Use file mapping to share data

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I am trying to share transient data between instances of a DLL by means of File Mapping. The first time this is done, everything needs to be initialised. How does an instance know whether CreateFileMapping() actually creates the object, or alternatively maps an existing object?

Posted 23-May-10 13:01pm

Bram van Kampen

Updated 23-May-10 14:08pm

Moak

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Sauro Viti · Accepted Answer · 2010-05-23T21:59:00

From the MSDN library (see http://msdn.microsoft.com/en-us/library/aa366537(VS.85).aspx[^]):

Return Values

If the function succeeds, the return value is a handle to the file mapping object.

If the object exists before the function call, the function returns a handle to the existing object (with its current size, not the specified size), and GetLastError returns ERROR_ALREADY_EXISTS.

If the function fails, the return value is NULL. To get extended error information, call GetLastError.

Then you should do the following:

call CreateFileMapping to get an handle to the object required

if the returned handle is NULL, call GetLastError to get extended informations about the failure

if the returned handle is not NULL call GetLastError and test the returned code: if the code is ERROR_ALREADY_EXISTS, then you have got an handle to a shared object that already exists, otherwise the object has been created for the first time