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File Pointers and Offsets in Windows

Understanding how Windows manages file access is crucial for efficient and robust I/O operations. This involves grasping the concepts of file pointers and offsets, which dictate where read and write operations occur within a file.

What is a File Pointer?

A file pointer, often referred to as a current position indicator, is a logical marker within a file that determines the point at which the next read or write operation will take place. Each open file handle has its own independent file pointer. When you open a file, the pointer is typically positioned at the beginning of the file, ready for the first operation.

Understanding File Offsets

A file offset is a numerical value that represents the distance, in bytes, from the beginning of the file to a specific point. Offsets are used in conjunction with file pointers to precisely locate data within a file. The offset is a relative position, and the file pointer tracks the absolute current position.

Key File I/O Operations and Pointer Movement

Several core file I/O functions in the Windows API directly interact with and modify the file pointer:

• Reading Data: When you read data from a file using functions like ReadFile, the file pointer automatically advances by the number of bytes read. If you read 100 bytes, the pointer moves forward by 100 bytes.
• Writing Data: Similarly, after a successful write operation using WriteFile, the file pointer is advanced by the number of bytes written.
• Seeking: The SetFilePointerEx (or the older SetFilePointer) function is used to explicitly reposition the file pointer. This function takes an offset and a move method (beginning, current, or end of file) as parameters. This is fundamental for random access to file data.

Working with SetFilePointerEx

The SetFilePointerEx function is the modern and preferred way to manipulate file pointers. It allows you to move the pointer relative to a specified origin:

• FILE_BEGIN: Moves the pointer to an offset from the beginning of the file.
• FILE_CURRENT: Moves the pointer relative to its current position.
• FILE_END: Moves the pointer relative to the end of the file.

#include <windows.h>
#include <stdio.h>

int main() {
    HANDLE hFile = CreateFile(
        L"example.txt",
        GENERIC_READ | GENERIC_WRITE,
        0,
        NULL,
        OPEN_ALWAYS,
        FILE_ATTRIBUTE_NORMAL,
        NULL
    );

    if (hFile == INVALID_HANDLE_VALUE) {
        printf("Error creating or opening file: %lu\n", GetLastError());
        return 1;
    }

    LARGE_INTEGER liNewPosition;
    DWORD dwNewPosition;
    BOOL bSuccess;

    // Move pointer to the beginning of the file
    liNewPosition.QuadPart = 0;
    bSuccess = SetFilePointerEx(hFile, liNewPosition, NULL, FILE_BEGIN);
    if (!bSuccess) {
        printf("Failed to set file pointer to beginning: %lu\n", GetLastError());
    } else {
        printf("Pointer set to the beginning.\n");
    }

    // Move pointer 10 bytes from the current position
    liNewPosition.QuadPart = 10;
    bSuccess = SetFilePointerEx(hFile, liNewPosition, NULL, FILE_CURRENT);
    if (!bSuccess) {
        printf("Failed to move pointer 10 bytes forward: %lu\n", GetLastError());
    } else {
        printf("Pointer moved 10 bytes forward.\n");
    }

    // Get the current position (optional, usually done after a read/write)
    // For demonstration, let's set it again from the end and then get it
    liNewPosition.QuadPart = -20; // Move 20 bytes from the end
    bSuccess = SetFilePointerEx(hFile, liNewPosition, &liNewPosition, FILE_END);
    if (!bSuccess) {
        printf("Failed to move pointer from the end: %lu\n", GetLastError());
    } else {
        printf("Pointer is now at offset: %lld (from end)\n", liNewPosition.QuadPart);
    }


    CloseHandle(hFile);
    return 0;
}
            

Important Considerations

• Overlapping I/O: When using asynchronous I/O (overlapped I/O), the file pointer may not be updated automatically after an operation. You must manage the pointer explicitly.
• File Locking: File locks can affect pointer operations. If a region of a file is locked by another process, your attempts to seek or read/write within that region might fail.
• Large Files: For files larger than 4GB, ensure you use 64-bit data types (like LARGE_INTEGER) for offsets and positions, and functions that support them (e.g., SetFilePointerEx).

Mastering file pointers and offsets is a foundational skill for any developer working with file systems on Windows. It enables precise control over data manipulation and is essential for building efficient storage solutions.

See Also

• File Input and Output
• SetFilePointerEx Function
• File Handles