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HDF5 1.14.5
API Reference
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Attributes

Attributes

Creating "Large" HDF5 Attributes

Problem
You would like to use HDF5 attributes the size of whose values exceeds a few dozen kilobytes
Solution
A file format change in the HDF5 1.8.x family of library releases made it possible to have attributes larger than about 64 KiB. Ensure that the lower library version bound for new HDF5 item creation is at least 1.8.x, and create larger attributes as usual.
In the example below, we create an attribute whose value occupies 4 MiB.
Note
This feature is only supported in HDF5 1.8.x+
15 {
16 __label__ fail_attr, fail_aspace, fail_fapl, fail_file;
17 hid_t fapl, file, aspace, attr;
18
19 if ((fapl = H5Pcreate(H5P_FILE_ACCESS)) < 0) {
20 ret_val = EXIT_FAILURE;
21 goto fail_fapl;
22 }
23#if H5_VERSION_GE(1, 10, 0)
25#elif H5_VERSION_GE(1, 8, 0)
27#else
28#error Only HDF5 1.8.x and later supported.
29#endif
30 ret_val = EXIT_FAILURE;
31 goto fail_file;
32 }
33 if ((file = H5Fcreate("large_attribute.h5", H5F_ACC_TRUNC, H5P_DEFAULT, fapl)) < 0) {
34 ret_val = EXIT_FAILURE;
35 goto fail_file;
36 }
37
38 if ((aspace = H5Screate_simple(1, (hsize_t[]){1024 * 1024}, NULL)) < 0) {
39 ret_val = EXIT_FAILURE;
40 goto fail_aspace;
41 }
42 if ((attr = H5Acreate(file, "4MiB", H5T_IEEE_F32LE, aspace, H5P_DEFAULT, H5P_DEFAULT)) < 0) {
43 ret_val = EXIT_FAILURE;
44 goto fail_attr;
45 }
46
47 H5Aclose(attr);
48fail_attr:
49 H5Sclose(aspace);
50fail_aspace:
51 H5Fclose(file);
52fail_file:
53 H5Pclose(fapl);
54fail_fapl:;
55 }
@ H5F_LIBVER_V18
Definition H5Fpublic.h:187
#define H5F_LIBVER_LATEST
Definition H5Fpublic.h:194
#define H5F_ACC_TRUNC
Definition H5Fpublic.h:50
int64_t hid_t
Definition H5Ipublic.h:60
#define H5P_FILE_ACCESS
Definition H5Ppublic.h:64
#define H5P_DEFAULT
Definition H5Ppublic.h:228
uint64_t hsize_t
Definition H5public.h:301
herr_t H5Pset_libver_bounds(hid_t plist_id, H5F_libver_t low, H5F_libver_t high)
Controls the range of library release versions used when creating objects in a file.
#define H5Acreate
Definition H5version.h:692
herr_t H5Aclose(hid_t attr_id)
Closes the specified attribute.
herr_t H5Fclose(hid_t file_id)
Terminates access to an HDF5 file.
hid_t H5Fcreate(const char *filename, unsigned flags, hid_t fcpl_id, hid_t fapl_id)
Creates an HDF5 file.
herr_t H5Sclose(hid_t space_id)
Releases and terminates access to a dataspace.
hid_t H5Screate_simple(int rank, const hsize_t dims[], const hsize_t maxdims[])
Creates a new simple dataspace and opens it for access.
#define H5T_IEEE_F32LE
Definition H5Tpublic.h:272
herr_t H5Pclose(hid_t plist_id)
Terminates access to a property list.
hid_t H5Pcreate(hid_t cls_id)
Creates a new property list as an instance of a property list class.
Discussion
Large attributes are supported only in HDF5 versions 1.8.x and higher. This has implications for the accessibility of your HDF5 files and is your call.
Since there are no size limitations for large attributes, it might seem tempting to mistake them for dataset stand-ins. This is not the case, for at least two reasons:
  1. Attributes decorate HDF5 objects, have their own local namespace, and can't be link targets.
  2. Attribute I/O treats the attribute value as atomic, i.e., there is no support for partial I/O. A large attribute will always be read or written in its entirety.
See Also
See Maintaining Compatibility with other HDF5 Library Versions for HDF5 compatibility implications.