createfilemapping size constraints

GF 21

What is the maximum size that can be used with CreateFileMapping with INVALID_HANDLE_VALUE and SEC_RESERVE, for a virtual paged memory backed file, on windows 11 with 32 gb physical memory I can specify for example 64 gb but on windows 2016 it seems limited to about 4 gb (from a 32 bit program but it shouldn't matter since it's done by the o.s.) are there any limitations on size allocatable, should paging file be turned on or what caveats ?.

It seems to happen on a vmware hosted W2016, with say 8-10 gb ram allocated, but not on physical W11, could there be any CreateFileMapping constraints specific to windows as guest vm, that the o.s. detects being hosted and perhaps restricts size, I have the page file set to auto.

Jeanine Zhang-MSFT 11,106 Reputation points Microsoft External Staff

2025-03-04T06:44:33.2033333+00:00

According to the Blog：Creating a shared memory block that can grow in size

When you create a shared memory block, you can pass the SEC_RESERVE flag to CreateFileMapping, then the size you pass to the function is treated as a maximum rather than an exact size.

If the file mapping object is backed by the operating system paging file (the hfile parameter is INVALID_HANDLE_VALUE), specifies that when a view of the file is mapped into a process address space, the entire range of pages is reserved for later use by the process rather than committed.

Reserved pages can be committed in subsequent calls to the VirtualAlloc function. Once the memory is committed, it cannot be decommitted.

In my opinion, the maximum size depends on the operating system and system resources.
RLWA32 47,521 Reputation points

2025-03-04T08:15:06.9933333+00:00

Even on a 64-bit system the maximum address space size for a 32-bit process is 4GB.

I haven't tested your issue but it makes sense to me that Windows would not allow a 32-bit process to reserve memory amounts that exceed the size of the process address space.
Darran Rowe 1,511 Reputation points

2025-03-04T13:45:16.8333333+00:00

@RLWA32

I would be surprised though. Creating the mapping isn't the same as mapping it. There should be no difference from creating an 8GiB file on disk and creating a mapping object based on that. It is MapViewOfFile that determines the offset into the file and how much to map into the address space.

A 32 bit process is certainly able to reserve and commit as much as is available. On my system, I was able to reserve and commit 52GiB, and this is about what Task Manager reports as available.
RLWA32 47,521 Reputation points

2025-03-04T14:07:59.94+00:00

@Darran Rowe Like I said, I hadn't done a test. Wouldn't be the first time that I was wrong about a guess. :)

Did you get the same results when creating the file mapping object backed by the page file vs. an actual file on disk?
Darran Rowe 1,511 Reputation points

2025-03-04T14:53:57.2+00:00

@RLWA32

Yes. Windows is happy enough to hand out anything unused in the page file.
RLWA32 47,521 Reputation points

2025-03-04T14:56:58.93+00:00

@Darran Rowe Thanks for the follow-up.
RLWA32 47,521 Reputation points

2025-03-04T14:59:44.1766667+00:00

should paging file be turned on or what caveats ?.

@GF, How much physical memory was on the Windows 2016 system where you encountered the issue? Had you disabled the page file?
GF 21 Reputation points

2025-03-04T17:52:04.69+00:00

It seems to happen on a vmware hosted W2016, with say 8-10 gb ram allocated, but not on physical W11, could there be any CreateFileMapping constraints specific to windows as guest vm, that the o.s. detects being hosted and perhaps restricts size, I have the page file set to auto.
Jeanine Zhang-MSFT 11,106 Reputation points Microsoft External Staff

2025-03-07T03:31:47.1033333+00:00

Could you please provide a minimum sample to help us reproduce the issue?

Whether have you tried to reproduce the issue on physical w2016? Whether this issue only occurs on the specific to windows as guest vm?

GF 21

there isn't much to it, could it be a vmware restriction on the windows guest :

LONGLONG heapsize=(LONGLONG) 1024*1024*1024*8; // 8 gb

HANDLE heap= CreateFileMapping(INVALID_HANDLE_VALUE,


            NULL,// security

            PAGE_READWRITE | SEC_RESERVE,

            HIDWORD(heapsize), // hi

            LODWORD(heapsize),// lo

            NULL);

Darran Rowe 1,511 Reputation points

2025-03-08T14:12:28.74+00:00

What values are task manager reporting for memory, in particular the Committed values.

The result of using CreateFileMapping with a null handle is to have the mapping backed by physical RAM and page file, and the Committed value is how much is backed by the page file and memory versus total page file and memory available. My system has 64 GiB of RAM and the page file is set to 4 GiB (the default) which is why around 53 GiB is available.

Any requests for a file mapping less than this ~53 GiB will succeed, any requests greater will fail.

So does the VM guest have enough RAM + page file to allow the request to go through?

Accepted answer

RLWA32 47,521 Reputation points

2025-03-08T09:04:03.0333333+00:00
Give this a try -

ULARGE_INTEGER uli{}; uli.QuadPart = (ULONGLONG)1024 * 1024 * 1024 * 8; // 8 gb HANDLE heap = CreateFileMapping(INVALID_HANDLE_VALUE, NULL,// security PAGE_READWRITE | SEC_RESERVE, uli.HighPart, // hi uli.LowPart,// lo NULL); return 0;
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