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MODULE E1

WIN32 AND C PROGRAMMING 6

What are in this Module?

1. Locking and Unlocking Byte Ranges in Files

2. Flushing System-Buffered I/O Data to Disk

3. Appending One File to Another File

4. File Encryption

5. File Compression and Decompression

6. File Compression

7. File Decompression

8. Sparse Files

9. File and Directory Linking

10. Directory

• Attempts to read from or write to a region locked by another process always fail.

• The LockFileEx() function allows an application to specify either a shared lock or anexclusive lock.

• An exclusive lock denies all other processes both read and write access to the specified region of the file.

• A shared lock denies all processes write access to the specified region of the file, including the process that first locks the region.  This can be used to create a read-only region in a file.

• The application unlocks the region by using the UnlockFile() or UnlockFileEx() function. An application should unlock all locked areas before closing a file.

• The following Table lists the needed information in order to use the UnlockFile() function.

Flushing System-Buffered I/O Data to Disk

• Windows stores the data in file read and write operations in system-maintained data buffers to optimize disk performance.

• When an application writes to a file, the system usually buffers the data and writes the data to the disk on a regular basis.

• An application can force the operating system to write the contents of these data buffers to the disk by using theFlushFileBuffers() function.

• The following Table lists the needed information in order to use the FlushFileBuffers() function.

• Alternatively, an application can specify that write operations are to bypass the data buffer and write directly to the disk by setting theFILE_FLAG_NO_BUFFERING flag when the file is created or opened using theCreateFile() function.

• If there is data in the internal buffer when the file is closed, the operating system does not automatically write the contents of the buffer to the disk before closing the file.

• If the application does not force the operating system to write the buffer to disk before closing the file, the caching algorithm determines when the buffer is written.

• Accessing a data buffer while a read or write operation is using it may corruption the buffer. Applications must not read from, write to, reallocate, or free the data buffer that a read or write operation is using until the operation completes.

Appending One File to Another File

• The following program example appends one file to the end of another file.  The application opens two files by usingCreateFile():  The testfile.txt is opened for reading, and testfiletwo.txt is opened for writing.

• Then the application appends the contents of testfile.txt to the end of testfiletwo.txt by reading and writing 4K blocks by usingReadFile() and WriteFile().

• Before writing to the second file, the application sets the second file's pointer to the end of the file by usingSetFilePointer() and locks the area to be written by usingLockFile().

• This prevents another process from accessing the area while the write is in progress.  After each write operation,UnlockFile() unlocks the locked area.

#include <windows.h>

#include <stdio.h>

 

int main()

{

HANDLE hFile;

HANDLE hAppend;

DWORD  dwBytesRead, dwBytesWritten, dwPos;

char fname[30] = "c:\\testfile.txt";

char fname2[30] = "c:\\testfiletwo.txt";

char buff[4096];

 

// open the existing file.

hFile = CreateFile(fname,         //open testfile.txt

    GENERIC_READ,                 //open for reading

    0,                                            //do not share

    NULL,                                     //default security

    OPEN_EXISTING,                //existing file only

    FILE_ATTRIBUTE_NORMAL,    //normal file

    NULL);                                    //no attribute template

 

if(hFile == INVALID_HANDLE_VALUE)

    printf("Could not open %s lol!.\n", fname);

else

    printf("%s opened successfully.\n", fname);

// open the existing file, or if the file does not exist, create a new file.

hAppend = CreateFile(fname2,   //open testfiletwo.txt

    GENERIC_WRITE,             //open for writing

    0,                                          //do not share

    NULL,                                  //default security

    OPEN_ALWAYS,               //open or create

    FILE_ATTRIBUTE_NORMAL,     //normal file

    NULL);                                 //no attribute template

 

if(hAppend == INVALID_HANDLE_VALUE)

    printf("Could not open %s lol!.\n", fname2);

else

{

printf("%s opened/created successfully.\n", fname2);

printf("\nAppending %s\'s content to %s\'s content\n", fname, fname2);

printf("Check the %s content lol!\n\n", fname2);

}

 

// append the first file to the end of the second file. Lock the second file to prevent another process from

// accessing it while writing to it. Unlock the file when writing is finished.

do

{

    if(ReadFile(hFile, buff, 4096, &dwBytesRead, NULL))

    {

        dwPos = SetFilePointer(hAppend, 0, NULL, FILE_END);

        if(LockFile(hAppend, dwPos, 0, dwBytesRead, 0) != 0)

              printf("Locking %s...\n", fname2);

        WriteFile(hAppend, buff, dwBytesRead, &dwBytesWritten, NULL);

        if(UnlockFile(hAppend, dwPos, 0, dwBytesRead, 0) != 0)

              printf("Unlocking %s...\n", fname2);

    }

} while (dwBytesRead == 4096);

 

// close both files.

if(CloseHandle(hFile) != 0)

  printf("\nFile handle closed successfully!\n");

if(CloseHandle(hAppend) != 0)

  printf("File's append handle closed successfully!\n");

return 0;

}

 

The output:

 

c:\testfile.txt opened successfully.

c:\testfiletwo.txt opened/created successfully.

 

Appending c:\testfile.txt's content to c:\testfiletwo.txt's content

Check the c:\testfiletwo.txt content lol!

 

Locking c:\testfiletwo.txt...

Unlocking c:\testfiletwo.txt...

Locking c:\testfiletwo.txt...

Unlocking c:\testfiletwo.txt...

Locking c:\testfiletwo.txt...

Unlocking c:\testfiletwo.txt...

Locking c:\testfiletwo.txt...

Unlocking c:\testfiletwo.txt...

Locking c:\testfiletwo.txt...

Unlocking c:\testfiletwo.txt...

 

File handle closed successfully!

File's append handle closed successfully!

Press any key to continue

File Encryption

• The Encrypted File System, or EFS, was introduced in version 5 of NTFS to provide an additional level of security for files and directories.

• It provides cryptographic protection of individual files on NTFS volumes using a public-key system. Typically, the access control to file and directory objects provided by the Windows security model is sufficient to protect unauthorized access to sensitive information.

• However, if a laptop containing sensitive data is lost or stolen, the security protection of that data may be compromised.  Encrypting the files increases security in this scenario.

• To determine whether a file system supports file encryption for files and directories, call the GetVolumeInformation() function and examine the FS_FILE_ENCRYPTION bit flag.  Note that the following items cannot be encrypted:

1. Compressed files.

2. System files.

3. System directories.

4. Root directories.

• Sparse files may be encrypted.

File Compression and Decompression

• NTFS volumes support file compression on an individual file basis.  The file compression algorithm used by NTFS is Lempel-Ziv compression.

• This is a lossless compression algorithm, which means that no data is lost when compressing and decompressing the file, as opposed to lossy compression algorithms such as JPEG, where some data is lost each time data compression and decompression occur.

File Compression

• Data compression reduces the size of a file by minimizing redundant data.  In a text file, redundant data can be frequently occurring characters, such as the space character, or common vowels, such as the letters e anda; it can also be frequently occurring character strings.

• Data compression creates a compressed version of a file by minimizing this redundant data.

• Each type of data-compression algorithm minimizes redundant data in a unique manner.  For example, the Huffman encoding algorithm assigns a code to characters in a file based on how frequently those characters occur.

• Another compression algorithm, called run-length encoding, generates a two-part value for repeated characters: the first part specifies the number of times the character is repeated, and the second part identifies the character.

• Another compression algorithm, known as the Lempel-Ziv algorithm, converts variable-length strings into fixed-length codes that consume less space than the original strings.

File Decompression

• Typically, an application calls functions in LzExpand.dll to decompress data.

• Applications can call the functions in LzExpand.dll to decompress files that have been compressed by the use of the Lempel-Ziv compression algorithm.  The functions can also process uncompressed files without attempting to decompress them.

• You can use the functions in LzExpand.dll to decompress single or multiple files. You can also use them to decompress compressed files a portion at a time.

Sparse Files

• A file in which much of the data is zeros is said to contain a sparse data set.  Files like these are typically very large.

• For example, a file containing image data to be processed or a matrix within a high-speed database. The problem with files containing sparse data sets is that the majority of the file does not contain useful data and, because of this, they are an inefficient use of disk space.

• The file compression introduced in version 3.51 of NTFS is a partial solution to the problem.  All data in the file that is not explicitly written is explicitly set to zero.

• File compression compacts these ranges of zeros.  However, a drawback of file compression is that access time may increase due to data compression and decompression.

• Support for sparse files was introduced in NTFS as another way to make the disk space usage more efficient.

• When the sparse file functionality is enabled, the system does not allocate hard drive space to a file except in regions where it contains nonzero data.  When a write operation is attempted where a large amount of the data in the buffer is zeros, the zeros are not written to the file.

• Instead, the file system creates an internal list containing the locations of the zeros in the file, and this list is consulted during all read operations.

• When a read operation is performed in areas of the file where zeros were located, the file system returns the appropriate number of zeros in the buffer allocated for the read operation. In this way, maintenance of the sparse file is transparent to all processes that access it.

• The default data value of a sparse file is zero; however, it can be set to other values.

File and Directory Linking

• NTFS provides the ability to create a system representation of a file or directory in a location in the directory structure that is different from the file or directory object that is being linked to.

• This process is calling linking.  There are two types of links supported in NTFS: hard links and junctions.

• NTFS also provides the distributed link tracking service, which automatically tracks links as they are moved.

Directory

• The following Table lists the information needed in order to use the CreateDirectory() function.

BOOL CreateDirectory( LPCTSTR  lpPathName, LPSECURITY_ATTRIBUTES  lpSecurityAttributes);

char szDirPath[20] = "c:\\testdir";

CreateDirectory(szDirPath, NULL);

• And the following is an information for RemoveDirectory() function.

char szDirPath[20] = "c:\\testdir";

• The RemoveDirectory() function marks a directory for deletion on close.  Therefore, the directory is not removed until the last handle to the directory is closed.

• To recursively delete the files in a directory, use the SHFileOperation() function.

• The following program example uses CreateDirectory(), CreateFile(), CloseHandle(), DeleteFile() andRemoveDirectory() functions.

#include <windows.h>

#include <stdio.h>

 

int main()

{

char szDirPath[20] = "c:\\testdir";

HANDLE hFile;

char fname[50] = "c:\\testdir\\testfile.txt";

 

// create a new directory.

if(!CreateDirectory(szDirPath, NULL))

    printf("Couldn't create %s directory.\n", szDirPath);

else

printf("%s directory successfully created.\n", szDirPath);

 

// create a file

hFile = CreateFile(fname,           //file to be opened

                GENERIC_READ,       //open for writing

                FILE_SHARE_READ,    //share for writing

                NULL,               //default security

                CREATE_ALWAYS,      //create new file only

                FILE_ATTRIBUTE_ARCHIVE | SECURITY_IMPERSONATION, //archive and impersonate client

                NULL);              //no attribute template

 

// check the handle, then open...

if(hFile == INVALID_HANDLE_VALUE)

    printf("Could not open %s file (error %d)\n", fname, GetLastError());

else

{

    printf("%s file HANDLE is OK!\n", fname);

    printf("%s opened successfully!\n", fname);

}

// close the handle...

if(CloseHandle(hFile) != 0)

    printf("CloseHandle() for %s file succeeded!\n", fname);

if(DeleteFile(fname) != 0)

    printf("%s file successfully deleted!\n", fname);

// delete the directory...

if(RemoveDirectory(szDirPath) != 0)

    printf("%s directory successfully deleted.\n", szDirPath);

 

return 0;

}

 

The output:

c:\testdir directory successfully created.

c:\testdir\testfile.txt file HANDLE is OK!

c:\testdir\testfile.txt opened successfully!

CloseHandle() for c:\testdir\testfile.txt file succeeded!

c:\testdir\testfile.txt file successfully deleted!

c:\testdir directory successfully deleted.

Press any key to continue

• A sample output using VC++ 2005.

• The following Table lists the information needed in order to use the GetCurrentDirectory() function.

DWORD GetCurrentDirectory( DWORD  nBufferLength, LPTSTR lpBuffer);

• The following Table lists the information needed in order to use the SetCurrentDirectory() function.

BOOL SetCurrentDirectory(LPCTSTR  lpPathName);

• Applications should store code in the Program Files folder and persistent data in the Application Data folder in the user's profile.

• Applications should not create files in the system directory.  If the user is running a shared version of the operating system, the application does not have write access to the system directory.

• This function is provided primarily for compatibility.

• Instead, you should use the CSIDL values in conjunction with one of four Shell functions,SHGetFolderLocation(), SHGetFolderPath(), SHGetSpecialFolderLocation(), and SHGetSpecialFolderPath(), to retrieve a special folder's path or pointer to an item identifier list (PIDL).

• CSIDL values provide a unique system-independent way to identify special folders used frequently by applications, but which may not have the same name or location on any given system.  For example, the system folder may be "C:\Windows" (Windows Xp) on one machine and "C:\Winnt" (Windows 2000) on another.

• The following Table lists the information needed in order to use the GetSystemDirectory() function.

UINT GetSystemDirectory(LPTSTR  lpBuffer, UINT  uSize);

• The Windows directory is the directory where an application should store initialization and help files.

• If the user is running a shared version of the system, the Windows directory is guaranteed to be private for each user.

• If an application creates other files that it wants to store on a per-user basis, it should place them in the directory specified by theHOMEPATH environment variable.

• This directory will be different for each user, if so specified by an administrator, through the User Manager administrative tool.

• HOMEPATH always specifies either the user's home directory, which is guaranteed to be private for each user, or a default directory (for example,C:\USERS\DEFAULT) where the user will have all access.

• The following Table lists the information needed in order to use the GetWindowsDirectory() function.

UINT GetWindowsDirectory(LPTSTR  lpBuffer, UINT  uSize);

• The following program example uses GetCurrentDirectory(), SetCurrentDirectory(), GetWindowsDirectory() and GetSystemDirectory() functions.

#include <windows.h>

#include <stdio.h>

#define BUFFER_SIZE 200

 

int main()

{

  TCHAR infoBuf[BUFFER_SIZE];

  // Change accordingly to other path of your machine

  TCHAR lpPathName[200] = "D:\\mydvd\\Adobe Acrobat 3D 8.1";

 // get the current working directory

  if(!GetCurrentDirectory(BUFFER_SIZE, infoBuf))

    printf("GetCurrentDirectory() failed!\n");

  printf("Your current directory is: %s\n", infoBuf);

  printf("Changing directory...\n");

 // set to current working directory

  if(!SetCurrentDirectory(lpPathName))

    printf("SetCurrentDirectory() failed!\n");

 // do some verification...

  if(!GetCurrentDirectory(BUFFER_SIZE, infoBuf))

    printf("GetCurrentDirectory() failed!\n");

  printf("Your current directory is: %s\n", infoBuf);

 // get and display the Windows directory.

  if(!GetWindowsDirectory(infoBuf, BUFFER_SIZE))

    printf("GetWindowsDirectory() failed!\n");

  printf("Your Windows directory is: %s\n", infoBuf);

 // get and display the system directory.

  if(!GetSystemDirectory(infoBuf, BUFFER_SIZE))

    printf("GetSystemDirectory() failed!\n");

  printf("Your system directory is: %s\n", infoBuf);

 

  return 0;

}

 

The output:

 

Your current directory is: g:\vcnetprojek\win32prog

Changing directory...

Your current directory is: D:\Program Files\Microsoft sql server

Your Windows directory is: C:\WINDOWS

Your system directory is: C:\WINDOWS\system32

Press any key to continue

Further reading and digging:

1. Check the best selling C, C++ and Windows books at Amazon.com.

2. Microsoft Visual C++, online MSDN.

3. ReactOS - Windows binary compatible OS - C/C++ source code repository, Doxygen.

4. Notation used in MSDN is Hungarian Notation instead of CamelCase and is discussedVC++ programming notations.

5. Windows data type information is inData types used in Windows programming.

6. For Multi bytes, Unicode characters and Localization please refer to Locale, wide characters & Unicode (Story) and Windows users & groups programming tutorials (Implementation).

7. Structure, enum, union and typedef story can be foundC/C++ struct, enum, union & typedef.

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