IO Completion Ports and OVERLAPPED management

debugcn Published at Dev

Edward83

How win32 manages instances of OVERLAPPED struct in context of two functions:

GetQueuedCompletionStatus
PostQueuedCompletionStatus

When I call GetQueuedCompletionStatus does win32 free instance of OVERLAPPED struct or I must do it by my self?
When I send data with PostQueuedCompletionStatus does win32 copy it to internal structs? When I must free memory of sent data?
Where I could find some picture with scheme of processing of OVERLAPPED data between GetQueuedCompletionStatus, PostQueuedCompletionStatus and IOCP queue?

Len Holgate

The OVERLAPPED structure must exist from when a successful I/O operation (or manual PostQueuedCompletionStatus()) executes until the OVERLAPPED emerges from a call to GetQueuedCompletionStatus().

You are responsible for the lifetime of the structure.

You'll see from the MSDN docs that GetQueuedCompletionStatus() actually takes "a pointer to a variable that receives the address of the OVERLAPPED structure that was specified when the completed I/O operation was started.". What you actually get out of that call is a pointer to the original OVERLAPPED that you passed when you made the PostQueuedCompletionStatus() call (or initiated an overlapped I/O operation).

This is all actually very useful as the "normal" way to use the OVERLAPPED structure is to place it inside a larger structure which holds all of the 'per operation' information that you might need - so it's the ideal way to navigate directly from the limited information that you're given when you call GetQueuedCompletionStatus() to, for example, the data buffer that you used in your overlapped read call...

I find the best way to deal with OVERLAPPED structures is to a) embed them in the buffer you're using for read/write b) reference count them and c) return them to a pool for reuse when the ref count drops to 0.

I have some source code that you could download (here) which may make this a little easier to understand (it's a full IOCP server example so it's a little complex, but it works and shows how these things can be used).

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edited at2021-06-20

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