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

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VOL Mapping (H5M)

## Detailed Description

The interface can only be used with the HDF5 VOL connectors that implement map objects. The native HDF5 library does not support this feature.

While the HDF5 data model is a flexible way to store data, some applications require a more general way to index information. HDF5 effectively uses key-value stores internally for a variety of purposes, but it does not expose a generic key-value store to the API. The Map APIs provide this capability to the HDF5 applications in the form of HDF5 map objects. These Map objects contain application-defined key-value stores, to which key-value pairs can be added, and from which values can be retrieved by key.

HDF5 VOL connectors with support for map objects:

- DAOS

Example:
  // NOTE: This example requires a VOL connector with map support (e.g., DAOS)
  hid_t file_id, fapl_id, map_id, vls_type_id;
  const char *names[2] = {"Alice", "Bob"};
  uint64_t IDs[2] = {25385486, 34873275};
  uint64_t val_out;
  herr_t ret;
  // Setup file access property list with VOL connector that supports maps
  fapl_id = H5Pcreate(H5P_FILE_ACCESS);
  // ... configure VOL connector in fapl_id (see connector documentation) ...
  vls_type_id = H5Tcopy(H5T_C_S1);
  H5Tset_size(vls_type_id, H5T_VARIABLE);
  file_id = H5Fcreate("file.h5", H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
  map_id = H5Mcreate(file_id, "map", vls_type_id, H5T_NATIVE_UINT64,
  H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
  H5Mput(map_id, vls_type_id, &names[0], H5T_NATIVE_UINT64, &IDs[0], H5P_DEFAULT);
  H5Mput(map_id, vls_type_id, &names[1], H5T_NATIVE_UINT64, &IDs[1], H5P_DEFAULT);
  ret = H5Mget(map_id, vls_type_id, &names[0], H5T_NATIVE_UINT64, &val_out, H5P_DEFAULT);
  if(ret < 0 || val_out != IDs[0]) {
  fprintf(stderr, "Map retrieval failed\n");
  // Handle error...
  }
  H5Mclose(map_id);
  H5Tclose(vls_type_id);
  H5Pclose(fapl_id);
  H5Fclose(file_id);
  H5F_ACC_TRUNC
  #define H5F_ACC_TRUNC
  Definition H5Fpublic.h:30
  hid_t
  int64_t hid_t
  Definition H5Ipublic.h:60
  H5P_FILE_ACCESS
  #define H5P_FILE_ACCESS
  Definition H5Ppublic.h:56
  H5P_DEFAULT
  #define H5P_DEFAULT
  Definition H5Ppublic.h:220
  H5T_VARIABLE
  #define H5T_VARIABLE
  Definition H5Tpublic.h:209
  herr_t
  int herr_t
  Definition H5public.h:269
  H5Fclose
  herr_t H5Fclose(hid_t file_id)
  Terminates access to an HDF5 file.
  H5Fcreate
  hid_t H5Fcreate(const char *filename, unsigned flags, hid_t fcpl_id, hid_t fapl_id)
  Creates an HDF5 file.
  H5Mget
  herr_t H5Mget(hid_t map_id, hid_t key_mem_type_id, const void *key, hid_t val_mem_type_id, void *value, hid_t dxpl_id)
  Retrieves a key-value pair from a map object.
  H5Mput
  herr_t H5Mput(hid_t map_id, hid_t key_mem_type_id, const void *key, hid_t val_mem_type_id, const void *value, hid_t dxpl_id)
  Adds a key-value pair to a map object.
  H5Mcreate
  hid_t H5Mcreate(hid_t loc_id, const char *name, hid_t key_type_id, hid_t val_type_id, hid_t lcpl_id, hid_t mcpl_id, hid_t mapl_id)
  Creates a map object.
  H5Mclose
  herr_t H5Mclose(hid_t map_id)
  Terminates access to a map object.
  H5Tset_size
  herr_t H5Tset_size(hid_t type_id, size_t size)
  Sets size for a datatype.
  H5Tcopy
  hid_t H5Tcopy(hid_t type_id)
  Copies an existing datatype.
  H5Tclose
  herr_t H5Tclose(hid_t type_id)
  Releases a datatype.
  H5T_NATIVE_UINT64
  #define H5T_NATIVE_UINT64
  Definition H5Tpublic.h:1267
  H5T_C_S1
  #define H5T_C_S1
  Definition H5Tpublic.h:646
  H5Pclose
  herr_t H5Pclose(hid_t plist_id)
  Terminates access to a property list.
  H5Pcreate
  hid_t H5Pcreate(hid_t cls_id)
  Creates a new property list as an instance of a property list class.

## Functions
herr_t | H5Mclose (hid_t map_id)
| Terminates access to a map object.
hid_t | H5Mcreate (hid_t loc_id, const char *name, hid_t key_type_id, hid_t val_type_id, hid_t lcpl_id, hid_t mcpl_id, hid_t mapl_id)
| Creates a map object.
hid_t | H5Mcreate_anon (hid_t loc_id, hid_t key_type_id, hid_t val_type_id, hid_t mcpl_id, hid_t mapl_id)
| Creates a map object without linking it into a file.
herr_t | H5Mdelete (hid_t map_id, hid_t key_mem_type_id, const void *key, hid_t dxpl_id)
| Deletes a key-value pair from a map object.
herr_t | H5Mexists (hid_t map_id, hid_t key_mem_type_id, const void *key, bool *exists, hid_t dxpl_id)
| Checks if provided key exists in a map object.
herr_t | H5Mget (hid_t map_id, hid_t key_mem_type_id, const void *key, hid_t val_mem_type_id, void *value, hid_t dxpl_id)
| Retrieves a key-value pair from a map object.
hid_t | H5Mget_access_plist (hid_t map_id)
| Gets access property list for a map object.
herr_t | H5Mget_count (hid_t map_id, hsize_t *count, hid_t dxpl_id)
| Retrieves the number of key-value pairs in a map object.
hid_t | H5Mget_create_plist (hid_t map_id)
| Gets creation property list for a map object.
hid_t | H5Mget_key_type (hid_t map_id)
| Gets key datatype for a map object.
hid_t | H5Mget_val_type (hid_t map_id)
| Gets value datatype for a map object.
herr_t | H5Miterate (hid_t map_id, hsize_t *idx, hid_t key_mem_type_id, H5M_iterate_t op, void *op_data, hid_t dxpl_id)
| Iterates over all key-value pairs in a map object.
herr_t | H5Miterate_by_name (hid_t loc_id, const char *map_name, hsize_t *idx, hid_t key_mem_type_id, H5M_iterate_t op, void *op_data, hid_t dxpl_id, hid_t lapl_id)
| Iterates over all key-value pairs in a map object.
hid_t | H5Mopen (hid_t loc_id, const char *name, hid_t mapl_id)
| Opens a map object.
herr_t | H5Mput (hid_t map_id, hid_t key_mem_type_id, const void *key, hid_t val_mem_type_id, const void *value, hid_t dxpl_id)
| Adds a key-value pair to a map object.

## Function Documentation

## ◆ H5Mclose()

herr_t H5Mclose | ( | hid_t | map_id | ) |

Terminates access to a map object.

Parameters

  [in] | map_id | Map identifier

Returns
  Returns a non-negative value if successful; otherwise, returns a negative value.

H5Mclose() closes access to a map object specified by `map_id` and releases resources used by it.

It is illegal to subsequently use that same map identifier in calls to other map functions.

Since
  1.12.0

## ◆ H5Mcreate()

hid_t H5Mcreate | ( | hid_t | loc_id,
|  | const char * | name,
|  | hid_t | key_type_id,
|  | hid_t | val_type_id,
|  | hid_t | lcpl_id,
|  | hid_t | mcpl_id,
|  | hid_t | mapl_id )

Creates a map object.

Parameters

  [in] | loc_id | Location identifier. The identifier may be that of a file, group, dataset, named datatype, or attribute.
  [in] | name | Map object name
  [in] | key_type_id | Datatype identifier
  [in] | val_type_id | Datatype identifier
  [in] | lcpl_id | Link creation property list identifier
  [in] | mcpl_id | Map creation property list identifier
  [in] | mapl_id | Map access property list identifier

Returns
  Returns a map object identifier if successful; otherwise returns H5I_INVALID_HID.

H5Mcreate() creates a new map object for storing key-value pairs. The in-file datatype for keys is defined by `key_type_id` and the in-file datatype for values is defined by `val_type_id`. `loc_id` specifies the location to create the map object and `name` specifies the name of the link to the map object relative to `loc_id`.

Since
  1.12.0

## ◆ H5Mcreate_anon()

hid_t H5Mcreate_anon | ( | hid_t | loc_id,
|  | hid_t | key_type_id,
|  | hid_t | val_type_id,
|  | hid_t | mcpl_id,
|  | hid_t | mapl_id )

Creates a map object without linking it into a file.

Parameters

  [in] | loc_id | Location identifier. The identifier may be that of a file, group, dataset, named datatype, or attribute.
  [in] | key_type_id | Datatype identifier
  [in] | val_type_id | Datatype identifier
  [in] | mcpl_id | Map creation property list identifier
  [in] | mapl_id | Map access property list identifier

Returns
  Returns a map object identifier if successful; otherwise returns H5I_INVALID_HID. The resulting ID should be linked into the file with H5Olink or it will be deleted when closed.

H5Mcreate_anon() creates a new map object for storing key-value pairs. The in-file datatype for keys is defined by `key_type_id` and the in-file datatype for values is defined by `val_type_id`. `loc_id` specifies the file to create the map object, but no link to the object is created. Other options can be specified through the property lists `mcpl_id` and `mapl_id`.

The new map should be linked into the group hierarchy before being closed or it will be deleted. The map should be closed when the caller no longer requires it.

Since
  1.12.0

## ◆ H5Mdelete()

herr_t H5Mdelete | ( | hid_t | map_id,
|  | hid_t | key_mem_type_id,
|  | const void * | key,
|  | hid_t | dxpl_id )

Deletes a key-value pair from a map object.

Parameters

  [in] | map_id | Map identifier
  [in] | key_mem_type_id | Datatype identifier
  [in] | key | Pointer to key buffer
  [in] | dxpl_id | Dataset transfer property list identifier

Returns
  Returns a non-negative value if successful; otherwise, returns a negative value.

H5Mdelete() deletes a key-value pair from the map object specified by `map_id`. `key_mem_type_id` specifies the datatype for the provided key buffer key, and if different from that used to create the map object, the key will be internally converted to the datatype for the map object.

Any further options can be specified through the property list `dxpl_id`.

Since
  1.12.0

## ◆ H5Mexists()

herr_t H5Mexists | ( | hid_t | map_id,
|  | hid_t | key_mem_type_id,
|  | const void * | key,
|  | bool * | exists,
|  | hid_t | dxpl_id )

Checks if provided key exists in a map object.

Parameters

  [in] | map_id | Map identifier
  [in] | key_mem_type_id | Datatype identifier
  [in] | key | Pointer to key buffer
  [out] | exists | Pointer to a buffer to return the existence status
  [in] | dxpl_id | Dataset transfer property list identifier

Returns
  Returns a non-negative value if successful; otherwise, returns a negative value.

H5Mexists() checks if the provided key is stored in the map object specified by `map_id`. If `key_mem_type_id` is different from that used to create the map object the key will be internally converted to the datatype for the map object for the query.

Any further options can be specified through the property list `dxpl_id`.

Since
  1.12.0

## ◆ H5Mget()

herr_t H5Mget | ( | hid_t | map_id,
|  | hid_t | key_mem_type_id,
|  | const void * | key,
|  | hid_t | val_mem_type_id,
|  | void * | value,
|  | hid_t | dxpl_id )

Retrieves a key-value pair from a map object.

Parameters

  [in] | map_id | Map identifier
  [in] | key_mem_type_id | Datatype identifier
  [in] | key | Pointer to key buffer
  [in] | val_mem_type_id | Datatype identifier
  [out] | value | Pointer to value buffer
  [in] | dxpl_id | Dataset transfer property list identifier

Returns
  Returns a non-negative value if successful; otherwise, returns a negative value.

H5Mget() retrieves from a map object specified by `map_id`, the value associated with the provided key `key`. `key_mem_type_id` and `val_mem_type_id` specify the datatypes for the provided key and value buffers. If if the datatype specified by `key_mem_type_id` is different from that used to create the map object the key will be internally converted to the datatype for the map object for the query, and if the datatype specified by `val_mem_type_id` is different from that used to create the map object the returned value will be converted to have a datatype as specified by `val_mem_type_id` before the function returns.

Any further options can be specified through the property list `dxpl_id`.

Since
  1.12.0

## ◆ H5Mget_access_plist()

hid_t H5Mget_access_plist | ( | hid_t | map_id | ) |

Gets access property list for a map object.

Parameters

  [in] | map_id | Map identifier

Returns
  Returns a map access property list identifier if successful; otherwise returns H5I_INVALID_HID.

H5Mget_access_plist() returns an identifier for a copy of the access property list for a map object specified by `map_id`.

Since
  1.12.0

## ◆ H5Mget_count()

herr_t H5Mget_count | ( | hid_t | map_id,
|  | hsize_t * | count,
|  | hid_t | dxpl_id )

Retrieves the number of key-value pairs in a map object.

Parameters

  [in] | map_id | Map identifier
  [out] | count | The number of key-value pairs stored in the map object
  [in] | dxpl_id | Dataset transfer property list identifier

Returns
  Returns a non-negative value if successful; otherwise, returns a negative value.

H5Mget_count() retrieves the number of key-value pairs stored in a map specified by map_id.

Since
  1.12.0

## ◆ H5Mget_create_plist()

hid_t H5Mget_create_plist | ( | hid_t | map_id | ) |

Gets creation property list for a map object.

Parameters

  [in] | map_id | Map identifier

Returns
  Returns a map creation property list identifier if successful; otherwise returns H5I_INVALID_HID.

H5Mget_create_plist() returns an identifier for a copy of the creation property list for a map object specified by `map_id`.

The creation property list identifier should be released with H5Pclose() to prevent resource leaks.

Since
  1.12.0

## ◆ H5Mget_key_type()

hid_t H5Mget_key_type | ( | hid_t | map_id | ) |

Gets key datatype for a map object.

Parameters

  [in] | map_id | Map identifier

Returns
  Returns a datatype identifier if successful; otherwise returns H5I_INVALID_HID.

H5Mget_key_type() retrieves key datatype as stored in the file for a map object specified by `map_id` and returns identifier for the datatype.

Since
  1.12.0

## ◆ H5Mget_val_type()

hid_t H5Mget_val_type | ( | hid_t | map_id | ) |

Gets value datatype for a map object.

Parameters

  [in] | map_id | Map identifier

Returns
  Returns a datatype identifier if successful; otherwise returns H5I_INVALID_HID.

H5Mget_val_type() retrieves value datatype as stored in the file for a map object specified by `map_id` and returns identifier for the datatype .

Since
  1.12.0

## ◆ H5Miterate()

herr_t H5Miterate | ( | hid_t | map_id,
|  | hsize_t * | idx,
|  | hid_t | key_mem_type_id,
|  | H5M_iterate_t | op,
|  | void * | op_data,
|  | hid_t | dxpl_id )

Iterates over all key-value pairs in a map object.

Parameters

  [in] | map_id | Map identifier
  [in,out] | idx | iteration index
  [in] | key_mem_type_id | Datatype identifier
  [in] | op | User-defined iterator function
  [in,out] | op_data | User-defined callback function context
  [in] | dxpl_id | Dataset transfer property list identifier

Returns
  Returns a non-negative value if successful; otherwise, returns a negative value.

H5Miterate() iterates over all key-value pairs stored in the map object specified by `map_id`, making the callback specified by `op` for each. The `idx` parameter is an in/out parameter that may be used to restart a previously interrupted iteration. At the start of iteration `idx` should be set to 0, and to restart iteration at the same location on a subsequent call to H5Miterate(), `idx` should be the same value as returned by the previous call. Iterate callback is defined as:

typedef herr_t (*H5M_iterate_t)(hid_t map_id, const void *key, void *op_data);

The `key` parameter is the buffer for the key for this iteration, converted to the datatype specified by `key_mem_type_id`. The `op_data` parameter is a simple pass through of the value passed to H5Miterate(), which can be used to store application-defined data for iteration. A negative return value from this function will cause H5Miterate() to issue an error, while a positive return value will cause H5Miterate() to stop iterating and return this value without issuing an error. A return value of zero allows iteration to continue.

Any further options can be specified through the property list `dxpl_id`.

Warning
  Adding or removing key-value pairs to the map during iteration will lead to undefined behavior.

Attention
  Leaving callback functions:
  The callback function must return normally, even in the case of error. Returning with H5_ITER_ERROR, instead of leaving by means of exceptions, exit() function, etc... will allow the HDF5 library to manage its resources and maintain a consistent state. See C++ Developers using HDF5 C-API functions warning for detail.

Since
  1.12.0

## ◆ H5Miterate_by_name()

herr_t H5Miterate_by_name | ( | hid_t | loc_id,
|  | const char * | map_name,
|  | hsize_t * | idx,
|  | hid_t | key_mem_type_id,
|  | H5M_iterate_t | op,
|  | void * | op_data,
|  | hid_t | dxpl_id,
|  | hid_t | lapl_id )

Iterates over all key-value pairs in a map object.

Parameters

  [in] | loc_id | Location identifier
  [in] | map_name | Map object name relative to the location specified by `loc_id`
  [in,out] | idx | Iteration index
  [in] | key_mem_type_id | Datatype identifier
  [in] | op | User-defined iterator function
  [in,out] | op_data | User-defined callback function context
  [in] | dxpl_id | Dataset transfer property list identifier
  [in] | lapl_id | Link access property list identifier

Returns
  Returns a non-negative value if successful; otherwise, returns a negative value.

H5Miterate_by_name() iterates over all key-value pairs stored in the map object specified by `map_id`, making the callback specified by `op` for each. The `idx` parameter is an in/out parameter that may be used to restart a previously interrupted iteration. At the start of iteration `idx` should be set to 0, and to restart iteration at the same location on a subsequent call to H5Miterate(), `idx` should be the same value as returned by the previous call. Iterate callback is defined as:

typedef herr_t (*H5M_iterate_t)(hid_t map_id, const void *key, void *op_data);

The`key` parameter is the buffer for the key for this iteration, converted to the datatype specified by `key_mem_type_id`. The `op_data` parameter is a simple pass through of the value passed to H5Miterate(), which can be used to store application-defined data for iteration. A negative return value from this function will cause H5Miterate() to issue an error, while a positive return value will cause H5Miterate() to stop iterating and return this value without issuing an error. A return value of zero allows iteration to continue.

Any further options can be specified through the property list `dxpl_id`.

Warning
  Adding or removing key-value pairs to the map during iteration will lead to undefined behavior.

Attention
  Leaving callback functions:
  The callback function must return normally, even in the case of error. Returning with H5_ITER_ERROR, instead of leaving by means of exceptions, exit() function, etc... will allow the HDF5 library to manage its resources and maintain a consistent state. See C++ Developers using HDF5 C-API functions warning for detail.

Since
  1.12.0

## ◆ H5Mopen()

hid_t H5Mopen | ( | hid_t | loc_id,
|  | const char * | name,
|  | hid_t | mapl_id )

Opens a map object.

Parameters

  [in] | loc_id | Location identifier. The identifier may be that of a file, group, dataset, named datatype, or attribute.
  [in] | name | Map object name relative to `loc_id`
  [in] | mapl_id | Map access property list identifier

Returns
  Returns a map object identifier if successful; otherwise returns H5I_INVALID_HID.

H5Mopen() finds a map object specified by `name` under the location specified by `loc_id`. The map object should be close with H5Mclose() when the application is not longer interested in accessing it.

Since
  1.12.0

## ◆ H5Mput()

herr_t H5Mput | ( | hid_t | map_id,
|  | hid_t | key_mem_type_id,
|  | const void * | key,
|  | hid_t | val_mem_type_id,
|  | const void * | value,
|  | hid_t | dxpl_id )

Adds a key-value pair to a map object.

Parameters

  [in] | map_id | Map identifier
  [in] | key_mem_type_id | Datatype identifier
  [in] | key | Pointer to key buffer
  [in] | val_mem_type_id | Datatype identifier
  [in] | value | Pointer to value buffer
  [in] | dxpl_id | Dataset transfer property list identifier

Returns
  Returns a non-negative value if successful; otherwise, returns a negative value.

H5Mput() adds a key-value pair to a map object specified by `map_id`, or updates the value for the specified key if one was set previously.

`key_mem_type_id` and `val_mem_type_id` specify the datatypes for the provided key and value buffers, and if different from those used to create the map object, the key and value will be internally converted to the datatypes for the map object.

Any further options can be specified through the property list `dxpl_id`.

Since
  1.12.0