Magic Buffers and io_uring Registered Buffers

# Magic Buffers and io_uring Registered Buffers – Some Assembly Required Source: [https://www.mindfruit.co.uk/posts/2025/10/magic-buffers-and-io-uring-write-fixed/](https://www.mindfruit.co.uk/posts/2025/10/magic-buffers-and-io-uring-write-fixed/) There’s a really cool little`mmap`gadget that maps the same underlying memory region into two contiguous virtual memory address ranges\. When you write past the end of the first mapped region, the remaining bytes end up in the second mapped region \(*and*at the start of the first one\)\. Fabien Giesen calls this a “[Magic Ring Buffer](https://fgiesen.wordpress.com/2012/07/21/the-magic-ring-buffer/)”, which is good enough for me\. I wondered whether the virtual memory contortions would survive contact with IO Uring’s registered buffers\. It turns out \(spoiler alert\) that they do, and it’s virtual memory all the way down\. I wrote a little test application, and have published it on my Github with some more comments[here](https://github.com/michael-g/mgkdb/tree/master/iou)\. Essentially, what the app does is: 1. Constructs a Magic Buffer \(using my`MgCircularBuffer`implementation[here](https://github.com/michael-g/mgkdb/blob/master/io/src/MgCircularBuffer.cpp), I know … it’s not circular\) 2. Uses`io\_uring\_register\_buffers`to register the buffer’s “double extent” with the kernel 3. Writes a kdb IPC message across the “seam” in the magic buffer 4. Uses`io\_uring\_prep\_write\_fixed`to send the message to a couple of connected KDB instances It works as expected\.