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:

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);
  }

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?

Originally Posted by Yarin

Does the code work outside of a sys serv?

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.

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

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

Does the serv have file handling permissions?

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:

    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

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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