MapViewOfFileEx - valid lpBaseAddress

Question

In answer to a question about mapping non-contiguous blocks of files into contiguous memory, here, it was suggested by one respondent that I should use VirtualAllocEx() with MEM_RESERVE in order to establish a 'safe' value for the final (lpBaseAddress) parameter for MapViewOfFileEx().

Further investigation revealed that this approach causes MapViewofFileEx() to fail with error 487: "Attempt to access invalid address." The MSDN page says:

"No other memory allocation can take place in the region that is used for mapping, including the use of the VirtualAlloc or VirtualAllocEx function to reserve memory."

While the documentation might be considered ambiguous with respect to valid sequences of calls, experimentation suggests that it is not valid to reserve memory for MapViewOfFileEx() using VirtualAllocEx().

On the web, I've found examples with hard-coded values - example:

#define BASE_MEM     (VOID*)0x01000000

...

hMap = MapViewOfFileEx( hFile, FILE_MAP_WRITE, 0, 0, 0, BASE_MEM );

To me, this seems inadequate and unreliable... It is far from clear to me why this address is safe, or how many blocks can be safely be mapped there. It seems even more shaky given that I need my solution to work in the context of other allocations... and that I need my source to compile and work in both 32 and 64 bit contexts.

What I'd like to know is if there is any way to reliably reserve a pool of address space in order that - subsequently - it can be reliably used by MapViewOfFileEx to map blocks to explicit memory addresses.

Answer 1

You almost got to the solution by yourself but fell short of the last small step. As you figured, use VirtualAlloc (with MEM_RESERVE) to find room in your address space, but after that (and before MapViewOfFileEx) use VirtualFree (with MEM_RELEASE). Now the address range will be free again. Then use the same memory address (returned by VirtualAlloc) with MapViewOfFileEx.