Membuat Tampilan Dalam File

Jika Anda ingin melihat sebagian file yang tidak dimulai di awal file, Anda harus membuat objek pemetaan file. Objek ini adalah ukuran bagian dari file yang ingin Anda lihat ditambah offset ke dalam file. Misalnya, jika Anda ingin melihat 1 kilobyte (1K) yang dimulai 131.072 byte (128K) ke dalam file, Anda harus membuat objek pemetaan file dengan ukuran setidaknya 132.096 byte (129K). Tampilan dimulai 131.072 byte (128K) ke dalam file dan diperluas setidaknya 1.024 byte. Contoh ini mengasumsikan granularitas alokasi file 64K.

Granularitas alokasi file memengaruhi tempat tampilan peta dapat dimulai. Tampilan peta harus dimulai dari offset ke dalam file yang merupakan kelipatan granularitas alokasi file. Jadi data yang ingin Anda lihat mungkin modul offset file granularitas alokasi ke dalam tampilan. Ukuran tampilan adalah offset modul data granularitas alokasi, ditambah ukuran data yang ingin Anda periksa.

Misalnya, misalkan fungsi GetSystemInfo menunjukkan granularitas alokasi 64K. Untuk memeriksa 1K data yaitu 138.240 byte (135K) ke dalam file, lakukan hal berikut:

1. Buat objek pemetaan file dengan ukuran setidaknya 139.264 byte (136K).
2. Buat tampilan file yang dimulai pada offset file yang merupakan kelipatan terbesar dari granularitas alokasi file kurang dari offset yang Anda butuhkan. Dalam hal ini, tampilan file dimulai dari offset 131.072 (128K) ke dalam file. Tampilannya 139264 byte (136K) dikurangi 131.072 byte (128K), atau 8.192 byte (8K), dalam ukuran.
3. Buat offset pointer 7K ke dalam tampilan untuk mengakses 1K tempat Anda tertarik.

Jika data yang Anda inginkan mengalihkan batas granularitas alokasi file, Anda dapat membuat tampilan lebih besar dari granularitas alokasi file. Ini menghindari memecah data menjadi beberapa bagian.

Program berikut mengilustrasikan contoh kedua di atas.

/*
   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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Membuat Tampilan File