对一个文件使用内存映射文件

类别: 内存映射文件

题目:对一个文件使用内存映射文件

Demo:

 1:创建或打开一个文件内核对象:

 // Open the file for reading and writing.
 HANDLE hFile = CreateFile(pszPathname, GENERIC_WRITE | GENERIC_READ, 0,
   NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

 // 由于hFile即使为INVALID_HANDLE_VALUE,下面的CreateFileMapping仍然可以正常运行,
 // 所以这里一定要对hFile进行检查!
 if (hFile == INVALID_HANDLE_VALUE) {
   chMB("File could not be opened.");
   return(FALSE);
 }

 2:创建一个文件映射内核对象:

 // Get the size of the file (I assume the whole file can be mapped).
 DWORD dwFileSize = GetFileSize(hFile, NULL);

 // Create the file-mapping object. The file-mapping object is 1 character
 // bigger than the file size so that a zero character can be placed at the
 // end of the file to terminate the string (file). Because I don't yet know
 // if the file contains ANSI or Unicode characters, I assume worst case
 // and add the size of a WCHAR instead of CHAR.
 HANDLE hFileMap = CreateFileMapping(hFile, NULL, PAGE_READWRITE,
   0,
   dwFileSize + sizeof(WCHAR), // 如果该文件小于设定的大小,本函数将扩展该文件的大小,
          // 使磁盘上的文件变大。这样当以后将该文件作为内存映射
          // 文件使用时,物理存储器就已经存在了。
   NULL f// 这个文件映射对象的名字用于与其他进程共享该对象,这里我们还用不到。
   );

 if (hFileMap == NULL) {
   chMB("File map could not be opened.");
   CloseHandle(hFile);
   return(FALSE);
 }

 3:将文件数据映射到进程的地址空间:
 当创建了一个文件映射对象之后,仍然必须让系统为文件的数据保留一个地址空间区域,
 并将文件的数据作为映射到该区域的物理存储器进行提交。

 // Get the address where the first byte of the file is mapped into memory.
 // the return value is the starting address of the mapped view:
 PVOID pvFile = MapViewOfFile(hFileMap, FILE_MAP_WRITE, 0, 0, 0);

 if (pvFile == NULL) {
   chMB("Could not map view of file.");
   CloseHandle(hFileMap);
   CloseHandle(hFile);
   return(FALSE);
 }

 4:既然我们通过pvFile得到了映象视图的起始地址,那么可以对视图做一些操作了:
  
 ANSI版本:
 PSTR pchANSI = (PSTR) pvFile;
 UNICODE版本:
 PWSTR pchUnicode = (PWSTR) pvFile;

 5:从进程的地址空间中撤销文件数据的映象:

 // Clean up everything before exiting.
 UnmapViewOfFile(pvFile);

 6:关闭文件映射对象和文件对象:

 CloseHandle(hFileMap);
 CloseHandle(hFile);

Definition:

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
);
LPVOID MapViewOfFile(
  HANDLE hFileMappingObject,   // handle to file-mapping object
  DWORD dwDesiredAccess,       // access mode
  DWORD dwFileOffsetHigh,      // high-order DWORD of offset
  DWORD dwFileOffsetLow,       // low-order DWORD of offset
  SIZE_T dwNumberOfBytesToMap  // number of bytes to map
);
BOOL UnmapViewOfFile(
  LPCVOID lpBaseAddress   // starting address
);

Tips:
也可以尽量早地把对象关闭,以消除资源泄漏的可能性,如:
HANDLE hFile = CreateFile(...);
HANDLE hFileMapping = CreateFileMapping(hFile,...);
CloseHandle(hFile);

PVOID pvFile = MapViewOfFile(hFileMapping,...);
CloseHandle(hFileMapping);

 // use the memory-mapped file.

UnmapViewOfFile(pvFile);

Trackback: http://tb.blog.csdn.net/TrackBack.aspx?PostId=12650