CreateFileMapping到底做了什么

[SniperPoint]

我知道对程序员来说,mapviewofFile()和virtulAlloc()功能相似,都是在线形地址得到空间,但是mapviewofFile()之前的CreateFileMapping到底做了什么工作呢?是把磁盘文件映射到虚拟内存吗?

[jingzhongrong]

CreateFileMapping根据传入参数不同最终在系统中创建了Section对象

virtulAlloc在进程地址空间申请一块virtual memory

[scq2099yt]

创建命名或匿名的映射文件对象

[scq2099yt]

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

HANDLE CreateFileMapping(
HANDLE hFile, // handle to file to map
LPSECURITY_ATTRIBUTES lpFileMappingAttributes,
// optional security attributes
DWORD flProtect, // protection for mapping object
DWORD dwMaximumSizeHigh, // high-order 32 bits of object size
DWORD dwMaximumSizeLow, // low-order 32 bits of object size
LPCTSTR lpName // name of file-mapping object
);

Parameters
hFile
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. It is recommended, though not required, that files you intend to map be opened for exclusive access.
If hFile is (HANDLE)0xFFFFFFFF, the calling process must also specify a mapping object size in the dwMaximumSizeHigh and dwMaximumSizeLow parameters. The function 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.

lpFileMappingAttributes
Pointer to a SECURITY_ATTRIBUTES structure that determines whether the returned handle can be inherited by child processes. If lpFileMappingAttributes is NULL, the handle cannot be inherited.
Windows NT: The lpSecurityDescriptor member of the structure specifies a security descriptor for the new file-mapping object. If lpFileMappingAttributes is NULL, the file-mapping object gets a default security descriptor.

flProtect
Specifies the protection desired for the file view, when the file is mapped. This parameter can be one of the following 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 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 non-cacheable. 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 (HANDLE)0xFFFFFFFF; that is, a file mapping object backed by the operating sytem paging file.



dwMaximumSizeHigh
Specifies the high-order 32 bits of the maximum size of the file-mapping object.
dwMaximumSizeLow
Specifies the low-order 32 bits of the maximum size of the file-mapping object. If this parameter and dwMaximumSizeHig are zero, the maximum size of the file-mapping object is equal to the current size of the file identified by hFile.
lpName
Pointer to a null-terminated string specifying the name of the mapping object. The name can contain any character except the backslash 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.

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, the function fails and the GetLastError function returns ERROR_INVALID_HANDLE. This occurs because these objects share the same name space.

Return Values
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.


Example
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;