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MapViewOfFile info  Overview  Group

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

LPVOID MapViewOfFile(

Parameters

hFileMappingObject

Identifies an open handle of a file-mapping object. The CreateFileMapping and OpenFileMapping functions return this handle.

dwDesiredAccess

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: 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

Specifies the high-order 32 bits of the file offset where mapping is to begin.

dwFileOffsetLow

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

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.

Once obtained, the handle to the memory-mapped file object is used to map views of the file to your process's address space. Views can be mapped and unmapped at will while the MMF object exists. When a view of the file is mapped, system resources are finally allocated. A contiguous range of addresses, large enough to span the size of the file view, are now committed in your process's address space. Yet, even though the addresses have been committed for the file view, physical pages of memory are still only committed on a demand basis when using the memory. So, the only way to allocate a page of physical memory for a committed page of addresses in your memory-mapped file view is to generate a page fault for that page. This is done automatically the first time you read or write to any address in the page of memory.

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: If the file-mapping object is backed by the paging file (handle = 0xFFFFFFFF), the paging file must be large enough to hold the entire mapping. If it is not, MapViewOfFile fails.

See Also

CreateFileMapping, DuplicateHandle, GetSystemInfo, MapViewOfFileEx, OpenFileMapping, UnmapViewOfFile, SYSTEM_INFO