H5T: Datatype Interface

Datatype Object API Functions

Name: H5Tcreate

Signature:

hid_t H5Tcreate(H5T_class_t class, size_tsize )

Description:

This function creates a new dataype of the specified class with the specified number of bytes. Currently, only the H5T_COMPOUND datatype class is supported with this function, use H5Tcopy to create integer or floating-point datatypes. The datatype ID returned from this function should be released with H5Tclose or resource leaks will result.

Parameters:

H5T_class_t class

Class of datatype to create.

size_t size

The number of bytes in the datatype to create.

Returns:

Datatype ID on success, negative on failure.

Name: H5Tcopy

Signature:

hid_t H5Tcopy(hid_t type_id )

Description:

This function copies an existing datatype. The datatype ID returned should be released with H5Tclose or resource leaks will occur. Native datatypes supported by the library are:

H5T_NATIVE_CHAR

Native character type, declare dataset array as 'char'

H5T_NATIVE_UCHAR

Native unsigned character type, declare dataset array as 'unsigned char'

H5T_NATIVE_SHORT

Native short type, declare dataset array as 'short'

H5T_NATIVE_USHORT

Native unsigned short type, declare dataset array as 'unsigned short'

H5T_NATIVE_INT

Native int type, declare dataset array as 'int'

H5T_NATIVE_UINT

Native unsigned int type, declare dataset array as 'unsigned int'

H5T_NATIVE_LONG

Native long type, declare dataset array as 'unsigned long'

H5T_NATIVE_ULONG

Native unsigned long type, declare dataset array as 'unsigned long'

H5T_NATIVE_LLONG

Native long long type, declare dataset array as 'unsigned long long'

H5T_NATIVE_ULLONG

Native unsigned long long type, declare dataset array as 'unsigned long long'

H5T_NATIVE_INT8

Native signed 8-bit type, declare dataset array as 'int8'

H5T_NATIVE_UINT8

Native unsigned 8-bit type, declare dataset array as 'uint8'

H5T_NATIVE_INT16

Native signed 16-bit type, declare dataset array as 'int16'

H5T_NATIVE_UINT16

Native unsigned 16-bit type, declare dataset array as 'uint16'

H5T_NATIVE_INT32

Native signed 32-bit type, declare dataset array as 'int32'

H5T_NATIVE_UINT32

Native unsigned 32-bit type, declare dataset array as 'uint32'

H5T_NATIVE_INT64

Native signed 64-bit type, declare dataset array as 'uint64'

H5T_NATIVE_UINT64

Native unsigned 64-bit type, declare dataset array as 'uint64'

H5T_NATIVE_FLOAT

Native single-precision float type, declare dataset array as 'float'

H5T_NATIVE_DOUBLE

Native double-precision float type, declare dataset array as 'double'

Parameters:

hid_t type_id

ID of datatype to copy.

Returns:

Datatype ID on success, negative on failure.

Name: H5Tequal

Signature:

hbool_t H5Tequal(hid_t type_id1, hid_ttype_id2 )

Description:

This function determines if two datatype IDs refer to the same datatype.

Parameters:

hid_t type_id1

ID of datatype to compare.

hid_t type_id2

ID of datatype to compare.

Returns:

TRUE/FALSE/negative

Name: H5Tlock

Signature:

herr_t H5Tlock(hid_t type_id )

Description:

This function locks a type, making it read-only and non-destrucible. This is normally done by the library for predefined data types so the application doesn't inadvertently change or delete a predefined type. Once a data type is locked it can never be unlocked.

Parameters:

hid_t type_id

ID of datatype to lock.

Returns:

zero/negative

Name: H5Tget_class

Signature:

H5T_class_t H5Tget_class(hid_t type_id )

Description:

This function returns the base class of a datatype.

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Non-negative type class on success, negative on failure.

Name: H5Tget_size

Signature:

size_t H5Tget_size(hid_t type_id )

Description:

This function returns the size of a datatype in bytes.

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Positve size in bytes on success, 0 on failure.

Name: H5Tset_size

Signature:

herr_t H5Tset_size(hid_t type_id, size_tsize )

Description:

This function sets the total size in bytes for an atomic data type (this operation is not permitted on compound data types). If the size is decreased so that the significant bits of the data type extend beyond the edge of the new size, then the `offset' property is decreased toward zero. If the `offset' becomes zero and the significant bits of the data type still hang over the edge of the new size, then the number of significant bits is decreased. Adjusting the size of an H5T_STRING automatically sets the precision to 8*size. All data types have a positive size.

Parameters:

hid_t type_id

ID of datatype to change size.

size_t size

Size in bytes to modify datatype.

Returns:

zero/negative

Name: H5Tget_order

Signature:

H5T_order_t H5Tget_order(hid_t type_id )

Description:

This function returns the byte order of an atomic datatype.

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Byte order constant on success, negative on failure

Name: H5Tset_order

Signature:

herr_t H5Tset_order(hid_t type_id, H5T_order_torder )

Description:

This function sets the byte ordering of an atomic datatype. Byte orderings currently supported are:

H5T_ORDER_LE

Little-endian byte ordering (default)

H5T_ORDER_BE

Big-endian byte ordering

H5T_ORDER_Vax

VAX-endianness byte ordering (not currently supported)

Parameters:

hid_t type_id

ID of datatype to set.

H5T_order_t order

Byte ordering constant.

Returns:

zero/negative

Name: H5Tget_precision

Signature:

size_t H5Tget_precision(hid_t type_id )

Description:

This function returns the precision of an atomic data type. The precision is the number of significant bits which, unless padding is present, is 8 times larger than the value returned by H5Tget_size().

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Number of significant bits on success, 0 on failure

Name: H5Tset_precision

Signature:

herr_t H5Tset_precision(hid_t type_id, size_tprecision )

Description:

This function sets the precision of an atomic data type. The precision is the number of significant bits which, unless padding is present, is 8 times larger than the value returned by H5Tget_size().

If the precision is increased then the offset is decreased and then the size is increased to insure that significant bits do not "hang over" the edge of the data type.

Changing the precision of an H5T_STRING automatically changes the size as well. The precision must be a multiple of 8.

When decreasing the precision of a floating point type, set the locations and sizes of the sign, mantissa, and exponent fields first.

Parameters:

hid_t type_id

ID of datatype to set.

size_t precision

Number of bits of precision for datatype.

Returns:

zero/negative

Name: H5Tget_offset

Signature:

size_t H5Tget_offset(hid_t type_id )

Description:

This function retrieves the bit offset of the first significant bit. The signficant bits of an atomic datum can be offset from the beginning of the memory for that datum by an amount of padding. The `offset' property specifies the number of bits of padding that appear to the "right of" the value. That is, if we have a 32-bit datum with 16-bits of precision having the value 0x1122 then it will be layed out in memory as (from small byte address toward larger byte addresses):

Byte Position	Big-Endian Offset=0	Big-Endian Offset=16	Little-Endian Offset=0	Little-Endian Offset=16
0:	[ pad]	[0x11]	[0x22]	[ pad]
1:	[ pad]	[0x22]	[0x11]	[ pad]
2:	[0x11]	[ pad]	[ pad]	[0x22]
3:	[0x22]	[ pad]	[ pad]	[0x11]

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Positive offset value on success, 0 on failure.

Name: H5Tset_offset

Signature:

herr_t H5Tset_offset(hid_t type_id, size_t offset )

Description:

This function sets the bit offset of the first significant bit. The signficant bits of an atomic datum can be offset from the beginning of the memory for that datum by an amount of padding. The `offset' property specifies the number of bits of padding that appear to the "right of" the value. That is, if we have a 32-bit datum with 16-bits of precision having the value 0x1122 then it will be layed out in memory as (from small byte address toward larger byte addresses):

Byte Position	Big-Endian Offset=0	Big-Endian Offset=16	Little-Endian Offset=0	Little-Endian Offset=16
0:	[ pad]	[0x11]	[0x22]	[ pad]
1:	[ pad]	[0x22]	[0x11]	[ pad]
2:	[0x11]	[ pad]	[ pad]	[0x22]
3:	[0x22]	[ pad]	[ pad]	[0x11]

If the offset is incremented then the total size is incremented also if necessary to prevent significant bits of the value from hanging over the edge of the data type.

The offset of an H5T_STRING cannot be set to anything but zero.

Parameters:

hid_t type_id

ID of datatype to set.

size_t offset

Offset of first significant bit.

Returns:

zero/negative

Name: H5Tget_pad

Signature:

herr_t H5Tget_pad(hid_t type_id, H5T_pad_t * lsb, H5T_pad_t * msb )

Description:

This function retrieves the padding type of the least and most-significant bit padding. Valid types are:

H5T_PAD_ZERO

Set background to zeros.

H5T_PAD_ONE

Set background to ones.

H5T_PAD_BACKGROUND

Leave background alone.

Parameters:

hid_t type_id

ID of datatype to query.

H5T_pad_t * lsb

Pointer to location to return least-significant bit padding type.

H5T_pad_t * msb

Pointer to location to return most-significant bit padding type.

Returns:

zero/negative

Name: H5Tset_pad

Signature:

herr_t H5Tset_pad(hid_t type_id, H5T_pad_t lsb, H5T_pad_t msb )

Description:

This function sets the least and most-significant bits padding types.

H5T_PAD_ZERO

Set background to zeros.

H5T_PAD_ONE

Set background to ones.

H5T_PAD_BACKGROUND

Leave background alone.

Parameters:

hid_t type_id

ID of datatype to set.

H5T_pad_t lsb

Padding type for least-significant bits.

H5T_pad_t msb

Padding type for most-significant bits.

Returns:

zero/negative

Name: H5Tget_sign

Signature:

H5T_sign_t H5Tget_sign(hid_t type_id )

Description:

This function retrieves the sign type for an integer type. Valid types are:

H5T_SGN_NONE

Unsigned integer type.

H5T_SGN_2

Two's complement signed integer type.

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Non-negative sign type on success, negative on failure

Name: H5Tset_sign

Signature:

herr_t H5Tset_sign(hid_t type_id, H5T_sign_t sign )

Description:

This function sets the sign proprety for an integer type.

H5T_SGN_NONE

Unsigned integer type.

H5T_SGN_2

Two's complement signed integer type.

Parameters:

hid_t type_id

ID of datatype to set.

H5T_sign_t sign

Sign type.

Returns:

zero/negative

Name: H5Tget_fields

Signature:

herr_t H5Tget_fields(hid_t type_id, size_t * epos, size_t * esize, size_t * mpos, size_t * msize )

Description:

This function retrieves information about the locations of the various bit fields of a floating point data type. The field positions are bit positions in the significant region of the data type. Bits are numbered with the least significant bit number zero. Any (or even all) of the arguments can be null pointers.

Parameters:

hid_t type_id

ID of datatype to query.

size_t * epos

Pointer to location to return exponent bit-position.

size_t * esize

Pointer to location to return size of exponent in bits.

size_t * mpos

Pointer to location to return mantissa bit-position.

size_t * msize

Pointer to location to return size of mantissa in bits.

Returns:

zero/negative

Name: H5Tset_fields

Signature:

herr_t H5Tset_fields(hid_t type_id, size_t epos, size_t esize, size_t mpos, size_t msize )

Description:

This function sets the locations and sizes of the various floating point bit fields. The field positions are bit positions in the significant region of the data type. Bits are numbered with the least significant bit number zero.

Fields are not allowed to extend beyond the number of bits of precision, nor are they allowed to overlap with one another.

Parameters:

hid_t type_id

ID of datatype to set.

size_t epos

Exponent bit position.

size_t esize

Size of exponent in bits.

size_t mpos

Mantissa bit position.

size_t msize

Size of mantissa in bits.

Returns:

zero/negative

Name: H5Tget_ebias

Signature:

size_t H5Tget_ebias(hid_t type_id )

Description:

This function retrieves the exponent bias of a floating-point type.

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Positive value on success, 0 on failure.

Name: H5Tset_ebias

Signature:

herr_t H5Tset_ebias(hid_t type_id, size_t ebias )

Description:

This function sets the exponent bias of a floating-point type.

Parameters:

hid_t type_id

ID of datatype to set.

size_t ebias

Exponent bias value.

Returns:

zero/negative

Name: H5Tget_norm

Signature:

H5T_norm_t H5Tget_norm(hid_t type_id )

Description:

This function retrieves the mantissa normalization of a floating-point datatype. Valid normalization values are:

H5T_NORM_IMPLIED

MSB of mantissa isn't stored, always 1

H5T_NORM_MSBSET

MSB of mantissa is always 1

H5T_NORM_NONE

Mantissa is not normalized

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Non-negative normalization type on success, negative on failure

Name: H5Tset_norm

Signature:

herr_t H5Tset_norm(hid_t type_id, H5T_norm_t norm )

Description:

This function sets the mantissa normalization of a floating-point datatype. Valid normalization values are:

H5T_NORM_IMPLIED

MSB of mantissa isn't stored, always 1

H5T_NORM_MSBSET

MSB of mantissa is always 1

H5T_NORM_NONE

Mantissa is not normalized

Parameters:

hid_t type_id

ID of datatype to set.

H5T_norm_t norm

Mantissa normalization type.

Returns:

zero/negative

Name: H5Tget_inpad

Signature:

H5T_pad_t H5Tget_inpad(hid_t type_id )

Description:

This function retrieves the internal padding type for unused bits in floating-point datatypes. Valid padding values are:

H5T_PAD_ZERO

Set background to zeros.

H5T_PAD_ONE

Set background to ones.

H5T_PAD_BACKGROUND

Leave background alone.

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Non-negative padding type on success, negative on failure

Name: H5Tset_inpad

Signature:

herr_t H5Tset_inpad(hid_t type_id, H5T_pad_t inpad )

Description:

If any internal bits of a floating point type are unused (that is, those significant bits which are not part of the sign, exponent, or mantissa) then they will be filled according to the value of this property. Valid padding values are:

H5T_PAD_ZERO

Set background to zeros.

H5T_PAD_ONE

Set background to ones.

H5T_PAD_BACKGROUND

Leave background alone.

Parameters:

hid_t type_id

ID of datatype to modify.

H5T_pad_t pad

Padding type.

Returns:

zero/negative

Name: H5Tget_cset

Signature:

H5T_cset_t H5Tget_cset(hid_t type_id )

Description:

This function retrieves the character set type of a string datatype. Valid character set values are:

H5T_CSET_ASCII

Character set is US ASCII

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Non-negative character set type on success, negative on failure

Name: H5Tset_cset

Signature:

herr_t H5Tset_cset(hid_t type_id, H5T_cset_t cset )

Description:

HDF5 is able to distinguish between character sets of different nationalities and to convert between them to the extent possible. Valid character set values are:

H5T_CSET_ASCII

Character set is US ASCII

Parameters:

hid_t type_id

ID of datatype to modify.

H5T_cset_t cset

Character set type.

Returns:

zero/negative

Name: H5Tget_strpad

Signature:

H5T_str_t H5Tget_strpad(hid_t type_id )

Description:

This function retrieves the string padding method for a string datatype. Valid string padding values are:

H5T_STR_NULL

Pad with zeros (as C does)

H5T_STR_SPACE

Pad with spaces (as FORTRAN does)

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Non-negative string padding type on success, negative on failure

Name: H5Tset_strpad

Signature:

herr_t H5Tset_strpad(hid_t type_id, H5T_str_t strpad )

Description:

The method used to store character strings differs with the programming language: C usually null terminates strings while Fortran left-justifies and space-pads strings. This property defines the storage mechanism for the string. Valid string padding values are:

H5T_STR_NULL

Pad with zeros (as C does)

H5T_STR_SPACE

Pad with spaces (as FORTRAN does)

Parameters:

hid_t type_id

ID of datatype to modify.

H5T_str_t strpad

String padding type.

Returns:

zero/negative

Name: H5Tget_nmembers

Signature:

intn H5Tget_nmembers(hid_t type_id )

Description:

This function retrieves the number of fields a compound datatype has.

Parameters:

hid_t type_id

ID of datatype to query.

Returns:

Number of members datatype has on success, negative on failure

Name: H5Tget_member_name

Signature:

char * H5Tget_member_name(hid_t type_id, intn fieldno )

Description:

This function retrieves the name of a field of a compound data type. Fields are stored in no particular order with numbers 0 through N-1 where N is the value returned by H5Tget_nmembers(). The name of the field is allocated with malloc() and the caller is responsible for freeing the memory used by the name.

Parameters:

hid_t type_id

ID of datatype to query.

intn fieldno

Field number (indexed from 0) of the field name to retrieve.

Returns:

Valid pointer on success, NULL on failure

Name: H5Tget_member_dims

Signature:

int H5Tget_member_dims(hid_t type_id, intn fieldno, size_t * dims, int * perm )

Description:

This function returns the dimensionality of the field. The dimensions and permuation vector are returned through arguments dims and perm, both arrays of at least four elements. Either (or even both) may be null pointers.

Parameters:

hid_t type_id

ID of datatype to query.

intn fieldno

Field number (indexed from 0) of the field dims to retrieve.

size_t * dims

Pointer to buffer to store the dimensions of the field.

int * perm

Pointer to buffer to store the permutation vector of the field.

Returns:

Number of dimensions on success, negative on failure.

Name: H5Tget_member_type

Signature:

hid_t H5Tget_member_type(hid_t type_id, intn fieldno )

Description:

This function returns the data type of the specified member. The caller should invoke H5Tclose() to release resources associated with the type.

Parameters:

hid_t type_id

ID of datatype to query.

intn fieldno

Field number (indexed from 0) of the field type to retrieve.

Returns:

The ID of a copy of the datatype of the field, negative on failure.

Name: H5Tinsert

Signature:

herr_t H5Tinsert(hid_t type_id, const char * name, off_t offset, hid_t field_id )

Description:

This function adds another member to the compound data type type_id. The new member has a name which must be unique within the compound data type. The offset argument defines the start of the member in an instance of the compound data type, and field_id is the type of the new member.

Note: All members of a compound data type must be atomic; a compound data type cannot have a member which is a compound data type.

Parameters:

hid_t type_id

ID of compound datatype to modify.

const char * name

Name of the field to insert.

off_t offset

Offset in memory structure of the field to insert.

hid_t field_id

Datatype ID of the field to insert.

Returns:

zero/negative

Name: H5Tpack

Signature:

herr_t H5Tpack(hid_t type_id )

Description:

This function recursively removes padding from within a compound datatype to make it more efficient (space-wise) to store that data.

Parameters:

hid_t type_id

ID of datatype to modify.

Returns:

zero/negative

Name: H5Tregister_hard

Signature:

herr_t H5Tregister_hard(const char * name, hid_t src_id, hid_t dst_id, H5T_conv_t func )

Description:

This function registers a hard conversion function for a data type conversion path. The path is specified by the source and destination datatypes src_id and dst_id. A conversion path can only have one hard function, so func replaces any previous hard function.

If func is the null pointer then any hard function registered for this path is removed from this path. The soft functions are then used when determining which conversion function is appropriate for this path. The name argument is used only for debugging and should be a short identifier for the function.

The type of the conversion function pointer is declared as: typedef herr_t (*H5T_conv_t) (hid_t src_id, hid_t dst_id, H5T_cdata_t *cdata, size_t nelmts, void *buf, void *bkg);

Parameters:

const char * name

Name displayed in diagnostic output.

hid_t src_id

ID of source datatype.

hid_t dst_id

ID of destination datatype.

H5T_conv_t func

Function to convert between source and destination datatypes.

Returns:

zero/negative

Name: H5Tregister_soft

Signature:

herr_t H5Tregister_soft(const char * name, hid_t src_id, hid_t dst_id, H5T_conv_t func )

Description:

This function registers a soft conversion function by adding it to the end of the master soft list and replacing the soft function in all applicable existing conversion paths. The name is used only for debugging and should be a short identifier for the function.

The type of the conversion function pointer is declared as: typedef herr_t (*H5T_conv_t) (hid_t src_id, hid_t dst_id, H5T_cdata_t *cdata, size_t nelmts, void *buf, void *bkg);

Parameters:

const char * name

Name displayed in diagnostic output.

hid_t src_id

ID of source datatype.

hid_t dst_id

ID of destination datatype.

H5T_conv_t func

Function to convert between source and destination datatypes.

Returns:

zero/negative

Name: H5Tunregister

Signature:

herr_t H5Tunregister(H5T_conv_t func )

Description:

This function removes a conversion function from all conversion paths.

The type of the conversion function pointer is declared as: typedef herr_t (*H5T_conv_t) (hid_t src_id, hid_t dst_id, H5T_cdata_t *cdata, size_t nelmts, void *buf, void *bkg);

Parameters:

H5T_conv_t func

Function to remove from conversion paths.

Returns:

zero/negative

Name: H5Tclose

Signature:

herr_t H5Tclose(hid_t type_id )

Description:

This function releases a datatype. Further access through the datatype ID is illegal. Failure to release a datatype with this call will result in resource leaks.

Parameters:

hid_t type_id

ID of datatype to release.

Returns:

zero/negative

Elena> Datatype Interface:
Elena> Do we have description of the named datatypes somewhere?

>From Datatypes.html...

html> 7. Sharing Data Types among Datasets
html> 
html> If a file has lots of datasets which have a common data type
html> then the file could be made smaller by having all the datasets
html> share a single data type.  Instead of storing a copy of the data
html> type in each dataset object header, a single data type is stored
html> and the object headers point to it. The space savings is
html> probably only significant for datasets with a compound data type
html> since the simple data types can be described with just a few
html> bytes anyway.
html> 
html> To create a bunch of datasets that share a single data type just
html> create the datasets with a committed (named) data type.
html> 
html>  To create two datasets that share a common data type one just
html>  commits the data type, giving it a name, and then uses that
html>  data type to create the datasets.
html> 
html>  hid_t t1 = ...some transient type...;
html>  H5Tcommit (file, "shared_type", t1);
html>  hid_t dset1 = H5Dcreate (file, "dset1", t1, space, H5P_DEFAULT);
html>  hid_t dset2 = H5Dcreate (file, "dset2", t1, space, H5P_DEFAULT);
html>                
html> 
html>  And to create two additional datasets later which share the
html>  same type as the first two datasets:
html> 
html>  hid_t dset1 = H5Dopen (file, "dset1");
html>  hid_t t2 = H5Dget_type (dset1);
html>  hid_t dset3 = H5Dcreate (file, "dset3", t2, space, H5P_DEFAULT);
html>  hid_t dset4 = H5Dcreate (file, "dset4", t2, space, H5P_DEFAULT);
html>                
html> 
html>                                     Example: Shared Types

Mail from Quincey summarizing shared data types:

Quincey> Hi Robb,
Quincey>     Everything looks good, I just have a couple of minor comments below:
Quincey> 
Quincey> > A very quick data types summary (so I can remember it next week :-)
Quincey> > 
Quincey> >    * Handles to named types are immutable.
Quincey> > 
Quincey> >    * A transient type handle can be converted to a named type handle
Quincey> >      by calling H5Tcommit().  This can only be called for transient
Quincey> >      types which are not locked or predefined.
Quincey> > 
Quincey> >    * H5Topen() returns a handle to a named immutable type.
Quincey> > 
Quincey> >    * H5Tcopy() returns a handle to a transient type.
Quincey>     H5Tcreate also returns a handle to a transient type.
Quincey> 
Quincey> >    * Using a named type in H5Dcreate() causes the dataset object
Quincey> >      header to point to the named type (shared).  The link count on
Quincey> >      the named type is incremented.
Quincey> > 
Quincey> >    * Using a transient type in H5Dcreate() causes the type to be
Quincey> >      copied and stored in the dataset header (unshared).
Quincey> > 
Quincey> >    * Type handles returned from H5Dget_type() are immutable.
Quincey> > 
Quincey> >    * If the dataset is using a shared type (dataset object header
Quincey> >      points to some other object header with a type message, e.g., a
Quincey> >      named type) then H5Dget_type() returns a handle to that named
Quincey> >      type.
Quincey> > 
Quincey> >    * If the dataset has a private type (data type is stored in the
Quincey> >      dataset object header) then H5Dget_type() returns a handle to a
Quincey> >      transient immutable type.
Quincey> > 
Quincey> >    * The name of a data type can be removed from a group, but unless
Quincey> >      the reference count becomes zero the type continues to exist.
Quincey> >      (Other objects work this way too).
Quincey> > 
Quincey> >    * H5Tcopy() applied to a dataset returns a transient, modifiable
Quincey> >      copy of that dataset's data type.
Quincey> > 
Quincey> >    * H5Topen() applied to a dataset returns either a transient
Quincey> >      immutable or named immutable data type depending on whether the
Quincey> >      dataset has a shared data type.
Quincey>     Hmm, do we want to allow this?  It makes a certain amount of sense, but
Quincey> is a little unusual... :-)
Quincey> 

Elena, we decided not not to allow H5Topen() on a dataset.

Quincey> 
Quincey> >    * The H5Tshare() and H5Tis_shared() will be removed.  Data types
Quincey> >      will not be stored in the global heap.  A new type of shared
Quincey> >      message header will be added to the object headers that points to 
Quincey> >      another object header instead of the global heap
Quincey> 
Quincey> >    * Still to discuss: Attributes on named data types?
Quincey>     I think we should all them.
Quincey> 

Elena, attributes work for named data types just like they do for
datasets.

Quincey> 
Quincey> >    * Still to discuss: compound types whose members point to other types.
Quincey>     I like this concept a lot and this we should figure out a way to do it.
Quincey> This allows the "is a" relationship to be used very nicely for named datatypes.
Quincey> 
Quincey> >    * Still to discuss: datasets that point to data types in other
Quincey> >      files by symbolic link.
Quincey>     Probably a good idea also, just ugly to implement.
Quincey>