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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 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 "mount point" in the first file (parent). The "mount point" in
HDF5 is a group ( it can also be the root group ).
<LI>Use the HDF5 API function H5Fmount to mount the second file (child) in
the first one.
<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 the H5Funmount function 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 with a group in it, and another
file with 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 picture
illustrates this concept.
<PRE>
'FILE1' 'FILE2'
-------------------- --------------------
! ! ! !
! / ! ! / !
! | ! ! | !
! | ! ! | !
! V ! ! V !
! -------- ! ! ---------- !
! ! Group ! ! ! ! Dataset! !
! --------- ! ! ---------- !
!------------------! !------------------!
</PRE>
After mounting the second file, 'FILE2', under the group in the file, 'FILE1',
the parent has the following structure:
<PRE>
'FILE1'
--------------------
! !
! / !
! | !
! | !
! V !
! -------- !
! ! Group ! !
! --------- !
! | !
! | !
! V !
! ----------- !
! ! Dataset ! !
! !---------- !
! !
!------------------!
</PRE>
[ <A HREF="examples/h5_mount.c">Download h5_mount.c</A> ]
<PRE>
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
#include<hdf5.h>
#define FILE1 "mount1.h5"
#define FILE2 "mount2.h5"
#define RANK 2
#define NX 4
#define NY 5
int main(void)
{
hid_t fid1, fid2, gid; /* Files and group identifiers */
hid_t did, tid, sid; /* Dataset and datatype identifiers */
herr_t status;
hsize_t dims[] = {NX,NY}; /* Dataset dimensions */
int i, j;
int bm[NX][NY], bm_out[NX][NY]; /* Data buffers */
/*
* Initialization of buffer matrix "bm"
*/
for(i =0; i < NX; i++) {
for(j = 0; j < NY; j++)
bm[i][j] = i + j;
}
/*
* Create first file and a group in it.
*/
fid1 = H5Fcreate(FILE1, H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
gid = H5Gcreate(fid1, "/G", 0);
/*
* Close group and file
*/
H5Gclose(gid);
H5Fclose(fid1);
/*
* Create second file and dataset "D" in it.
*/
fid2 = H5Fcreate(FILE2, H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
dims[0] = NX;
dims[1] = NY;
sid = H5Screate_simple(RANK, dims, NULL);
did = H5Dcreate(fid2, "D", H5T_NATIVE_INT, sid, H5P_DEFAULT);
/*
* Write data to the dataset.
*/
status = H5Dwrite(did, H5T_NATIVE_INT, H5S_ALL, H5S_ALL, H5P_DEFAULT, bm);
/*
* Close all identifiers.
*/
H5Sclose(sid);
H5Dclose(did);
H5Fclose(fid2);
/*
* Reopen both files
*/
fid1 = H5Fopen(FILE1, H5F_ACC_RDONLY, H5P_DEFAULT);
fid2 = H5Fopen(FILE2, H5F_ACC_RDONLY, H5P_DEFAULT);
/*
* Mount second file under G in the first file.
*/
H5Fmount(fid1, "/G", fid2, H5P_DEFAULT);
/*
* Access dataset D in the first file under /G/D name.
*/
did = H5Dopen(fid1,"/G/D");
tid = H5Dget_type(did);
status = H5Dread(did, tid, H5S_ALL, H5S_ALL, H5P_DEFAULT, bm_out);
/*
* Print out the data.
*/
for(i=0; i < NX; i++){
for(j=0; j < NY; j++)
printf(" %d", bm_out[i][j]);
printf("\n");
}
/*
* Close all identifers
*/
H5Tclose(tid);
H5Dclose(did);
/*
* Unmounting second file
*/
H5Funmount(fid1, "/G");
/*
* Close both files
*/
H5Fclose(fid1);
H5Fclose(fid2);
return 0;
}
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
</PRE>
<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 with the H5F_ACC_RDONLY access flag since no
objects will be modified. The child should be opened with H5F_ACC_RDWR if
the dataset is to be modified.
<P>
<LI> The second file is mounted in the first using the H5Fmount function.
<PRE>
herr_t H5Fmount(hid_t loc_id, const char *name, hid_t file_id, hid_t plist_id)
</PRE>
<UL>
<LI> The first two arguments specify the location of the "mount point"
( a group ). In this example the "mount point" is a group "/G" in the
file specified by its handle <I>fid1</I>. Since group G is in the root
group of the first file, one can also use just "G" to identify it.
<P>
Below is a description of another scenario:<BR>
Suppose group G was a member of group D in the first file (<I>fid1</I>).
Then mounting point G can be specified in two different ways:
<P>
<UL>
<LI> <I>loc_id</I> is <I>fid1</I><BR>
<I>name</I> is "D/G"
<P>
<LI> <I>loc_id</I> is an identifier of the group "D"<BR>
<I>name</I> is just "G"
</UL>
<P>
<LI> The third argument is an identifier of the file which will be mounted.
Only one file can be mounted per "mount point".
<P>
<LI> The fourth argument is an identifier of the property list to be used.
Currently, only the default property list, H5P_DEFAULT, can be used.
</UL>
<P>
<LI> In this example we just read data from the dataset D. One can modify
data also. If the dataset is modified while the file is mounted, it becomes
modified in the original file too after the file is unmounted.
<P>
<LI> The file is unmounted with the H5Funmount function:
<PRE>
herr_t H5Funmount(hid_t loc_id, const char *name)
</PRE>
Arguments to this function specify the location of the "mount point".
In our example <I>loc_id</I> is an identifier of the first file, and
<I>name</I> is the name of group G, "/G". One could also use "G"
instead of "/G" since G is a member of the root group in the file
<I>fid1</I>. Notice that H5Funmount does not close files. They are closed
with the respective calls to the H5Fclose function. Closing the parent
automatically unmounts the child.
<P>
<LI>The h5dump utility cannot display files in memory, therefore no output
of FILE1 after FILE2 was mounted is provided.
</UL>
</UL>
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<BR> <H6>Last Modified: August 27, 1999</H6><BR>
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