MapViewOfFileEx function (memoryapi.h)

Maps a view of a file mapping into the address space of a calling process. A caller can optionally specify a suggested base memory address for the view.

To specify the NUMA node for the physical memory, see MapViewOfFileExNuma.


LPVOID MapViewOfFileEx(
  [in]           HANDLE hFileMappingObject,
  [in]           DWORD  dwDesiredAccess,
  [in]           DWORD  dwFileOffsetHigh,
  [in]           DWORD  dwFileOffsetLow,
  [in]           SIZE_T dwNumberOfBytesToMap,
  [in, optional] LPVOID lpBaseAddress


[in] hFileMappingObject

A handle to a file mapping object. The CreateFileMapping and OpenFileMapping functions return this handle.

[in] dwDesiredAccess

The type of access to a file mapping object, which determines the page protection of the pages. This parameter can be one of the following values, or a bitwise OR combination of multiple values where appropriate.

Value Meaning
A read/write view of the file is mapped. The file mapping object must have been created with PAGE_READWRITE or PAGE_EXECUTE_READWRITE protection.

When used with the MapViewOfFileEx function, FILE_MAP_ALL_ACCESS is equivalent to FILE_MAP_WRITE.

A read-only view of the file is mapped. An attempt to write to the file view results in an access violation.

The file mapping object must have been created with PAGE_READONLY, PAGE_READWRITE, PAGE_EXECUTE_READ, or PAGE_EXECUTE_READWRITE protection.

A read/write view of the file is mapped. The file mapping object must have been created with PAGE_READWRITE or PAGE_EXECUTE_READWRITE protection.

When used with MapViewOfFileEx, (FILE_MAP_WRITE | FILE_MAP_READ) and FILE_MAP_ALL_ACCESS are equivalent to FILE_MAP_WRITE.


Using bitwise OR, you can combine the values above with these values.

Value Meaning
A copy-on-write view of the file is mapped. The file mapping object must have been created with PAGE_READONLY, PAGE_READ_EXECUTE, PAGE_WRITECOPY, PAGE_EXECUTE_WRITECOPY, PAGE_READWRITE, or PAGE_EXECUTE_READWRITE protection.

When a process writes to a copy-on-write page, the system copies the original page to a new page that is private to the process. The new page is backed by the paging file. The protection of the new page changes from copy-on-write to read/write.

When copy-on-write access is specified, the system and process commit charge taken is for the entire view because the calling process can potentially write to every page in the view, making all pages private. The contents of the new page are never written back to the original file and are lost when the view is unmapped.

Starting with Windows 10, version 1703, this flag specifies that the view should be mapped using large page support. The size of the view must be a multiple of the size of a large page reported by the GetLargePageMinimum function, and the file-mapping object must have been created using the SEC_LARGE_PAGES option. If you provide a non-null value for lpBaseAddress, then the value must be a multiple of GetLargePageMinimum.
An executable view of the file is mapped (mapped memory can be run as code). The file mapping object must have been created with PAGE_EXECUTE_READ, PAGE_EXECUTE_WRITECOPY, or PAGE_EXECUTE_READWRITE protection.

Windows Server 2003 and Windows XP:  This value is available starting with Windows XP with SP2 and Windows Server 2003 with SP1.

Sets all the locations in the mapped file as invalid targets for Control Flow Guard (CFG). This flag is similar to PAGE_TARGETS_INVALID. Use this flag in combination with the execute access right FILE_MAP_EXECUTE. Any indirect call to locations in those pages will fail CFG checks, and the process will be terminated. The default behavior for executable pages allocated is to be marked valid call targets for CFG.

For file-mapping objects created with the SEC_IMAGE attribute, the dwDesiredAccess parameter has no effect, and should be set to any valid value such as FILE_MAP_READ.

For more information about access to file mapping objects, see File Mapping Security and Access Rights.

[in] dwFileOffsetHigh

The high-order DWORD of the file offset where the view is to begin.

[in] dwFileOffsetLow

The low-order DWORD of the file offset where the view is to begin. The combination of the high and low offsets must specify an offset within the file mapping. They must also match the memory allocation granularity of the system. That is, the offset must be a multiple of the allocation granularity. To obtain the memory allocation granularity of the system, use the GetSystemInfo function, which fills in the members of a SYSTEM_INFO structure.

[in] dwNumberOfBytesToMap

The number of bytes of a file mapping to map to a view. All bytes must be within the maximum size specified by CreateFileMapping. If this parameter is 0 (zero), the mapping extends from the specified offset to the end of the file mapping.

[in, optional] lpBaseAddress

A pointer to the memory address in the calling process address space where mapping begins. This must be a multiple of the system's memory allocation granularity, or the function fails. To determine the memory allocation granularity of the system, use the GetSystemInfo function. If there is not enough address space at the specified address, the function fails.

If lpBaseAddress is NULL, the operating system chooses the mapping address. In this scenario, the function is equivalent to the MapViewOfFile function.

While it is possible to specify an address that is safe now (not used by the operating system), there is no guarantee that the address will remain safe over time. Therefore, it is better to let the operating system choose the address. In this case, you would not store pointers in the memory mapped file, you would store offsets from the base of the file mapping so that the mapping can be used at any address.

Return value

If the function succeeds, the return value is the starting address of the mapped view.

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


Mapping a file makes the specified portion of the file visible in the address space of the calling process.

For files that are larger than the address space, you can only map a small portion of the file data at one time. When the first view is complete, then you unmap it and map a new view.

To obtain the size of a view, use the VirtualQueryEx function.

The initial contents of the pages in a file mapping object backed by the page file are 0 (zero).

Typically, the suggested address is used to specify that a file should be mapped at the same address in multiple processes. This requires the region of address space to be available in all involved processes. 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.

If the lpBaseAddress parameter specifies a base offset, the function succeeds if the specified memory region is not already in use by the calling process. The system does not ensure that the same memory region is available for the memory mapped file in other 32-bit processes.

Multiple views of a file (or a file mapping object and its mapped file) are coherent if they contain identical data at a specified time. This occurs if the file views are derived from the same file mapping object. A process can duplicate a file mapping object handle into another process by using the DuplicateHandle function, or another process can open a file mapping object by name by using the OpenFileMapping function.

With one important exception, file views derived from any file mapping object that is backed by the same file are coherent or identical at a specific time. Coherency is guaranteed for views within a process and for views that are mapped by different processes.

The exception is related to remote files. Although MapViewOfFileEx works with remote files, it does not keep them coherent. For example, if two computers both map a file as writable, and both change the same page, each computer only sees its own writes to the page. When the data gets updated on the disk, it is not merged.

A mapped view of a file is not guaranteed to be coherent with a file being accessed by the ReadFile or WriteFile function.

To guard against EXCEPTION_IN_PAGE_ERROR exceptions, use structured exception handling to protect any code that writes to or reads from a memory mapped view of a file other than the page file. For more information, see Reading and Writing From a File View.

When modifying a file through a mapped view, the last modification timestamp may not be updated automatically. If required, the caller should use SetFileTime to set the timestamp.

To have a file with executable permissions, an application must call CreateFileMapping with either PAGE_EXECUTE_READWRITE or PAGE_EXECUTE_READ, and then call MapViewOfFileEx with FILE_MAP_EXECUTE | FILE_MAP_WRITE or FILE_MAP_EXECUTE | FILE_MAP_READ.

In Windows Server 2012, this function is supported by the following technologies.

Technology Supported
Server Message Block (SMB) 3.0 protocol Yes
SMB 3.0 Transparent Failover (TFO) Yes
SMB 3.0 with Scale-out File Shares (SO) Yes
Cluster Shared Volume File System (CsvFS) Yes
Resilient File System (ReFS) Yes


Requirement Value
Minimum supported client Windows XP [desktop apps only]
Minimum supported server Windows Server 2003 [desktop apps only]
Target Platform Windows
Header memoryapi.h (include Windows.h, Memoryapi.h)
Library onecore.lib
DLL Kernel32.dll

See also


Creating a File View


File Mapping Functions