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<html>
  <head>
    <title>Mounting Files</title>
    
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<!-- #EndLibraryItem --><h1>Mounting Files</h1>

    <h2>Purpose</h2>

    <p>This document contrasts two methods for mounting an hdf5 file
      on another hdf5 file: the case where the relationship between
      files is a tree and the case where it's a graph.  The tree case
      simplifies current working group functions and allows symbolic
      links to point into ancestor files whereas the graph case is
      more consistent with the organization of groups within a
      particular file.

    <h2>Definitions</h2>

    <p>If file <code>child</code> is mounted on file
      <code>parent</code> at group <code>/mnt</code> in
      <code>parent</code> then the contents of the root group of
      <code>child</code> will appear in the group <code>/mnt</code> of
      <code>parent</code>. The group <code>/mnt</code> is called the
      <em>mount point</em> of the child in the parent.

    <h2>Common Features</h2>

    <p>These features are common to both mounting schemes.

    <ul>
      <li>The previous contents of <code>/mnt</code> in
	<code>parent</code> is temporarily hidden.  If objects in that
	group had names from other groups then the objects will still
	be visible by those other names.

      <li>The mount point is actually an OID (not a name) so if there
	are other names besides <code>/mnt</code> for that group then
	the root group of the child will be visible in all those
	names.

      <li>At most one file can be mounted per mount point but a parent 
	can have any number of mounted children.

      <li>Name lookups will entail a search through the mount table at
	each stage of the lookup.  The search will be O(log
	<em>N</em>) where <em>N</em> is the number of children mounted
	on that file.

      <li>Files open for read-only can be mounted on other files that
	are open for read-only.  Mounting a file in no way changes the
	contents of the file.

      <li>Mounting a child may hide mount points that exist below the
	child's mount point, but it does not otherwise affect mounted
	files.

      <li>Hard links cannot cross file boundaries.  An object cannot
	be moved or renamed with <code>H5Gmove()</code> in such a way
	that the new location would be in a different file than the
	original location.

      <li>The child can be accessed in a manner different from the
	parent. For instance, a read-write child in a read-only
	parent, a parallel child in a serial parent, <em>etc</em>.

      <li>If some object in the child is open and the child is
	unmounted and/or closed, the object will remain open and
	accessible until explicitly closed. As in the mountless case,
	the underlying UNIX file will be held open until all member
	objects are closed.

      <li>Current working groups that point into a child will remain
	open and usable even after the child has been unmounted and/or 
	closed.

      <li>Datasets that share a committed datatype must reside in the 
	same file as the datatype.

    </ul>

    <h2>Contrasting Features</h2>

    <center>
      <table border width="90%">
	<tr>
	  <th width="50%">Tree</th>
	  <th width="50%">Graph</th>
	</tr>

	<tr valign=top>
	  <td>The set of mount-related files makes a tree.</td>
	  <td>The set of mount-related files makes a directed
	    graph.</td>
	</tr>

	<tr valign=top>
	  <td>A file can be mounted at only one mount point.</td>
	  <td>A file can be mounted at any number of mount points.</td>
	</tr>

	<tr valign=top>
	  <td>Symbolic links in the child that have a link value which 
	    is an absolute name can be interpreted with respect to the 
	    root group of either the child or the root of the mount
	    tree, a property which is determined when the child is
	    mounted.</td>
	  <td>Symbolic links in the child that have a link value which 
	    is an absolute name are interpreted with respect to the
	    root group of the child.</td>
	</tr>

	<tr valign=top>
	  <td>Closing a child causes it to be unmounted from the
	    parent.</td>
	  <td>Closing a child has no effect on its relationship with
	    the parent.  One can continue to access the child contents 
	    through the parent.</td>
	</tr>

	<tr valign=top>
	  <td>Closing the parent recursively unmounts and closes all 
	    mounted children.</td>
	  <td>Closing the parent unmounts all children but 
	    does not close them or unmount their children.</td>
	</tr>

	<tr valign=top>
	  <td>The current working group functions
	    <code>H5Gset()</code>, <code>H5Gpush()</code>, and
	    <code>H5Gpop()</code> operate on the root of the mount
	    tree.</td>
	  <td>The current working group functions operate on the file
	    specified by their first argument.</td>
	</tr>

	<tr valign=top>
	  <td>Absolute name lookups (like for <code>H5Dopen()</code>)
	    are always performed with respect to the root of the mount 
	    tree.</td>
	  <td>Absolute name lookups are performed with respect to the
	    file specified by the first argument.</td>
	</tr>

	<tr valign=top>
	  <td>Relative name lookups (like for <code>H5Dopen()</code>)
	    are always performed with respect to the specified group
	    or the current working group of the root of the mount
	    tree.</td>
	  <td>Relative name lookups are always performed with respect
	    to the specified group or the current working group of the 
	    file specified by the first argument.</td>
	</tr>

	<tr valign=top>
	  <td>Mounting a child temporarily hides the current working
	    group stack for that child</td>
	  <td>Mounting a child has no effect on its current working
	    group stack.</td>
	</tr>

	<tr valign=top>
	  <td>Calling <code>H5Fflush()</code> will flush all files of
	    the mount tree regardless of which file is specified as
	    the argument.</td>
	  <td>Calling <code>H5Fflush()</code> will flush only the
	    specified file.</td>
	</tr>

	
      </table>
    </center>


    <h2>Functions</h2>

    <dl>
      <dt><code>herr_t H5Fmount(hid_t <em>loc</em>, const char
	  *<em>name</em>, hid_t <em>child</em>, hid_t
	  <em>plist</em>)</code>
      <dd>The file <em>child</em> is mounted at the specified location 
	in the parent. The <em>loc</em> and <em>name</em> specify the
	mount point, a group in the parent.  The <em>plist</em>
	argument is an optional mount property list. The call will
	fail if some file is already mounted on the specified group.

	<table border>
	  <tr>
	    <th width="50%">Tree</th>
	    <th width="50%">Graph</th>
	  </tr>
	  <tr valign=top>
	    <td>The call will fail if the child is already mounted
	      elsewhere.</td>
	    <td>A child can be mounted at numerous mount points.</td>
	  </tr>
	  <tr valign=top>
	    <td>The call will fail if the child is an ancestor of the
	      parent.</td>
	    <td>The mount graph is allowed to have cycles.</td>
	  </tr>
	  <tr valign=top>
	    <td>Subsequently closing the child will cause it to be
	      unmounted from the parent.</td>
	    <td>Closing the child has no effect on its mount
	      relationship with the parent.</td>
	  </tr>
	</table>

	<br><br>
      <dt><code>herr_t H5Funmount(hid_t <em>loc</em>, const char
	  *<em>name</em>)</code>
      <dd>Any file mounted at the group specified by <em>loc</em> and
	<em>name</em> is unmounted.  The child is not closed.  This
	function fails if no child is mounted at the specified point.

	<br><br>
      <dt><code>hid_t H5Pcreate(H5P_MOUNT)</code>
      <dd>Creates and returns a new mount property list initialized
	with default values.

	<br><br>
      <dt><code>herr_t H5Pset_symlink_locality(hid_t <em>plist</em>,
	  H5G_symlink_t <em>locality</em>)</code>
      <dt><code>herr_t H5Pget_symlink_locality(hid_t <em>plist</em>,
	  H5G_symlink_t *<em>locality</em>)</code>
      <dd>These functions exist only for the tree scheme.  They set or 
	query the property that determines whether symbolic links with 
	absolute name value in the child are looked up with respect to 
	the child or to the mount root.  The possible values are
	<code>H5G_SYMLINK_LOCAL</code> or
	<code>H5G_SYMLINK_GLOBAL</code> (the default).

	<br><br>
      <dt><code>hid_t H5Freopen(hid_t <em>file</em>)</code>
      <dd>A file handle is reopened, creating an additional file
	handle. The new file handle refers to the same file but has an 
	empty current working group stack.

	<table border>
	  <tr>
	    <th width="50%">Tree</th>
	    <th width="50%">Graph</th>
	  </tr>
	  <tr valign=top>
	    <td>The new handle is not mounted but the old handle
	      continues to be mounted.</td>
	    <td>The new handle is mounted at the same location(s) as
	      the original handle.</td>
	  </tr>
	</table>
    </dl>

    <h2>Example</h2>

    <p>A file <code>eos.h5</code> contains data which is constant for
      all problems.  The output of a particular physics application is
      dumped into <code>data1.h5</code> and <code>data2.h5</code> and
      the physics expects various constants from <code>eos.h5</code>
      in the <code>eos</code> group of the two data files.  Instead of
      copying the contents of <code>eos.h5</code> into every physics
      output file we simply mount <code>eos.h5</code> as a read-only
      child of <code>data1.h5</code> and <code>data2.h5</code>. 

      <center>
	<table border width="90%">
	  <tr><td><h3>Tree</h3><code><pre>
/* Create data1.h5 */
data1 = H5Fcreate("data1.h5", H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
H5Gclose(H5Gcreate(data1, "/eos", 0));
H5Gset_comment(data1, "/eos", "EOS mount point");

/* Create data2.h5 */
data2 = H5Fcreate("data2.h5", H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
H5Gclose(H5Gcreate(data2, "/eos", 0));
H5Gset_comment(data2, "/eos", "EOS mount point");

/* Open eos.h5 and mount it in both files */
eos1 = H5Fopen("eos.h5", H5F_ACC_RDONLY, H5P_DEFAULT);
H5Fmount(data1, "/eos", eos1, H5P_DEFAULT);
eos2 = H5Freopen(eos1);
H5Fmount(data2, "/eos", eos2, H5P_DEFAULT);

    ... physics output ...

H5Fclose(data1);
H5Fclose(data2);
	      </pre></code></td></tr>
	  <tr><td><h3>Graph</h3><code><pre>
/* Create data1.h5 */
data1 = H5Fcreate("data1.h5", H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
H5Gclose(H5Gcreate(data1, "/eos", 0));
H5Gset_comment(data1, "/eos", "EOS mount point");

/* Create data2.h5 */
data2 = H5Fcreate("data2.h5", H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
H5Gclose(H5Gcreate(data2, "/eos", 0));
H5Gset_comment(data2, "/eos", "EOS mount point");

/* Open eos.h5 and mount it in both files */
eos = H5Fopen("eos.h5", H5F_ACC_RDONLY, H5P_DEFAULT);
H5Fmount(data1, "/eos", eos, H5P_DEFAULT);
H5Fmount(data2, "/eos", eos, H5P_DEFAULT);
H5Fclose(eos);

    ... physics output ...

H5Fclose(data1);
H5Fclose(data2);
	      </pre></code></td></tr>
	</table>
      </center>


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Describes HDF5 Release 1.5, Unreleased Development Branch
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