Global shared memory in a system service

I have a system service running that created a global shared memory buffer using file mapping. The service runs fine, but when i try to have an application access the file mapping, the call to CreateFileMapping() returns 5, access denied. Here is the code

Code:

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hCommandBuffer = CreateFileMapping((HANDLE)INVALID_HANDLE_VALUE , NULL , PAGE_READWRITE | SEC_COMMIT , 0 , sizeof(COMMANDBUFFER) , "Elysia Makes Me Hot Buffer");
if(hCommandBuffer == NULL){
  sprintf(szError , "%d\n" , GetLastError());
  MessageBox(NULL , szError , "Failed to allocate handle to global shared memory" , MB_OK);
  }
pCommandBuffer = (COMMANDBUFFER*)MapViewOfFile(hCommandBuffer , FILE_MAP_WRITE , 0 , 0 , sizeof(COMMANDBUFFER));
if(pCommandBuffer == NULL){
  sprintf(szError , "%d\n" , GetLastError());
  MessageBox(NULL , szError , "Failed to allocate global shared memory" , MB_OK);
  }

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If I run the application without the system service installed, it doesnt have a problem, So I think it might have something to do with system services running at higher (lower numerically) privledge level.

Does the code work outside of a sys serv?
Are you sure another instance of the serv isn't doesn't already have it mapped?
Does the serv have file handling permissions?

Quote:

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Originally Posted by Yarin

Does the code work outside of a sys serv?

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Yes, the application cretes the file mapping fine. The system service also creates it fine, but the application get denied access if the system service has already created a file mapping. Both the service and the application shoudl be able to map the file at the same time, to achieve interprocess memory sharing.

Quote:

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Are you sure another instance of the serv isn't doesn't already have it mapped?

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Wouldnt matter, since they are supposed to be able to share the mapping this way.

Quote:

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Does the serv have file handling permissions?

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I am assuming so, since the service runs fine. Only the application has a problem, and then only when the file mapping has already been created.

UPDATE

I solved the problem by having the application create the file mapping. This is a bit of a kludge since the system service has to keep trying to open the mapping, which consumes clock cycles until it find it, but at least i am getting proper function now.

The problem is most likely due to the service running under a different account, which causes different default permissions to be used when you pass NULL to "LPSECURITY_ATTRIBUTES" parameter(s).

Here are some options:
Create a Null DACL, which is basically "wide-open" security. Access will never be denied.

Code:

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    SECURITY_ATTRIBUTES SecAttr, *pSec = 0;
    SECURITY_DESCRIPTOR SecDesc;

    if (InitializeSecurityDescriptor(&SecDesc, SECURITY_DESCRIPTOR_REVISION) &&
        SetSecurityDescriptorDacl(&SecDesc, TRUE, (PACL)0, FALSE))
    {
        SecAttr.nLength = sizeof(SecAttr);
        SecAttr.lpSecurityDescriptor = &SecDesc;
        SecAttr.bInheritHandle = TRUE;
        pSec = &SecAttr;
    }//if

    // use pSec for any LPSECURITY_ATTRIBUTES parameters

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Or, here's sample code that'll give you a little more security:
http://msdn2.microsoft.com/en-us/lib...95(VS.85).aspx
or
http://msdn2.microsoft.com/en-us/lib...98(VS.85).aspx

gg

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