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| HDF5 Last Updated on 2026-07-28 The HDF5 Field Guide

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H5FDonion.h File Reference
`#include "H5FDpublic.h"`

## Data Structures
struct | H5FD_onion_fapl_info_t

## Macros
#define | H5FD_ONION (H5OPEN H5FD_ONION_id_g)
#define | H5FD_ONION_FAPL_INFO_COMMENT_MAX_LEN 255
#define | H5FD_ONION_FAPL_INFO_CREATE_FLAG_ENABLE_PAGE_ALIGNMENT (0x0001u)
#define | H5FD_ONION_FAPL_INFO_REVISION_ID_LATEST UINT64_MAX
#define | H5FD_ONION_FAPL_INFO_VERSION_CURR 1
#define | H5FD_ONION_VALUE H5_VFD_ONION

## Enumerations
enum | H5FD_onion_target_file_constant_t { H5FD_ONION_STORE_TARGET_ONION }

## Functions
herr_t | H5FDonion_get_revision_count (const char *filename, hid_t fapl_id, uint64_t *revision_count)
| get the number of revisions
herr_t | H5Pget_fapl_onion (hid_t fapl_id, H5FD_onion_fapl_info_t *fa_out)
| get the onion info from the file access property list
herr_t | H5Pset_fapl_onion (hid_t fapl_id, const H5FD_onion_fapl_info_t *fa)
| set the onion info for the file access property list

## Macro Definition Documentation

## ◆ H5FD_ONION

#define H5FD_ONION (H5OPEN H5FD_ONION_id_g)

ID for the onion VFD

## ◆ H5FD_ONION_FAPL_INFO_COMMENT_MAX_LEN

#define H5FD_ONION_FAPL_INFO_COMMENT_MAX_LEN 255

Max length of a comment. The buffer is defined to be this size + 1 to handle the NUL.

Since
  1.14.0

## ◆ H5FD_ONION_FAPL_INFO_CREATE_FLAG_ENABLE_PAGE_ALIGNMENT

#define H5FD_ONION_FAPL_INFO_CREATE_FLAG_ENABLE_PAGE_ALIGNMENT (0x0001u)

Onion history metadata will align to page_size. Partial pages of unused space will occur in the file, but may improve read performance from the backing store on some systems. If disabled (0), padding will not be inserted to align to page boundaries.

Since
  1.14.0

## ◆ H5FD_ONION_FAPL_INFO_REVISION_ID_LATEST

#define H5FD_ONION_FAPL_INFO_REVISION_ID_LATEST UINT64_MAX

Indicates that you want the latest revision.

## ◆ H5FD_ONION_FAPL_INFO_VERSION_CURR

#define H5FD_ONION_FAPL_INFO_VERSION_CURR 1

Current version of the onion VFD fapl info struct

## ◆ H5FD_ONION_VALUE

#define H5FD_ONION_VALUE H5_VFD_ONION

Identifier for the onion VFD

Since
  1.14.0

## Enumeration Type Documentation

## ◆ H5FD_onion_target_file_constant_t

enum H5FD_onion_target_file_constant_t

Indicates how the new onion data will be stored.

Enumerator
H5FD_ONION_STORE_TARGET_ONION | Onion history is stored in a single, separate "onion file". Shares filename and path as hdf5 file (if any), with only a different filename extension.