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H5L_EXISTS

Determine whether a link with the specified name exists in a group

Procedure:

H5L_EXISTS(loc_id, name, lapl_id)

Signature:

htri_t H5Lexists( hid_t loc_id, const char *name, hid_t lapl_id )

SUBROUTINE h5lexists_f(loc_id, name, link_exists, hdferr, lapl_id)
  IMPLICIT NONE
  INTEGER(HID_T), INTENT(IN) :: loc_id ! Identifier of file or group to query.
  CHARACTER(LEN=*), INTENT(IN) :: name ! Link name to check.
  LOGICAL, INTENT(OUT) :: link_exists  ! .TRUE. if exists, .FALSE. otherwise
  INTEGER, INTENT(OUT) :: hdferr       ! Error code:
                                       ! 0 on success and -1 on failure
  INTEGER(HID_T), OPTIONAL, INTENT(IN) :: lapl_id 
                                       ! Link access property list identifier.
END SUBROUTINE h5lexists_f

Parameters:
hid_t loc_idIN: Location identifier of link to query; may be a file, group, dataset, named datatype or attribute identifier
const char *name    IN: The name of the link to check
hid_t lapl_idIN: Link access property list identifier

Description:

H5L_EXISTS allows an application to determine whether the link name exists in the location specified by loc_id. The link may be of any type; only the presence of a link with that name is checked.

Note that H5L_EXISTS verifies only that the target link exists. If name includes either a relative path or an absolute path to the target link, intermediate steps along the path must be verified before the existence of the target link can be safely checked. If the path is not verified and an intermediate element of the path does not exist, H5L_EXISTS will fail. The example in the next paragraph illustrates one step-by-step method for verifying the existence of a link with a relative or absolute path.

Example: Use the following steps to verify the existence of the link datasetD in the group group1/group2/softlink_to_group3/, where group1 is a member of the group specified by loc_id:

 

  • First use H5Lexists to verify that group1 exists.
  • If group1 exists, use H5Lexists again, this time with name set to group1/group2, to verify that group2 exists.
  • If group2 exists, use H5Lexists with name set to group1/group2/softlink_to_group3 to verify that softlink_to_group3 exists. 
     
  • If softlink_to_group3 exists, you can now safely use H5L_EXISTS with name set to group1/group2/softlink_to_group3/datasetD to verify that the target link, datasetD, exists.

If the link to be verified is specified with an absolute path, the same approach should be used, but starting with the first link in the file’s root group. For instance, if datasetD were in /group1/group2/softlink_to_group3, the first call to H5L_EXISTS would have name set to /group1.

Note that this is an outline and does not include all necessary details. Depending on circumstances, for example, you may need to verify that an intermediate link points to a group and that a soft link points to an existing target.

The behavior of H5L_EXISTS was changed in the 1.10 release in the case where the root group, “/”, is the name of the link. This change is described below: 

  1. Let ‘file’ denote a valid HDF5 file identifier, and let ‘lapl’ denote a valid link access property list identifier. A call to H5L_EXISTS with arguments ‘file’, “/”, and ‘lapl’ returns a positive value; in other words, H5Lexists(file, "/", lapl) returns a positive value. In HDF5 version 1.8.16, this function returns 0.
  2. Let ‘root’ denote a valid HDF5 group identifier that refers to the root group of an HDF5 file, and let ‘lapl’ denote a valid link access property list identifier. A call to H5L_EXISTS with arguments ‘root’, “/”, and ‘lapl’ returns a positive value; in other words, H5Lexists(root, "/", lapl) returns a postive value. In HDF5 version 1.8.16, this function returns 0.


Note that the function accepts link names and path names. This is potentially misleading to callers, and we plan to separate the functionality for link names and path names in a future release. 

Returns:

Returns a positive value if the link exists. 
Returns 0 if the link does not exist. 
Returns a negative value when the function fails and may return a negative value if the link does not exist. See “Failure Modes” below.

Failure Modes:

H5L_EXISTS checks the existence of only the final element in a relative or absolute path; it does not check any other path elements. The function will therefore fail when both of the following conditions exist:

  • name is not local to the group specified by loc_id or, if loc_id is something other than a group identifier, name is not local to the root group.
  • Any element of the relative path or absolute path in name, except the target link, does not exist.

Example:

Coming Soon!

History:
Release    Change
1.10.0Function behavior changed in this release. See the “Note on Behavior Change” section above.
1.8.0Function introduced in this release.

--- Last Modified: April 25, 2019 | 12:41 PM

 

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