A group associates names with objects and provides a mechanism for mapping a name to an object. Since all objects appear in at least one group (with the possible exception of the root object) and since objects can have names in more than one group, the set of all objects in an HDF5 file is a directed graph. The internal nodes (nodes with out-degree greater than zero) must be groups while the leaf nodes (nodes with out-degree zero) are either empty groups or objects of some other type. Exactly one object in every non-empty file is the root object. The root object always has a positive in-degree because it is pointed to by the file boot block.
An object name consists of one or more components separated from one another by slashes. An absolute name begins with a slash and the object is located by looking for the first component in the root object, then looking for the second component in the first object, etc., until the entire name is traversed. A relative name does not begin with a slash and the traversal begins at the location specified by the calling function.
H5Gcreate(hid_t loc_id,
const char *name,
size_t size_hint
)
H5Gcreate creates a new group with the specified
name at the specified location, loc_id.
The location is identified by a file or group identifier.
The name, name, must not already be taken by some
other object and all parent groups must already exist.
size_hint is a hint for the number of bytes to
reserve to store the names which will be eventually added to
the new group. Passing a value of zero for size_hint
is usually adequate since the library is able to dynamically
resize the name heap, but a correct hint may result in better
performance.
If a non-positive value is supplied for size_hint,
then a default size is chosen.
The return value is a group identifier for the open group.
This group identifier should be closed by calling
H5Gclose() when it is no longer needed.
loc_id
name
size_hint
H5Gopen(hid_t loc_id,
const char *name
)
H5Gopen opens an existing group with the specified name at
the specified location, loc_id.
The location is identified by a file or
group identifier, and returns a group identifier for the group.
The obtained group identifier should be released by calling
H5Gclose() when it is no longer needed.
loc_id
name
H5Gclose(hid_t group_id)
H5Gclose releases resources used by a group which was
opened by H5Gcreate() or H5Gopen().
After closing a group, the group_id cannot be used again.
Failure to release a group with this call will result in resource leaks.
group_id
H5Glink(hid_t loc_id,
H5G_link_t link_type,
const char *current_name,
const char *new_name
)
new_name
to current_name.
H5Glink creates a new name for an object that has some current
name, possibly one of many names it currently has.
If link_type is H5G_LINK_HARD, then
current_name must name an existing object and both
names are interpreted relative to loc_id, which is
either a file identifier or a group identifier.
If link_type is H5G_LINK_SOFT, then
current_name can be anything and is interpreted at
lookup time relative to the group which contains the final
component of new_name. For instance, if
current_name is ./foo,
new_name is ./x/y/bar, and a request
is made for ./x/y/bar, then the actual object looked
up is ./x/y/./foo.
loc_id
link_type
H5G_LINK_HARD and H5G_LINK_SOFT.
current_name
new_name
H5Gunlink(hid_t loc_id,
const char *name
)
name from the group graph and
decrements the link count for the object to which name points
H5Gunlink removes an association between a name and an object.
Object headers keep track of how many hard links refer to the object;
when the hard link count reaches zero, the object can be removed
from the file. Objects which are open are not removed until all
identifiers to the object are closed.
If the link count reaches zero, all file-space associated with the object will be reclaimed. If the object is open, the reclamation of the file space is delayed until all handles to the object are closed.
loc_id
name
H5Giterate(hid_t loc_id,
const char *name,
int *idx,
H5G_operator_t operator,
void *operator_data
)
H5Giterate iterates over the members of
name in the file or group specified with
loc_id.
For each object in the group, the operator_data
and some additional information, specified below, are
passed to the operator function.
The iteration begins with the idx object in the
group and the next element to be processed by the operator is
returned in idx. If idx
is NULL, then the iterator starts at the first group member;
since no stopping point is returned in this case, the iterator
cannot be restarted if one of the calls to its operator returns
non-zero.
The prototype for H5G_operator_t is:
typedef herr_t *(H5G_operator_t)(hid_t group_id,
const char *member_name, void *operator_data/*in,out*/);
group_id, the name of the current
object within the group, member_name, and the
pointer to the operator data passed in to H5Giterate,
operator_data.
The return values from an operator are:
loc_id
*name
*idx
operator
*operator_data
H5Gmove(hid_t loc_id,
const char *src,
const char *dst
)
H5Gmove renames an object within an HDF5 file.
The original name, src, is unlinked from the
group graph and the new name, dst, is inserted
as an atomic operation. Both names are interpreted relative
to loc_id, which is either a file or a group
identifier.
loc_id
*src
*dst
H5Gget_objinfo(hid_t loc_id,
const char *name,
hbool_t follow_link,
H5G_stat_t *statbuf
)
H5Gget_objinfo returns information about the
specified object through the statbuf argument.
loc_id (a file, group, or dataset identifier) and
name together determine the object.
If the object is a symbolic link and follow_link is
zero (0), then the information returned is that for the link itself;
otherwise the link is followed and information is returned about
the object to which the link points.
If follow_link is non-zero but the final symbolic link
is dangling (does not point to anything), then an error is returned.
The statbuf fields are undefined for an error.
The existence of an object can be tested by calling this function
with a null statbuf.
H5Gget_objinfo() fills in the following data structure:
typedef struct H5G_stat_t {
unsigned long fileno[2];
unsigned long objno[2];
unsigned nlink;
H5G_type_t type;
time_t mtime;
size_t linklen;
} H5G_stat_t
The fileno and objno fields contain
four values which uniquely itentify an object among those
HDF5 files which are open: if all four values are the same
between two objects, then the two objects are the same
(provided both files are still open).
The nlink field is the number of hard links to
the object or zero when information is being returned about a
symbolic link (symbolic links do not have hard links but
all other objects always have at least one).
The type field contains the type of the object,
one of H5G_GROUP, H5G_DATASET,
or H5G_LINK.
The mtime field contains the modification time.
If information is being returned about a symbolic link then
linklen will be the length of the link value
(the name of the pointed-to object with the null terminator);
otherwise linklen will be zero.
Other fields may be added to this structure in the future.
mtime value of 0 (zero).
loc_id
*name
follow_link
*statbuf
statbuf
(if non-null) initialized.
Otherwise returns FAIL (-1).
H5Gget_linkval(hid_t loc_id,
const char *name,
size_t size,
char *value
)
H5Gget_linkval returns size
characters of the link value through the value
argument if loc_id (a file or group identifier)
and name specify a symbolic link.
If size is smaller than the link value, then
value will not be null terminated.
This function fails if the specified object is not a symbolic link.
The presence of a symbolic link can be tested by passing zero for
size and NULL for value.
Use H5Gget_objinfo() to get the size of a link value.
loc_id
name
size
value
to be returned.
value
value,
if successful.
Otherwise returns FAIL (-1).
H5Gset_comment(hid_t loc_id,
const char *name,
const char *comment
)
H5Gset_comment sets the comment for the the
object name to comment.
Any previously existing comment is overwritten.
If comment is the empty string or a
null pointer, the comment message is removed from the object.
Comments should be relatively short, null-terminated, ASCII strings.
Comments can be attached to any object that has an object header, e.g., data sets, groups, named data types, and data spaces, but not symbolic links.
loc_id
name
comment
H5Gget_comment(hid_t loc_id,
const char *name,
size_t bufsize,
char *comment
)
H5Gget_comment retrieves the comment for the the
object name. The comment is returned in the buffer
comment.
At most bufsize characters, including a null
terminator, are copied. The result is not null terminated
if the comment is longer than the supplied buffer.
If an object does not have a comment, the empty string is returned.
loc_id
name
bufsize
comment.
comment
bufsize.
Otherwise returns FAIL (-1).