在文件中创建视图
如果要查看未从文件开头开始的文件部分，则必须创建文件映射对象。 此对象是要查看的文件部分的大小加上文件中的偏移量。 例如，如果要查看从 131，072 字节 (128K 开始的 1 千字节 (1K) ) ，则必须创建大小至少为 132，096 字节 (129K) 的文件映射对象。 视图开始 131，072 个字节 (128K) 到文件中，并扩展至少 1，024 个字节。 此示例假定文件分配粒度为 64K。
文件分配粒度会影响地图视图的启动位置。 地图视图必须从文件分配粒度的倍数的偏移量开始。 因此，要查看的数据可能是文件偏移量，即视图中的分配粒度。 视图的大小是数据模式与分配粒度的偏移量，以及要检查的数据大小。
例如，假设 GetSystemInfo 函数指示分配粒度为 64K。 若要检查文件中 138，240 字节 (135K) 的 1K 数据，请执行以下操作：
- 创建大小至少为 139，264 字节 (136K) 的文件映射对象。
- 创建一个文件视图，该视图从文件分配粒度的最大倍数小于所需偏移量的文件偏移量开始。 在这种情况下，文件视图从 131，072 (128K) 到文件中的偏移量开始。 视图139264字节 (136K) 减去 131，072 字节 (128K) ，或 8，192 字节 (8K) 。
- 在视图中创建指针偏移量 7K，以访问感兴趣的 1K。
如果所需的数据跨越文件分配粒度边界，则可以使视图大于文件分配粒度。 这可避免将数据分解成碎片。
以下程序演示了上面的第二个示例。
/*
This program demonstrates file mapping, especially how to align a
view with the system file allocation granularity.
*/
#include <windows.h>
#include <stdio.h>
#include <tchar.h>
#define BUFFSIZE 1024 // size of the memory to examine at any one time
#define FILE_MAP_START 138240 // starting point within the file of
// the data to examine (135K)
/* The test file. The code below creates the file and populates it,
so there is no need to supply it in advance. */
TCHAR * lpcTheFile = TEXT("fmtest.txt"); // the file to be manipulated
int main(void)
{
HANDLE hMapFile; // handle for the file's memory-mapped region
HANDLE hFile; // the file handle
BOOL bFlag; // a result holder
DWORD dBytesWritten; // number of bytes written
DWORD dwFileSize; // temporary storage for file sizes
DWORD dwFileMapSize; // size of the file mapping
DWORD dwMapViewSize; // the size of the view
DWORD dwFileMapStart; // where to start the file map view
DWORD dwSysGran; // system allocation granularity
SYSTEM_INFO SysInfo; // system information; used to get granularity
LPVOID lpMapAddress; // pointer to the base address of the
// memory-mapped region
char * pData; // pointer to the data
int i; // loop counter
int iData; // on success contains the first int of data
int iViewDelta; // the offset into the view where the data
//shows up
// Create the test file. Open it "Create Always" to overwrite any
// existing file. The data is re-created below
hFile = CreateFile(lpcTheFile,
GENERIC_READ | GENERIC_WRITE,
0,
NULL,
CREATE_ALWAYS,
FILE_ATTRIBUTE_NORMAL,
NULL);
if (hFile == INVALID_HANDLE_VALUE)
{
_tprintf(TEXT("hFile is NULL\n"));
_tprintf(TEXT("Target file is %s\n"),
lpcTheFile);
return 4;
}
// Get the system allocation granularity.
GetSystemInfo(&SysInfo);
dwSysGran = SysInfo.dwAllocationGranularity;
// Now calculate a few variables. Calculate the file offsets as
// 64-bit values, and then get the low-order 32 bits for the
// function calls.
// To calculate where to start the file mapping, round down the
// offset of the data into the file to the nearest multiple of the
// system allocation granularity.
dwFileMapStart = (FILE_MAP_START / dwSysGran) * dwSysGran;
_tprintf (TEXT("The file map view starts at %ld bytes into the file.\n"),
dwFileMapStart);
// Calculate the size of the file mapping view.
dwMapViewSize = (FILE_MAP_START % dwSysGran) + BUFFSIZE;
_tprintf (TEXT("The file map view is %ld bytes large.\n"),
dwMapViewSize);
// How large will the file mapping object be?
dwFileMapSize = FILE_MAP_START + BUFFSIZE;
_tprintf (TEXT("The file mapping object is %ld bytes large.\n"),
dwFileMapSize);
// The data of interest isn't at the beginning of the
// view, so determine how far into the view to set the pointer.
iViewDelta = FILE_MAP_START - dwFileMapStart;
_tprintf (TEXT("The data is %d bytes into the view.\n"),
iViewDelta);
// Now write a file with data suitable for experimentation. This
// provides unique int (4-byte) offsets in the file for easy visual
// inspection. Note that this code does not check for storage
// medium overflow or other errors, which production code should
// do. Because an int is 4 bytes, the value at the pointer to the
// data should be one quarter of the desired offset into the file
for (i=0; i<(int)dwSysGran; i++)
{
WriteFile (hFile, &i, sizeof (i), &dBytesWritten, NULL);
}
// Verify that the correct file size was written.
dwFileSize = GetFileSize(hFile, NULL);
_tprintf(TEXT("hFile size: %10d\n"), dwFileSize);
// Create a file mapping object for the file
// Note that it is a good idea to ensure the file size is not zero
hMapFile = CreateFileMapping( hFile, // current file handle
NULL, // default security
PAGE_READWRITE, // read/write permission
0, // size of mapping object, high
dwFileMapSize, // size of mapping object, low
NULL); // name of mapping object
if (hMapFile == NULL)
{
_tprintf(TEXT("hMapFile is NULL: last error: %d\n"), GetLastError() );
return (2);
}
// Map the view and test the results.
lpMapAddress = MapViewOfFile(hMapFile, // handle to
// mapping object
FILE_MAP_ALL_ACCESS, // read/write
0, // high-order 32
// bits of file
// offset
dwFileMapStart, // low-order 32
// bits of file
// offset
dwMapViewSize); // number of bytes
// to map
if (lpMapAddress == NULL)
{
_tprintf(TEXT("lpMapAddress is NULL: last error: %d\n"), GetLastError());
return 3;
}
// Calculate the pointer to the data.
pData = (char *) lpMapAddress + iViewDelta;
// Extract the data, an int. Cast the pointer pData from a "pointer
// to char" to a "pointer to int" to get the whole thing
iData = *(int *)pData;
_tprintf (TEXT("The value at the pointer is %d,\nwhich %s one quarter of the desired file offset.\n"),
iData,
iData*4 == FILE_MAP_START ? TEXT("is") : TEXT("is not"));
// Close the file mapping object and the open file
bFlag = UnmapViewOfFile(lpMapAddress);
bFlag = CloseHandle(hMapFile); // close the file mapping object
if(!bFlag)
{
_tprintf(TEXT("\nError %ld occurred closing the mapping object!"),
GetLastError());
}
bFlag = CloseHandle(hFile); // close the file itself
if(!bFlag)
{
_tprintf(TEXT("\nError %ld occurred closing the file!"),
GetLastError());
}
return 0;
}
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