MapViewOfFile

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Windows Embedded CE 6.0 R3

4/8/2010

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

Syntax

LPVOID MapViewOfFile(
  HANDLE hFileMappingObject,
  DWORD dwDesiredAccess,
  DWORD dwFileOffsetHigh,
  DWORD dwFileOffsetLow,
  DWORD dwNumberOfBytesToMap
);

Parameters

• hFileMappingObject
  [in] Handle to an open handle of a file-mapping object returned by the CreateFileMapping function.

• dwDesiredAccess
  [in] Type of access to the file view and, therefore, the protection of the pages mapped by the file.

  The following table shows possible values.

  Value	Description
  FILE_MAP_ALL_ACCESS	Specifies 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	Specifies 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_WRITE	Specifies read-write access. The hFileMappingObject parameter must have been created with PAGE_READWRITE protection. A read-write view of the file is mapped.

• dwFileOffsetHigh
  [in] Specifies the high-order 32 bits of the file offset where mapping is to begin.

• dwFileOffsetLow
  [in] Specifies the low-order 32 bits 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 this parameter is set to zero, the entire file is mapped.

Return Value

The starting address of the mapped view indicates success. NULL indicates failure. To get extended error information, call GetLastError.

Remarks

The CreateFileMapping function can map files of a full 64-bit size. This function can create views as long as the current process has enough virtual address space to contain them. Resource constraints preven direct-ROM and non-pageable mapfiles that are larger than 4 GB.

If you intend to grow the file, specify the maximum file size so that the kernel can reserve the correct amount of memory.

The 64-KB alignment is not required for dwFileOffsetLow.

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. To create another view of a file-mapping object for a different process, use the CreateFileMapping function.

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

To guard against an access violation, use structured exception handling to protect any code that writes to or reads from a memory-mapped view.

The pages in the mapped views can become read/write if they are written to. Any pages that do not overlap remain read-only. Any pages that overlapped, but are not written to, remain read-only. If all read/write views are unmapped, unwritten pages remain read-only.

Requirements

See Also

Reference

File Mapping Functions
CreateFileMapping
UnmapViewOfFile

Other Resources

GetSystemInfo
SYSTEM_INFO