IOCP

IOCP stands for I/O Completion Port, a Windows kernel mechanism designed to enable scalable, asynchronous input/output. It allows a small pool of threads to efficiently handle thousands of outstanding I/O operations by delivering completion notifications instead of blocking per operation.

The core concept is to associate file handles and sockets with an I/O Completion Port (created via CreateIoCompletionPort). I/O operations are issued using an OVERLAPPED structure; when an operation finishes, Windows queues a completion packet to the IOCP. Each packet contains the number of bytes transferred, a user-defined completion key, and a pointer to the OVERLAPPED structure. Threads retrieve completed events by calling GetQueuedCompletionStatus (or GetQueuedCompletionStatusEx), which returns the associated completion key and overlapped pointer for identification.

A common usage pattern is a thread pool: worker threads wait on the IOCP and wake to process each completed I/O, then return to waiting. This model minimizes context switches and scales with the number of CPU cores, making it suitable for large numbers of concurrent I/O operations.

Typical applications include high-performance network servers and disk I/O workloads that use overlapped I/O. IOCP patterns frequently involve AcceptEx-based accept loops, concurrent ReadFile/WriteFile operations, and other asynchronous Windows I/O primitives.

Considerations include its Windows-specific nature and the need for careful management of resources. Developers must handle cancellation, error reporting, and cleanup of OVERLAPPED structures and associated handles. While there are alternative asynchronous models, IOCP remains a foundational mechanism for scalable I/O in Windows environments.