MapViewOfFile

The MapViewOfFile function maps a view of a file into the address space of the calling process.

To specify a suggested base address, use the MapViewOfFileEx function.

LPVOID MapViewOfFile(
  HANDLE hFileMappingObject,   // handle to file-mapping object
  DWORD dwDesiredAccess,       // access mode
  DWORD dwFileOffsetHigh,      // high-order DWORD of offset
  DWORD dwFileOffsetLow,       // low-order DWORD of offset
  SIZE_T dwNumberOfBytesToMap  // number of bytes to map
);

Parameters

hFileMappingObject

[in] Handle to an open handle of a file-mapping object. The CreateFileMapping and OpenFileMapping functions return this handle.

dwDesiredAccess

[in] Specifies the type of access to the file view and, therefore, the protection of the pages mapped by the file. This parameter can be one of the following values.

Value	Meaning
FILE_MAP_WRITE	Read/write access. The hFileMappingObject parameter must have been created with PAGE_READWRITE protection. A read/write view of the file is mapped.
FILE_MAP_READ	Read-only access. The hFileMappingObject parameter must have been created with PAGE_READWRITE or PAGE_READONLY protection. A read-only view of the file is mapped.
FILE_MAP_ALL_ACCESS	Same as FILE_MAP_WRITE.
FILE_MAP_COPY	Copy on write access. If you create the map with PAGE_WRITECOPY and the view with FILE_MAP_COPY, you will receive a view to file. If you write to it, the pages are automatically swappable and the modifications you make will not go to the original data file. Windows 95: You must pass PAGE_WRITECOPY to CreateFileMapping; otherwise, an error will be returned. If you share the mapping between multiple processes using DuplicateHandle or OpenFileMapping and one process writes to a view, the modification is propagated to the other process. The original file does not change. Windows NT/2000: There is no restriction as to how the hFileMappingObject parameter must be created. Copy on write is valid for any type of view. If you share the mapping between multiple processes using DuplicateHandle or OpenFileMapping and one process writes to a view, the modification is not propagated to the other process. The original file does not change.

dwFileOffsetHigh

[in] Specifies the high-order DWORD of the file offset where mapping is to begin.

dwFileOffsetLow

[in] Specifies the low-order DWORD of the file offset where mapping is to begin. The combination of the high and low offsets must specify an offset within the file that matches the system's memory allocation granularity, or the function fails. That is, the offset must be a multiple of the allocation granularity. Use the GetSystemInfo function, which fills in the members of a SYSTEM_INFO structure, to obtain the system's memory allocation granularity.

dwNumberOfBytesToMap

[in] Specifies the number of bytes of the file to map. If dwNumberOfBytesToMap is zero, the entire file is mapped.

Return Values

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.

Remarks

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

Multiple views of a file (or a file-mapping object and its mapped file) are said to be "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.

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

• Windows 95: MapViewOfFile may require the swapfile to grow. If the swapfile cannot grow, the function fails.
• Windows NT/2000: If the file-mapping object is backed by the paging file (hFile is INVALID_HANDLE_VALUE), the paging file must be large enough to hold the entire mapping. If it is not, MapViewOfFile fails.

Note  To guard against an access violation, use structured exception handling to protect any code that writes to or reads from a memory mapped view. For more information, see Reading and Writing.

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winbase.h; include Windows.h.
  Library: Use Kernel32.lib.

See Also

File Mapping Overview, File Mapping Functions, CreateFileMapping, DuplicateHandle, GetSystemInfo, MapViewOfFileEx, OpenFileMapping, UnmapViewOfFile, SYSTEM_INFO