File Kernel Object

The File kernel object is a fundamental concept in Windows for interacting with the file system. It represents an open instance of a file or device and provides a handle through which applications can perform operations like reading, writing, seeking, and querying file attributes.

Core Concepts

Handles: A file object is accessed through a handle obtained via functions like CreateFile. This handle is a unique identifier used in subsequent API calls.
Access Rights: When creating or opening a file, applications specify desired access rights (e.g., GENERIC_READ, GENERIC_WRITE) to control what operations are permitted.
Sharing Modes: Applications can define how the file can be shared with other processes when opening it, using sharing modes like FILE_SHARE_READ and FILE_SHARE_WRITE.
File Attributes: Files can have various attributes (e.g., FILE_ATTRIBUTE_READONLY, FILE_ATTRIBUTE_ARCHIVE) that describe their characteristics.
Asynchronous I/O: Windows supports asynchronous file operations, allowing applications to initiate I/O requests and continue processing without waiting for completion, improving performance for I/O-bound tasks.

Key Functions

Function	Description
`CreateFile`	Opens an existing file or creates a new one. This is the primary function for obtaining a file handle.
`ReadFile`	Reads data from a file into a buffer.
`WriteFile`	Writes data from a buffer to a file.
`SetFilePointer`	Moves the file pointer (the current read/write position) to a specified location within the file.
`GetFileSizeEx`	Retrieves the size of a file.
`CloseHandle`	Closes an open handle to a file object. Essential for releasing system resources.
`DeviceIoControl`	Sends control codes directly to a specified device driver, allowing for device-specific operations beyond standard file I/O.

`CreateFile`

HANDLE CreateFile( [in] LPCTSTR lpFileName, [in] DWORD dwDesiredAccess, [in] DWORD dwShareMode, [in, optional] LPSECURITY_ATTRIBUTES lpSecurityAttributes, [in] DWORD dwCreationDisposition, [in] DWORD dwFlagsAndAttributes, [in, optional] HANDLE hTemplateFile );

This function is the cornerstone for file operations. It can create, open, truncate, or replace a file. Key parameters include:

lpFileName: The name of the file.
dwDesiredAccess: The requested access to the file (e.g., GENERIC_READ, GENERIC_WRITE).
dwShareMode: How the file can be shared with other processes.
dwCreationDisposition: Action to take if the file exists or doesn't exist (e.g., OPEN_EXISTING, CREATE_ALWAYS).
dwFlagsAndAttributes: File attributes and flags (e.g., FILE_ATTRIBUTE_NORMAL, FILE_FLAG_OVERLAPPED for asynchronous I/O).

`ReadFile`

BOOL ReadFile( [in] HANDLE hFile, [out] LPVOID lpBuffer, [in] DWORD nNumberOfBytesToRead, [out, optional] LPDWORD lpNumberOfBytesRead, [in, out, optional] LPOVERLAPPED lpOverlapped );

Reads data from the file specified by hFile into the buffer pointed to by lpBuffer. The number of bytes read is stored in lpNumberOfBytesRead.

`WriteFile`

BOOL WriteFile( [in] HANDLE hFile, [in] LPCVOID lpBuffer, [in] DWORD nNumberOfBytesToWrite, [out, optional] LPDWORD lpNumberOfBytesWritten, [in, out, optional] LPOVERLAPPED lpOverlapped );

Writes data from the buffer pointed to by lpBuffer to the file specified by hFile. The number of bytes written is stored in lpNumberOfBytesWritten.

`SetFilePointer`

LONG SetFilePointer( [in] HANDLE hFile, [in] LONG lDistanceToMove, [in, out] PLONG lpDistanceToMoveHigh, [in] DWORD dwMoveMethod );

Repositions the file pointer to a specified location within a file. dwMoveMethod can be FILE_BEGIN, FILE_CURRENT, or FILE_END.

`GetFileSizeEx`

BOOL GetFileSizeEx( [in] HANDLE hFile, [out] PLARGE_INTEGER lpFileSize );

A more modern way to get the file size, returning it as a LARGE_INTEGER (64-bit value), which is essential for large files.

`CloseHandle`

BOOL CloseHandle( [in] HANDLE hObject );

Closes a handle to a kernel object, including file handles. Releasing handles prevents resource leaks.

`DeviceIoControl`

BOOL DeviceIoControl( [in] HANDLE hcDevice, [in] DWORD dwIoControlCode, [in, optional] LPVOID lpInBuffer, [in] DWORD nInBufferSize, [out, optional] LPVOID lpOutBuffer, [in] DWORD nOutBufferSize, [out, optional] LPDWORD lpBytesReturned, [in, out, optional] LPVOID lpOverlapped );

Used for performing device-specific operations. Many file system operations or driver interactions are exposed through this function using custom IOCTL codes.

Important: Always check the return values of file I/O functions and use GetLastError() to diagnose errors. Properly closing handles is crucial for system stability.

Tip: For improved performance with large files or when dealing with I/O-bound applications, consider using asynchronous I/O with the FILE_FLAG_OVERLAPPED flag and functions like ReadFileEx and WriteFileEx.

Security Consideration: Be cautious when constructing file paths from user input to prevent directory traversal vulnerabilities. Always validate and sanitize user-provided path components.