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<HTML><HEAD>
<TITLE>HDF5 Tutorial - Reading from and Writing to a Dataset
</TITLE> 
</HEAD>

<body bgcolor="#ffffff">

<!-- BEGIN MAIN BODY -->


 [ <A HREF="title.html"><I>HDF5 Tutorial Top</I></A> ]
<H1>
<BIG><BIG><BIG><FONT COLOR="#c101cd">Reading from and Writing to a Dataset</FONT>
</BIG></BIG></BIG></H1>

<hr noshade size=1>

<BODY>
<H2>Contents:</H2>
<UL>
 <LI><A HREF="#rdwr">Reading from and Writing to a Dataset</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="rdwr">
<H2>Reading from and Writing to a Dataset</h2>
<P>
During a dataset I/O operation, the library transfers raw data between memory
and the file. The data in memory can have a datatype different from that of 
the file and can also be of a different size 
(i.e., the data in memory is a subset of the dataset elements, or vice versa).
Therefore, to perform read or write operations, the application
program must specify:
<UL>
  <LI> The dataset

  <LI> The dataset's datatype in memory

  <LI> The dataset's dataspace in memory

  <LI> The dataset's dataspace in the file

  <LI>The dataset transfer property list 
      (The dataset transfer property list controls various aspects of the 
      I/O operations, such as the number of processes participating in a
      collective I/O request or hints to the library to control caching of
      raw data. In this tutorial, we use the default dataset transfer 
      property list.)

  <LI> The data buffer
</UL>


<P>
The steps to read from or write to a dataset are
as follows:
<OL>
  <LI> Obtain the dataset identifier.
  <LI> Specify the memory datatype.
  <LI> Specify the memory dataspace.
  <LI> Specify the file dataspace.
  <LI> Specify the transfer properties.
  <LI> Perform the desired operation on the dataset.
  <LI> Close the dataset.
  <LI> Close the dataspace, datatype, and property list if necessary.
</OL>

To read from or write to a dataset, 
the <code>H5Dread</code>/<code>h5dread_f</code> and 
<code>H5Dwrite</code>/<code>h5dwrite_f</code>
routines are used. <P> 
<I>C</I>:
<PRE>
   status = H5Dread (set_id, mem_type_id, mem_space_id, file_space_id,
                     xfer_prp, buf );
   status = H5Dwrite (set_id, mem_type_id, mem_space_id, file_space_id,
                     xfer_prp, buf);

</PRE> 
<I>FORTRAN</I>:
<PRE>
   CALL h5dread_f(dset_id, mem_type_id, buf, error, &
                     mem_space_id=mspace_id, file_space_id=fspace_id, &
                     xfer_prp=xfer_plist_id)
        <font face=times><i>or</i></font>
   CALL h5dread_f(dset_id, mem_type_id, buf, error)


   CALL h5dwrite_f(dset_id, mem_type_id, buf, error, &
                     mem_space_id=mspace_id, file_space_id=fspace_id, &
                     xfer_prp=xfer_plist_id)
        <font face=times><i>or</i></font>
   CALL h5dwrite_f(dset_id, mem_type_id, buf, error)
</PRE>


<P>
<H2> Programming Example</H2>
<A NAME="desc">
<H3><U>Description</U></H3>      
The following example shows how to read and write an existing dataset.
It opens the file created in the previous example, obtains the dataset 
identifier for the dataset <code>/dset</code>, 
writes the dataset to the file, then reads the dataset back from 
memory. It then closes the dataset and file. <BR>
<UL>
[ <A HREF="examples/h5_rdwt.c">C Example</A> ] - <code>h5_rdwt.c</code> <BR>
[ <A HREF="examples/rwdsetexample.f90">FORTRAN Example</A> ] - <code>rwdsetexample.f90</code><BR>
[ <A HREF="examples/java/DatasetRdWt.java">Java Example</A> ] - <code>DatasetRdWt.java</code> <BR>
</UL>

<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.

</PRE>
<A NAME="rem">
<H3><U>Remarks</U></H3>
<UL>
<LI><code>H5Fopen</code>/<code>h5fopen_f</code> opens an existing file and 
    returns a file identifier.
<PRE>
<I>C</I>:
  hid_t H5Fopen (const char *name, unsigned access_mode, hid_t access_prp) 

<I>FORTRAN</I>:
  h5fopen_f (name, access_mode, file_id, hdferr, access_prp)

        name         CHARACTER(LEN=*)
        access_mode  INTEGER
                     (Possible values: H5F_ACC_RDWR_F, H5F_ACC_RDONLY_F)
        file_id      INTEGER(HID_T)
        hdferr       INTEGER
                     (Possible values: 0 on success and -1 on failure)
        access_prp   INTEGER(HID_T), OPTIONAL

</PRE>
<UL>
   <LI> The argument <I>name</I> is the filename.
   <P>
   <LI> The <I>access_mode</I> parameter is the file access mode. 
        <code>H5F_ACC_RDWR</code> in C 
        (<code>H5F_ACC_RDWR_F</code> in FORTRAN) 
        allows read/write access 
        while <code>H5F_ACC_RDONLY</code> in C 
        (<code>H5F_ACC_RDONLY_F</code> in FORTRAN) 
        allows read-only access.

   <P>
   <LI> The <I>access_prp</I> parameter identifies the file access property list.
        <code>H5P_DEFAULT</code> in C and <code>H5P_DEFAULT_F</code> in FORTRAN
        specify the default file access property list.
        This parameter is optional in FORTRAN; if it is omitted, the default file
        access property list is used.

   <P>
   <LI>In FORTRAN, the return code is passed back in the <I>hdferr</I>
       parameter: 0 if successful, -1 if not.  In C, the function returns
       the file identifier if successful, and a negative value otherwise.
</UL>
<P>
<LI> <code>H5Dopen</code>/<code>h5dopen_f</code> opens an existing dataset 
     with the name specified by <i>name</i> at the location specified by 
     <i>loc_id</i>.
     For FORTRAN, the return value is passed in the <I>hdferr</I> parameter:
     0 if successful, -1 if not.  For C, the function returns the dataset
     identifier if successful, and a negative value if not.
 <P>
<I>C</I>:
<PRE>
  hid_t H5Dopen (hid_t loc_id, const char *name) 
</PRE>
<I>FORTRAN</I>:
<PRE>
  h5dopen_f(loc_id, name, hdferr) 

        loc_id   INTEGER(HID_T) 
        name     CHARACTER(LEN=*) 
        hdferr   INTEGER 
                 (Possible values: 0 on success and -1 on failure)
</PRE>

<P>
<LI><code>H5Dwrite</code>/<code>h5dwrite_f</code> writes raw data 
   from an application buffer to the specified
   dataset, converting from the datatype and dataspace of the dataset in
   memory to the datatype and dataspace of the dataset in the file.
<P>
<I>C</I>:
<PRE>
  herr_t H5Dwrite (hid_t dset_id, hid_t mem_type_id, hid_t mem_space_id, 
                   hid_t file_space_id, hid_t xfer_prp, const void * buf) 
</PRE>
<I>FORTRAN</I>:
<PRE>
  h5dwrite_f (dset_id, mem_type_id, buf, hdferr, mem_space_id, &
                                       file_space_id, xfer_prp)

        dset_id        INTEGER(HID_T)
        mem_type_id    INTEGER(HID_T)
        buf(*,...*)    TYPE
        hdferr         INTEGER 
                       (Possible values: 0 on success and -1 on failure)
        mem_space_id   INTEGER(HID_T), OPTIONAL
                       (Default value: H5S_ALL_F)
        file_space_id  INTEGER(HID_T), OPTIONAL
                       (Default value: H5S_ALL_F)
        xfer_prp       INTEGER(HID_T), OPTIONAL
                       (Default value: H5P_DEFAULT_F)
</PRE>
<UL>
   <LI> The <I>dset_id</I> is the dataset identifier.
   <P>

   <LI> The <I>mem_type_id</I> parameter is the identifier of the dataset's 
        memory datatype.  <code>H5T_NATIVE_INT</code> in C 
        (<code>H5T_NATIVE_INTEGER</code> in FORTRAN) is an integer datatype 
        for the machine on which the library was compiled.
   <P>

  <LI> The <I>mem_space_id</I> parameter is the identifier of the dataset's 
       memory dataspace. <code>H5S_ALL</code> in C (<code>H5S_ALL_F</code> 
       in FORTRAN) is the default value and indicates that the whole dataspace 
       in memory is selected for the I/O operation.
       This parameter is optional in FORTRAN; if it is omitted, the default 
       will be used.
   <P>

   <LI> The <I>file_space_id</I> parameter is the identifier of the 
        dataset's file dataspace.  
        <code>H5S_ALL</code> in C (<code>H5S_ALL_F</code> in FORTRAN)
        is the default value and indicates that the entire dataspace of 
        the dataset in the file is selected for the I/O operation.
        This parameter is optional in FORTRAN; if it is omitted, the default 
        will be used.
   <P>

   <LI> The <I>xfer_prp</I> parameter is the data transfer propery list 
        identifier.
        <code>H5P_DEFAULT</code> in C 
        (<code>H5P_DEFAULT_F</code> in FORTRAN) is the default value and
        indicates that the default data transfer property list is used. 
        This parameter is optional in FORTRAN; if it is omitted, the default 
        will be used.
   <P>

   <LI> The <I>buf</I> parameter is the data buffer to write.
   <P>

   <LI> In FORTRAN, the <I>hdferr</I> parameter is for the error code
        passed back:  0 if successful, -1 if not.  In C, this function
        returns a non-negative value if successful; otherwise it returns 
        a negative value. 
</UL>
<P>
<LI><code>H5Dread</code>/<code>h5dread_f</code> reads raw data from the 
    specified dataset to an application buffer,
   converting from the file datatype and dataspace to the memory datatype and
   dataspace.
<P>
<I>C</I>:
<PRE>
  herr_t H5Dread (hid_t dset_id, hid_t mem_type_id, hid_t mem_space_id, 
                  hid_t file_space_id, hid_t xfer_prp, void * buf) 
</PRE>
<I>FORTRAN</I>:
<PRE>
  h5dread_f (dset_id, mem_type_id, buf, hdferr, mem_space_id, &
                  file_space_id, xfer_prp)

        dset_id        INTEGER(HID_T)
        mem_type_id    INTEGER(HID_T)
        buf(*,...*)    TYPE
        hdferr         INTEGER 
                       (Possible values: 0 on success and -1 on failure)
        mem_space_id   INTEGER(HID_T), OPTIONAL
                       (Default value: H5S_ALL_F)
        file_space_id  INTEGER(HID_T), OPTIONAL
                       (Default value: H5S_ALL_F)
        xfer_prp       INTEGER(HID_T), OPTIONAL
                       (Default value: H5P_DEFAULT_F)

</PRE>

<p>
<UL>
   <LI>The <I>dset_id</I> parameter is the dataset identifier. 
   <P>

   <LI>The <I>mem_type_id</I> parameter is the identifier of the dataset's 
       memory datatype.  <code>H5T_NATIVE_INT</code> in C 
       (<code>H5T_NATIVE_INTEGER</code> in FORTRAN) is an integer datatype 
       for the machine on which the library was compiled.
   <P>

   <LI>The <I>mem_space_id</I> parameter is the identifier of the dataset's 
       memory dataspace.  <code>H5S_ALL</code> in C (<code>H5S_ALL_F</code> 
       in FORTRAN) is the default value and indicates that the whole dataspace 
       in memory is selected for the I/O operation.
       This parameter is optional in FORTRAN; if it is omitted, the default 
       will be used.
   <P>

   <LI>The <I>file_space_id</I> parameter is the identifier of the 
       dataset's file dataspace.
       <code>H5S_ALL</code> in C (<code>H5S_ALL_F</code> in FORTRAN)
       is the default value and indicates that the entire dataspace of 
       the dataset in the file is selected for the I/O operation.
       This parameter is optional in FORTRAN; if it is omitted, the default 
       will be used.

   <P>
   <LI>The <I>xfer_prp</I> parameter is the data transfer propery list 
       identifier.
       <code>H5P_DEFAULT</code> in C
       (<code>H5P_DEFAULT_F</code> in FORTRAN) is the default value and
       indicates that the default data transfer property list is used. 
       This parameter is optional in FORTRAN; if it is omitted, the default 
       will be used.
   <P>

   <LI> The <I>buf</I> parameter is the data buffer to read into.
   <P>

   <LI> In FORTRAN, the <I>hdferr</I> parameter is for the error code
        passed back:  0 if successful, -1 if not.  In C, this function
        returns a non-negative value if successful; otherwise it returns 
        a negative value.
</UL>
</UL>
<A NAME="fc">
<H3><U>File Contents</U></H3>
Figure 6.1a  shows the contents of <code>dset.h5</code> (created by the C program).
<BR>
Figure 6.1b  shows the contents of <code>dsetf.h5</code> (created by the FORTRAN 
program).  
<P>
      <B>Fig. 6.1a</B> &nbsp;  <I><code>dset.h5</code> in DDL</I>
<PRE>
      HDF5 "dset.h5" {
      GROUP "/" {
         DATASET "dset" {
            DATATYPE { H5T_STD_I32BE }
            DATASPACE { SIMPLE ( 4, 6 ) / ( 4, 6 ) }
            DATA {
               1, 2, 3, 4, 5, 6,
               7, 8, 9, 10, 11, 12,
               13, 14, 15, 16, 17, 18,
               19, 20, 21, 22, 23, 24
            }
         }
      }
      }
</PRE>
<P>
      <B>Fig. 6.1b</B> &nbsp;  <I><code>dsetf.h5</code> in DDL</I>
<PRE>
HDF5 "dsetf.h5" {
GROUP "/" {
   DATASET "dset" {
      DATATYPE { H5T_STD_I32BE }
      DATASPACE { SIMPLE ( 6, 4 ) / ( 6, 4 ) }
      DATA {
         1, 7, 13, 19,
         2, 8, 14, 20,
         3, 9, 15, 21,
         4, 10, 16, 22,
         5, 11, 17, 23,
         6, 12, 18, 24
      }
   }
}
}
</PRE>




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