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<HTML><HEAD>
<TITLE>HDF5 Tutorial - Mounting Files
</TITLE>
</HEAD>
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[ <A HREF="title.html"><I>HDF5 Tutorial Top</I></A> ]
<H1>
<BIG><BIG><BIG><FONT COLOR="#c101cd">Mounting Files</FONT>
</BIG></BIG></BIG></H1>
<hr noshade size=1>
<BODY>
<H2>Contents:</H2>
<UL>
<LI> <A HREF="#def">Mounting Files</A>
<LI> Programming Example
<UL>
<LI> <A HREF="#desc">Description</A>
<LI> <A HREF="#rem">Remarks</A>
<!--
<LI> <A HREF="#fc">File Contents</A>
-->
</UL>
</UL>
<HR>
<A NAME="def">
<H2>Mounting Files</H2>
HDF5 allows you to combine two or more HDF5 files in memory
in a manner similar to mounting files in UNIX.
The group structure and metadata from one file appear as though
they exist in another file. The following steps are involved:
<OL>
<LI>Open the files.
<LI>Choose the <strong>mount point</strong> in the first file
(the parent file). The mount point in
HDF5 is a group, which CANNOT be the root group.
<LI>Use the HDF5 routine <CODE>H5Fmount</CODE> / <CODE>h5fmount_f</CODE>
to mount the second file (the child file) in the first file.
<LI>Work with the objects in the second file as if they were members of
the mount point group in the first file. The previous contents of
the mount point group are temporarily hidden.
<LI>Unmount the second file using <CODE>H5Funmount</CODE> /
<CODE>h5funmount_f</CODE> when the work is done.
</OL>
<H2> Programming Example</H2>
<A NAME="desc">
<H3><U>Description</U></H3>
In the following example, we create one file containing a group and
another file containing a dataset.
Mounting is used to access the dataset from the second
file as a member of a group in the first file.
The following figures illustrate this concept.
<PRE>
FILE1 FILE2
-------------------- --------------------
! ! ! !
! / ! ! / !
! | ! ! | !
! | ! ! | !
! V ! ! V !
! -------- ! ! ---------- !
! ! Group ! ! ! ! Dataset! !
! --------- ! ! ---------- !
!------------------! !------------------!
</PRE>
After mounting <code>FILE2</code> under the group in <code>FILE1</code>,
the parent file has the following structure:
<PRE>
FILE1
--------------------
! !
! / !
! | !
! | !
! V !
! -------- !
! ! Group ! !
! --------- !
! | !
! | !
! V !
! ----------- !
! ! Dataset ! !
! !---------- !
! !
!------------------!
</PRE>
[ <A HREF="examples/h5_mount.c">C program</A> ]
- <code>h5_mount.c</code><BR>
[ <A HREF="examples/mount.f90">FORTRAN program</A> ]
- <code>mountexample.f90</code>
<P>
<B>NOTE:</B> To download a tar file of the examples, including a Makefile,
please go to the <A HREF="references.html">References</A> page.
<A NAME="rem">
<H3><U>Remarks</U></H3>
<UL>
<LI> The first part of the program creates a group in one file and creates
and writes a dataset to another file.
<P>
<LI> Both files are reopened and the second file is mounted in the first
using <CODE>H5Fmount</CODE> / <CODE>h5fmount_f</CODE>.
If no objects will be modified, the
files can be opened with <CODE>H5F_ACC_RDONLY</CODE>
(<CODE>H5F_ACC_RDONLY_F</CODE> in FORTRAN).
If the data is to be modified, the files should be opened with
<CODE>H5F_ACC_RDWR</CODE> (<CODE>H5F_ACC_RDWR_F</CODE> in FORTRAN).
<P>
<I><B>C:</B></I>
<pre>
herr_t H5Fmount (hid_t loc_id, const char *dsetname,
hid_t file_id, hid_t access_prp)
</pre>
<P>
<I><B>FORTRAN:</B></I>
<pre>
h5fmount_f (loc_id, dsetname, file_id, hdferr, access_prp)
loc_id IN: INTEGER (HID_T)
dsetname IN: CHARACTER (LEN=*)
file_id IN: INTEGER (HID_T)
hdferr OUT: INTEGER
access_prp IN: INTEGER (HID_T), OPTIONAL
(Default value: H5P_DEFAULT_F)
</pre>
<P>
<UL>
<LI> The <em>loc_id</em> and <em>dsetname</em> arguments
specify the location of the mount point.
In this example, the mount point is a group <code>/G</code> in the
specified file. Since the group <code>/G</code> is in the root
group of the first file, one can also use just <code>G</code> to
identify it.
<P>
Below is a description of another scenario:
<p>
Suppose the group <code>G</code> were a member of
the group <code>H</code> in the first file.
Then the mount point <code>G</code> can be specified in
two different ways:
<P>
<UL>
<LI> <em>loc_id</em> is the file identifier for the first file.<BR>
<em>dsetname</em> is <code>H/G</code>.
<P>
<LI> <em>loc_id</em> is the identifier for the group <code>H</code>.<BR>
<em>dsetname</em> is <code>G</code>.
</UL>
<P>
<LI> The <em>file_id</em> argument is the identifier for the file
which will be mounted.
Only one file can be mounted per mount point.
<P>
<LI> The <I>access_prp</I> argument is the identifier for the property list
to be used. Currently, only the default property list,
<CODE>H5P_DEFAULT</CODE>, can be used in C.
In FORTRAN, this argument can be omitted or
<CODE>H5P_DEFAULT_F</CODE> can be used.
<P>
<LI> The C function <CODE>H5Fmount</CODE> returns a non-negative
value if successful and a negative value otherwise.
With the FORTRAN routine, <CODE>h5fmount_f</CODE>,
the return value of the call is returned in <em>hdferr</em>:
0 if successful and -1 otherwise.
</UL>
<P>
<LI>In this example, we only read data from the dataset <code>D</code>.
One can also modify data.
If the dataset is modified while the file is mounted, it is
modified in the original file after the file is unmounted.
<P>
<LI> The file is unmounted with <CODE>H5Funmount</CODE> /
<CODE>h5funmount_f</CODE>:
<P>
<I><B>C:</B></I>
<pre>
herr_t H5Funmount (hid_t loc_id, const char *dsetname)
</pre>
<P>
<I><B>FORTRAN:</B></I>
<pre>
h5funmount_f (loc_id, dsetname, hdferr)
loc_id IN: INTEGER (HID_T)
dsetname IN: CHARACTER (LEN=*)
hdferr OUT: INTEGER
</pre>
<P>
<ul>
<li>The <I>loc_id</I> and <I>dsetname</I> arguments specify the location
of the mount point.
In our example <I>loc_id</I> is the first file's file identifier
and <I>dsetname</I> is the name of group <code>/G</code>.
</ul>
<P>
<li>Note that <CODE>H5Funmount</CODE> / <CODE>h5funmount_f</CODE>
does not close files. Files are closed with the respective calls to
the <CODE>H5Fclose</CODE> / <CODE>h5fclose_f</CODE> function.
<P>
<li>Closing the parent file automatically unmounts the child file.
<P>
<LI>The <code>h5dump</code> utility cannot display files in memory.
Therefore, no output of <code>FILE1</code> after <code>FILE2</code>
was mounted is provided.
</UL>
</UL>
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Describes HDF5 Release 1.2.2, June 2000
<BR> <H6>Last Modified: January 19, 2000</H6><BR>
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