| Platform SDK: Memory |
The CreateFileMapping function creates or opens a named or unnamed file-mapping object for the specified file.
HANDLE CreateFileMapping( HANDLE hFile, // handle to file LPSECURITY_ATTRIBUTES lpAttributes, // security DWORD flProtect, // protection DWORD dwMaximumSizeHigh, // high-order DWORD of size DWORD dwMaximumSizeLow, // low-order DWORD of size LPCTSTR lpName // object name );
If hFile is INVALID_HANDLE_VALUE, the calling process must also specify a mapping object size in the dwMaximumSizeHigh and dwMaximumSizeLow parameters. In this case, CreateFileMapping creates a file-mapping object of the specified size backed by the operating-system paging file rather than by a named file in the file system. The file-mapping object can be shared through duplication, through inheritance, or by name.
Windows NT/2000: The lpSecurityDescriptor member of the structure specifies a security descriptor for the new file-mapping object. If lpAttributes is NULL, the file-mapping object gets a default security descriptor.
| Value | Description |
|---|---|
| PAGE_READONLY | Gives read-only access to the committed region of pages. An attempt to write to or execute the committed region results in an access violation. The file specified by the hFile parameter must have been created with GENERIC_READ access. |
| PAGE_READWRITE | Gives read/write access to the committed region of pages. The file specified by hFile must have been created with GENERIC_READ and GENERIC_WRITE access. |
| PAGE_WRITECOPY | Gives copy on write access to the committed region of pages. The files specified by the hFile parameter must have been created with GENERIC_READ and GENERIC_WRITE access. |
In addition, an application can specify certain section attributes by combining (using the bitwise OR operator) one or more of the following section attribute values with one of the preceding page protection values.
| Value | Description |
|---|---|
| SEC_COMMIT | Allocates physical storage in memory or in the paging file on disk for all pages of a section. This is the default setting. |
| SEC_IMAGE | The file specified for a section's file mapping is an executable image file. Because the mapping information and file protection are taken from the image file, no other attributes are valid with SEC_IMAGE. |
| SEC_NOCACHE | All pages of a section are to be set as noncacheable. This attribute is intended for architectures requiring various locking structures to be in memory that is never fetched into the processor's. On 80x86 and MIPS machines, using the cache for these structures only slows down the performance as the hardware keeps the caches coherent. Some device drivers require noncached data so that programs can write through to the physical memory. SEC_NOCACHE requires either the SEC_RESERVE or SEC_COMMIT to also be set. |
| SEC_RESERVE | Reserves all pages of a section without allocating physical storage. The reserved range of pages cannot be used by any other allocation operations until it is released. Reserved pages can be committed in subsequent calls to the VirtualAlloc function. This attribute is valid only if the hFile parameter is INVALID_HANDLE_VALUE; that is, a file-mapping object backed by the operating system paging file. |
An attempt to map a file with a length of zero in this manner fails with an error code of ERROR_FILE_INVALID. Applications should test for files with a length of zero and reject such files.
If this parameter matches the name of an existing named mapping object, the function requests access to the mapping object with the protection specified by flProtect.
If this parameter is NULL, the mapping object is created without a name.
If lpName matches the name of an existing event, semaphore, mutex, waitable timer, or job object, the function fails and the GetLastError function returns ERROR_INVALID_HANDLE. This occurs because these objects share the same name space.
Terminal Services: The name can have a "Global\" or "Local\" prefix to explicitly create the object in the global or session name space. The remainder of the name can contain any character except the backslash character (\). For more information, see Kernel Object Name Spaces.
Windows 2000: On Windows 2000 systems without Terminal Services running, the "Global\" and "Local\" prefixes are ignored. The remainder of the name can contain any character except the backslash character.
Windows NT 4.0 and earlier, Windows 95/98: The name can contain any character except the backslash character.
If the function succeeds, the return value is a handle to the file-mapping object. If the object existed before the function call, the function returns a handle to the existing object (with its current size, not the specified size) and GetLastError returns ERROR_ALREADY_EXISTS.
If the function fails, the return value is NULL. To get extended error information, call GetLastError.
After a file-mapping object has been created, the size of the file must not exceed the size of the file-mapping object; if it does, not all of the file's contents will be available for sharing.
If an application specifies a size for the file-mapping object that is larger than the size of the actual named file on disk, the file on disk is grown to match the specified size of the file-mapping object. If the file cannot be grown, this results in a failure to create the file-mapping object. GetLastError will return ERROR_DISK_FULL.
The handle that CreateFileMapping returns has full access to the new file-mapping object. It can be used with any function that requires a handle to a file-mapping object. File-mapping objects can be shared either through process creation, through handle duplication, or by name. For information on duplicating handles, see DuplicateHandle. For information on opening a file-mapping object by name, see OpenFileMapping.
Windows 95: File handles that have been used to create file-mapping objects must not be used in subsequent calls to file I/O functions, such as ReadFile and WriteFile. In general, if a file handle has been used in a successful call to the CreateFileMapping function, do not use that handle unless you first close the corresponding file-mapping object.
Creating a file-mapping object creates the potential for mapping a view of the file but does not map the view. The MapViewOfFile and MapViewOfFileEx functions map a view of a file into a process's address space.
With one important exception, file views derived from a single file-mapping object are coherent, or identical, at a given time. If multiple processes have handles of the same file-mapping object, they see a coherent view of the data when they map a view of the file.
The exception has to do with remote files. Although CreateFileMapping 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 will only see its own writes to the page. When the data gets updated on the disk, it is not merged.
A mapped file and a file accessed by means of the input and output (I/O) functions (ReadFile and WriteFile) are not necessarily coherent.
To fully close a file-mapping object, an application must unmap all mapped views of the file-mapping object by calling UnmapViewOfFile, and close the file-mapping object handle by calling CloseHandle. The order in which these functions are called does not matter. The call to UnmapViewOfFile is necessary because mapped views of a file-mapping object maintain internal open handles to the object, and a file-mapping object will not close until all open handles to it are closed.
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.
To implement a mapping-object creation function that fails if the object already exists, an application can use the following code.
hMap = CreateFileMapping(...);
if (hMap != NULL && GetLastError() == ERROR_ALREADY_EXISTS)
{
CloseHandle(hMap);
hMap = NULL;
}
return hMap;
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.
Unicode: Implemented as Unicode and ANSI versions on Windows NT/2000.
File Mapping Overview, File Mapping Functions, CloseHandle, DuplicateHandle, MapViewOfFile, MapViewOfFileEx, OpenFileMapping, ReadFile, SECURITY_ATTRIBUTES, UnmapViewOfFile, VirtualAlloc, WriteFile