Windows I/O Subsystem

The Windows I/O subsystem provides a unified framework for performing input and output operations across a variety of devices, files, and communication channels. It abstracts low‑level details, enabling developers to interact with devices through a consistent set of APIs.

Key Concepts

File Handles – Opaque identifiers representing open resources such as files, sockets, or devices.
Overlapped I/O – Enables asynchronous operations without blocking the calling thread.
I/O Completion Ports (IOCP) – Scalable model for handling many simultaneous asynchronous I/O requests.
Device Drivers – Interact with hardware through the I/O manager using IRP (I/O Request Packets).

Sample: Reading a File Asynchronously

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

int main() {
    HANDLE hFile = CreateFileW(L"example.txt",
                               GENERIC_READ,
                               FILE_SHARE_READ,
                               NULL,
                               OPEN_EXISTING,
                               FILE_FLAG_OVERLAPPED,
                               NULL);
    if (hFile == INVALID_HANDLE_VALUE) {
        wprintf(L"Failed to open file (%%d)\n", GetLastError());
        return 1;
    }

    char buffer[256];
    OVERLAPPED ov = {0};
    ov.Offset = 0; // start at beginning

    BOOL result = ReadFile(hFile, buffer, sizeof(buffer) - 1, NULL, &ov);
    if (!result && GetLastError() != ERROR_IO_PENDING) {
        wprintf(L"ReadFile error (%%d)\n", GetLastError());
        CloseHandle(hFile);
        return 1;
    }

    // Wait for the operation to complete
    DWORD bytesRead;
    GetOverlappedResult(hFile, &ov, &bytesRead, TRUE);
    buffer[bytesRead] = '\0';
    printf("Data read: %%s\n", buffer);

    CloseHandle(hFile);
    return 0;
}

This example demonstrates how to open a file with the FILE_FLAG_OVERLAPPED flag, initiate an asynchronous read, and wait for completion using GetOverlappedResult.

Windows I/O Subsystem

Key Concepts

Sample: Reading a File Asynchronously

Further Reading