File Pointer Operations

This section details the functions and concepts related to manipulating file pointers in Windows. File pointers are essential for controlling the read/write position within a file.

Seeking File Pointers

Seeking allows you to reposition the current file pointer to a specific location within a file. This is crucial for random access to file data.

SetFilePointerEx

The SetFilePointerEx function sets the file pointer of the specified file to a desired location. It returns a 64-bit pointer value.

BOOL SetFilePointerEx(
  HANDLE hFile,
  LARGE_INTEGER liDistanceToMove,
  PLARGE_INTEGER lpNewFilePointer,
  DWORD dwMoveMethod
);
            

Parameters:

• hFile: A handle to the file.
• liDistanceToMove: The number of bytes to move the file pointer.
• lpNewFilePointer: An optional pointer to a LARGE_INTEGER that receives the new file pointer.
• dwMoveMethod: The starting point for the move. Can be one of:
  • FILE_BEGIN (0): Seek from the beginning of the file.
  • FILE_CURRENT (1): Seek from the current file pointer position.
  • FILE_END (2): Seek from the end of the file.

SetFilePointer

An older function for setting the file pointer. It works with 32-bit offsets, which may be limiting for large files.

DWORD SetFilePointer(
  HANDLE hFile,
  LONG lDistanceToMove,
  PLONG lpNewFilePointer,
  DWORD dwMoveMethod
);
            

Reading and Writing Positions

After performing read or write operations using functions like ReadFile and WriteFile, the file pointer is automatically advanced to the next available position.

Retrieving the Current File Pointer

To get the current position of the file pointer without moving it, you can use SetFilePointerEx with a distance of 0.

LARGE_INTEGER currentPosition;
if (SetFilePointerEx(hFile, {0}, ¤tPosition, FILE_CURRENT)) {
    // currentPosition.QuadPart now holds the current file pointer
    printf("Current file pointer position: %lld\n", currentPosition.QuadPart);
}
            

Locking and Unlocking Regions

File locking provides a mechanism to prevent other processes from accessing specific regions of a file while your process is working with it, ensuring data integrity.

LockFile

The LockFile function locks a specified region of bytes in an open file.

BOOL LockFile(
  HANDLE hFile,
  DWORD dwFileOffsetLow,
  DWORD dwFileOffsetHigh,
  DWORD nNumberOfBytesToLockLow,
  DWORD nNumberOfBytesToLockHigh
);
            

Parameters:

• hFile: Handle to the file.
• dwFileOffsetLow: Low-order part of the byte offset.
• dwFileOffsetHigh: High-order part of the byte offset.
• nNumberOfBytesToLockLow: Low-order part of the number of bytes to lock.
• nNumberOfBytesToLockHigh: High-order part of the number of bytes to lock.

UnlockFile

The UnlockFile function unlocks a previously locked region of bytes in an open file.

BOOL UnlockFile(
  HANDLE hFile,
  DWORD dwFileOffsetLow,
  DWORD dwFileOffsetHigh,
  DWORD nNumberOfBytesToUnlockLow,
  DWORD nNumberOfBytesToUnlockHigh
);
            

Understanding and effectively using file pointer operations is fundamental for efficient file I/O in Windows applications.