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Overview |
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Group |
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Quick Info
Windows NT
| Yes
| Win95
| Yes
| Win32s
| Yes
| Import Library
| kernel32.lib
| Header File
| winbase.h
| Unicode
| No
| Platform Notes
| Windows 95: Memory mapped files appear in the same address space in all
processes
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MapViewOfFile
The MapViewOfFile function maps a view of a file into the address space of the calling process.
LPVOID MapViewOfFile(
HANDLE hFileMappingObject,
| // file-mapping object to map into address space
| DWORD dwDesiredAccess,
| // access mode
| DWORD dwFileOffsetHigh,
| // high-order 32 bits of file offset
| DWORD dwFileOffsetLow,
| // low-order 32 bits of file offset
| DWORD dwNumberOfBytesToMap
| // number of bytes to map
| );
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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.
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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.
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
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