.com/Forums/en-US/vcgeneral/thread/b3cf8d9b-187d-4895-b831-89dbb1897b0a

Answer 5

If the pointer returned by MapViewOfFile can be called as Normal then why should the de-allocation  failed using delete operator?.So it behaves like special and difficult to manage as normal. As discussed earlier de-allocation works fine for those which is allocated using Malloc and HeapAlloc. But we used  MapViewOfFile to reserve the  pages  in virtual address range (using VirtualAlloc )and is unavailable to other allocations. The reserved pages can only be freed using the VirtualFree API. Reserved are released when the view is unmapped and the file mapping object is closed. Actually this is the only point I want to know. Thanks for your help.

Answer 6

Hi Kevin,

Thanks for your help.

The link you put seems my question asked in the same thread:).

Jay

Answer 7

Jaydhar wrote:

If the pointer returned by MapViewOfFile can be called as Normal then  why should the de-allocation failed using delete
operator?

Because the memory wasn't allocated with new operator.

So it behaves like special and difficult to manage as normal.

This is begging the question. You seem to have changed your definition  of the term "normal pointer" (to mean "a pointer to memory allocated  with new", as far as I can tell), introduced a new term "special  pointer" (to mean "a pointer to memory allocated by any menas other than  new", apparently), and then ask why special pointers don't behave like  normal pointers.

As discussed earlier de-allocation works fine for those
which is allocated using Malloc and HeapAlloc.

It is possible that, in a particular implemenation, free() simply  delegates to HeapFree, and operator delete simply delegates to free()  (and so ultimately calls HeapFree). This is an internal undocumented  implementation detail subject to change without notice (and in fact,  this has not always been the case, and is not the case now under certain  configuration settings). It is a bad idea to rely on being able to mix  and match allocation and deallocation functions this way.

But we used MapViewOfFile to reserve the pages in virtual address  range (using
VirtualAlloc )

I don't understand this statement. What exactly is the relationship  between MapViewOfFile and VirtualAlloc in your program? How are the two  combined?

The reserved pages can only be freed using the VirtualFree API

As usual, allocation and deallocation functions come in pairs. Memory  allocated with VirtualAlloc should be deallcoated with VirtualFree. This  should come as no surprise by now.

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Igor Tandetnik