H5Z_REGISTER registers a new filter with the HDF5 library.

Making a new filter available to an application is a two-step process. The first step is to write the three filter callback functions described below: can_apply, set_local, and filter. This call to H5Z_REGISTER, registering the filter with the library, is the second step. The can_apply and set_local fields can be set to NULL if they are not required for the filter being registered.

 H5Z_REGISTER accepts a single parameter, a pointer to a buffer for the filter_class data structure. That data structure must conform to one of the following definitions:

        typedef struct H5Z_class1_t {
            H5Z_filter_t id;
            const char  *name;
            H5Z_can_apply_func_t can_apply;
            H5Z_set_local_func_t set_local;
            H5Z_func_t filter;
        } H5Z_class1_t;

        typedef struct H5Z_class2_t {
            int version;
            H5Z_filter_t id;
            unsigned encoder_present;
            unsigned decoder_present;
            const char  *name;
            H5Z_can_apply_func_t can_apply;
            H5Z_set_local_func_t set_local;
            H5Z_func_t filter;
        } H5Z_class2_t;
      

version is a library-defined value reporting the version number of the H5Z_class_t struct. This currently must be set to H5Z_CLASS_T_VERS.

id is the identifier for the new filter. This is a user-defined value between H5Z_FILTER_RESERVED and H5Z_FILTER_MAX. These values are defined in the HDF5 source file H5Zpublic.h, but the symbols H5Z_FILTER_RESERVED and H5Z_FILTER_MAX should always be used instead of the literal values.

encoder_present is a library-defined value indicating whether the filter’s encoding capability is available to the application.

decoder_present is a library-defined value indicating whether the filter’s encoding capability is available to the application.

name is a descriptive comment used for debugging, may contain a descriptive name for the filter, and may be the null pointer.

can_apply, described in detail below, is a user-defined callback function which determines whether the combination of the dataset creation property list values, the datatype, and the dataspace represent a valid combination to apply this filter to.

set_local, described in detail below, is a user-defined callback function which sets any parameters that are specific to this dataset, based on the combination of the dataset creation property list values, the datatype, and the dataspace.

filter, described in detail below, is a user-defined callback function which performs the action of the filter.

The statistics associated with a filter are not reset by this function; they accumulate over the life of the library.

H5Z_class_t is a macro which maps to either H5Z_class1_t or H5Z_class2_t, depending on the needs of the application. To affect only this macro, H5Z_class_t_vers may be defined to either 1 or 2. Otherwise, it will behave in the same manner as other API compatibility macros. See API Compatibility Macros in HDF5 for more information. H5Z_class1_t matches the H5Z_class_t structure that is used in the 1.6.x versions of the HDF5 library.

H5Z_REGISTER will automatically detect which structure type has been passed in, regardless of the mapping of the H5Z_class_t macro. However, the application must make sure that the fields are filled in according to the correct structure definition if the macro is used to declare the structure.

The callback functions 
Before H5Z_REGISTER  can link a filter into an application, three callback functions must be defined as described in the HDF5 library header file H5Zpublic.h.

When a filter is applied to the fractal heap for a group (e.g., when compressing group metadata) and if the can apply and set local callback functions have been defined for that filter, HDF5 passes the value -1 for all parameters for those callback functions. This is done to ensure that the filter will not be applied to groups if it relies on these parameters, as they are not applicable to group fractal heaps; to operate on group fractal heaps, a filter must be capable of operating on an opaque block of binary data.

The can apply callback function is defined as follows:

typedef htri_t (*H5Z_can_apply_func_t) (hid_t dcpl_id, hid_t type_id, hid_t space_id)

Before a dataset is created, the can apply callbacks for any filters used in the dataset creation property list are called with the dataset's dataset creation property list, dcpl_id, the dataset's datatype, type_id, and a dataspace describing a chunk, space_id, (for chunked dataset storage).

This callback must determine whether the combination of the dataset creation property list settings, the datatype, and the dataspace represent a valid combination to which to apply this filter. For example, an invalid combination may involve the filter not operating correctly on certain datatypes, on certain datatype sizes, or on certain sizes of the chunk dataspace. If this filter is enabled through H5P_SET_FILTER as optional and the can apply function returns 0, the library will skip the filter in the filter pipeline.

This callback can be the NULL pointer, in which case the library will assume that the filter can be applied to a dataset with any combination of dataset creation property list values, datatypes, and dataspaces.

The can apply callback function must return a positive value for a valid combination, zero for an invalid combination, and a negative value for an error.

The set local callback function is defined as follows:

typedef herr_t (*H5Z_set_local_func_t) (hid_t dcpl_id, hid_t type_id, hid_t space_id)

After the can apply callbacks are checked for a new dataset, the set local callback functions for any filters used in the dataset creation property list are called. These callbacks receive dcpl_id, the dataset's private copy of the dataset creation property list passed in to H5Dcreate (i.e. not the actual property list passed in to H5Dcreate); type_id, the datatype identifier passed in to H5Dcreate, which is not copied and should not be modified; and space_id, a dataspace describing the chunk (for chunked dataset storage), which should also not be modified.

The set local callback must set any filter parameters that are specific to this dataset, based on the combination of the dataset creation property list values, the datatype, and the dataspace. For example, some filters perform different actions based on different datatypes, datatype sizes, numbers of dimensions, or dataspace sizes.

The set local callback may be the NULL pointer, in which case, the library will assume that there are no dataset-specific settings for this filter.

The set local callback function must return a non-negative value on success and a negative value for an error.

The filter operation callback function, defining the filter's operation on the data, is defined as follows:

typedef size_t (*H5Z_func_t) (unsigned int flags, size_t cd_nelmts, const unsigned int cd_values[], size_t nbytes, size_t *buf_size, void **buf)

The parameters flags, cd_nelmts, and cd_values are the same as for the function H5Pset_filter. The one exception is that an additional flag, H5Z_FLAG_REVERSE, is set when the filter is called as part of the input pipeline.

The parameter *buf points to the input buffer which has a size of *buf_size bytes, nbytes of which are valid data.

The filter should perform the transformation in place if possible. If the transformation cannot be done in place, then the filter should allocate a new buffer with malloc() and assign it to *buf, assigning the allocated size of that buffer to *buf_size. The old buffer should be freed by calling free().

If successful, the filter operation callback function returns the number of valid bytes of data contained in *buf. In the case of failure, the return value is 0 (zero) and all pointer arguments are left unchanged.

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

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