# The number of bytes used for "Page Bits" in Extensible Array Indexing Information

**URL:** <https://forum.hdfgroup.org/t/the-number-of-bytes-used-for-page-bits-in-extensible-array-indexing-information/13725>\
**Category:** HDF5 Library\
**Created:** [March 26, 2026, 7:53am UTC](https://forum.hdfgroup.org/t/the-number-of-bytes-used-for-page-bits-in-extensible-array-indexing-information/13725 "2026-03-26T07:53:20Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![furutaka.kazuyoshi](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/f/e5b9ba/32.png) [@furutaka.kazuyoshi](https://forum.hdfgroup.org/u/furutaka.kazuyoshi)\
**Post date:** [March 26, 2026, 7:53am UTC](https://forum.hdfgroup.org/t/the-number-of-bytes-used-for-page-bits-in-extensible-array-indexing-information/13725/1 "2026-03-26T07:53:20Z")

</div>

Does the “Page Bits” in Extensible Array Indexing Information really use two bytes?

 ![PageBits_ExtensibleArray](https://dbkt.hdfgroup.org/original/2X/3/3f1b9068b62dc75c6de207fc24a1e4c6d15ca084.jpeg)

---

<div class="post-metadata">

**Author:** ![gheber](https://forum.hdfgroup.org/user_avatar/forum.hdfgroup.org/gheber/32/1919_2.png) [@gheber](https://forum.hdfgroup.org/u/gheber)\
**Post date:** [March 26, 2026, 1:36pm UTC](https://forum.hdfgroup.org/t/the-number-of-bytes-used-for-page-bits-in-extensible-array-indexing-information/13725/2 "2026-03-26T13:36:36Z")

</div>

Thanks for the report @furutaka.kazuyoshi . If I look at the URL in your screenshot ([https://support.hdfgroup.org/documentation/hdf5/latest/\_f\_m\_t4.html#title22](https://support.hdfgroup.org/documentation/hdf5/latest/_f_m_t4.html#title22)), I see something quite different

 ![image](https://dbkt.hdfgroup.org/original/2X/2/2254d37af6d5f67593b468d7ab3a94b1ad846e1a.png)

The extensible array index section looks like this:

 ![image](https://dbkt.hdfgroup.org/original/2X/5/5d4aa85ada9acf11127b1aaa1056944bfb85abdc.png)

Maybe you have a stale cached copy? Or maybe I do 🤔

---

<div class="post-metadata">

**Author:** ![furutaka](https://forum.hdfgroup.org/letter_avatar_proxy/v4/letter/f/ba8739/32.png) [@furutaka](https://forum.hdfgroup.org/u/furutaka)\
**Post date:** [March 27, 2026, 11:23am UTC](https://forum.hdfgroup.org/t/the-number-of-bytes-used-for-page-bits-in-extensible-array-indexing-information/13725/3 "2026-03-27T11:23:54Z")

</div>

Thanks for your response.

I couldn’t find any direct link to the corresponding tables (so the url seen is not what I want to point to, sorry), and I did find the tables by looking for the string “Extensible Array Indexing Information” by searching for the string, “Extensible Array Indexing Information”, which are highlighted in the image I attached (there are only two in the manual)

---

<div class="post-metadata">

**Author:** ![gheber](https://forum.hdfgroup.org/user_avatar/forum.hdfgroup.org/gheber/32/1919_2.png) [@gheber](https://forum.hdfgroup.org/u/gheber)\
**Post date:** [March 27, 2026, 1:57pm UTC](https://forum.hdfgroup.org/t/the-number-of-bytes-used-for-page-bits-in-extensible-array-indexing-information/13725/4 "2026-03-27T13:57:23Z")

</div>

> [@furutaka](#):
>
> Extensible Array Indexing Information

You are correct. _This is an error in the specification._ The original [RFC](https://support.hdfgroup.org/releases/hdf5/documentation/rfc/2015-09-28-RFC-HDF5-1.10.0-File-Format-Superblock-Changes-EP.docx.pdf) states on page 12 that the Extensible Array Indexing Information byte size is 5 (4+1). The source code shows the same picture. I’ll create a GitHub issue.

Thanks, G.

---

<div class="post-metadata">

**Author:** ![gheber](https://forum.hdfgroup.org/user_avatar/forum.hdfgroup.org/gheber/32/1919_2.png) [@gheber](https://forum.hdfgroup.org/u/gheber)\
**Post date:** [March 27, 2026, 4:01pm UTC](https://forum.hdfgroup.org/t/the-number-of-bytes-used-for-page-bits-in-extensible-array-indexing-information/13725/5 "2026-03-27T16:01:42Z")

</div>

For reference: [Error in the file format specification: Incorrect 'Page Bits' field size in Extensible Array Indexing Information · Issue #6325 · HDFGroup/hdf5 · GitHub](https://github.com/HDFGroup/hdf5/issues/6325)