CreateFileMapping

Article
11/18/2015

A version of this page is also available for

4/8/2010

This function creates a named or unnamed file-mapping object for the specified file.

Syntax

HANDLE CreateFileMapping(
  HANDLE hFile,
  LPSECURITY_ATTRIBUTES lpFileMappingAttributes,
  DWORD flProtect,
  DWORD dwMaximumSizeHigh,
  DWORD dwMaximumSizeLow,
  LPCTSTR lpName 
);

Parameters

hFile
[in] Handle to the file from which to create a mapping object. The file must be opened with an access mode compatible with the protection flags specified by the flProtect parameter. Open files that you intend to map for exclusive access. If two handles to the file are open at the same time, inconsistencies between the file and the mapped view of it can occur. Unless you are prepared to synchronize between the two handles, do not use FILE_SHARE_READ or FILE_SHARE_WRITE when you call the CreateFile function.

If this parameter is set to (HANDLE)INVALID_HANDLE_VALUE, the calling process must also specify a mapping object size in the dwMaximumSizeHigh and the dwMaximumSizeLow parameters. The function creates a file-mapping object of the specified size, backed by physical memory, rather than by a named file in the file system. The file-mapping object can be shared by name. In Windows Mobile, memory utilized for the mapping object is not part of the 32 MB virtual process space.

The value of this parameter can come from either CreateFile or the CreateFileForMapping function. However, the file handle differs between this function and the CloseHandle function.

lpFileMappingAttributes
[in] Ignored. Must be set to NULL.

flProtect
[in] Protection desired for the file view, when the file is mapped. The following table shows possible values.

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 hFile must have been created with GENERIC_READ access. You cannot specify PAGE_READONLY without a file handle.
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	Not supported.

In addition, an application can specify certain section attributes by combining one or more section attribute values with one of the preceding page protection values, using the bitwise OR operator. The following table shows possible section attribute 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	Not supported.
SEC_NOCACHE	Not supported.
SEC_RESERVE	Not supported.

dwMaximumSizeHigh
[in] Specifies the high-order 32 bits of the maximum size of the file-mapping object.

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

dwMaximumSizeLow
[in] Specifies the low-order 32 bits of the maximum size of the file-mapping object. If this parameter and dwMaximumSizeHigh are set to zero, the maximum size of the file-mapping object is equal to the current size of the file specified by hFile.

If dwMaximumSizeLow is set to zero, the function returns an error that indicates an invalid parameter.

lpName
[in] Long pointer to a null-terminated string specifying the name of the mapping object. The name can contain any character.

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.

Each object type, such as memory maps, semaphores, events, message queues, mutexes, and watchdog timers, has its own separate namespace. Empty strings ("") are handled as named objects. On Windows desktop-based platforms, synchronization objects all share the same namespace.

Return Value

A handle to the file-mapping object indicates success. 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. NULL indicates failure. To get extended error information, call GetLastError.

Remarks

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

A memory-mapped file should not be accessed by using the ReadFile or the WriteFile function. This can result in inconsistencies between the file and the memory-mapped file.

Two writable handles to the same file cannot be open at the same time to prevent inconsistencies between the file and the mapped view of it. For two processes to share the data in a memory-mapped file, they must both memory map the file.

If you call this function with a handle from CreateFileForMapping and it fails for any reason, the handle is closed. For example, any attempt to use the CloseHandle function on a file handle that was returned from CreateFileForMapping and then used in a failed call to this function automatically return FALSE. Instead, the kernel automatically closes the handle specified in hFile.

This function does not work on Windows Mobile devices that do not support demand paging.

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 file contents are available for sharing.

If an application specifies a size for the file-mapping object that is larger than the size of the 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, the file-mapping object is not created. GetLastError returns ERROR_DISK_FULL.

The handle that this function 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 through process creation.

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. Generally, if a file handle has been used in a successful call to this 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. MapViewOfFile maps a view of a file into the address space of a process.

With one important exception, file views derived from a single file-mapping object are coherent, or identical, at a specified 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 this function 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 sees only its own view as it writes to the page. When the data is updated on the disk, it is not merged.

To fully close a file-mapping object, an application must unmap all mapped views of the file-mapping object by calling the UnmapViewOfFile function, 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 does not close until all open handles to it are closed.

The CreateFileForMapping function opens a file and returns a file handle that is passed to this function. This function then returns a handle to the file mapping. For Windows Mobile, it is necessary only to close the mapping handle as the file handle is automatically closed when you close the mapping handle.

If the file being mapped is an uncompressed ROM file and it is mapped with PAGE_READONLY access, the file is accessed directly from ROM without using any RAM to cache the data.

Security 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. Information in a readable memory-mapped file can be read by other processes on the system. Do not store confidential information in a memory-mapped file. Information in a writable memory-mapped file can be written to by other processes on the system. You must validate all data that you read from a memory mapped file.

To guard against an access violation, use structured exception handling to protect any code that writes to or reads from a memory-mapped view. Information in a readable memory-mapped file can be read by other processes on the system. Do not store confidential information in a memory-mapped file. Information in a writable memory-mapped file can be written to by other processes on the system. You must validate all data that you read from a memory mapped file.

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;

Requirements

Header	winbase.h
Library	coredll.lib
Windows Embedded CE	Windows CE 1.01 and later
Windows Mobile	Windows Mobile Version 5.0 and later

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CreateFileMapping

Syntax

Parameters

Return Value

Remarks

Requirements

See Also

Reference

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