136 lines
6.4 KiB
HTML
136 lines
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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(see below). A copy of the license is included in the section entitled
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The GNU Fortran Compiler: CONVERT specifier</title>
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<meta name="description" content="The GNU Fortran Compiler: CONVERT specifier">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" rel="up" title="Extensions implemented in GNU Fortran">
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<link href="OpenMP.html#OpenMP" rel="next" title="OpenMP">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="CONVERT-specifier"></a>
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<div class="header">
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<p>
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Next: <a href="OpenMP.html#OpenMP" accesskey="n" rel="next">OpenMP</a>, Previous: <a href="Cray-pointers.html#Cray-pointers" accesskey="p" rel="prev">Cray pointers</a>, Up: <a href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" accesskey="u" rel="up">Extensions implemented in GNU Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="CONVERT-specifier-1"></a>
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<h4 class="subsection">6.1.15 <code>CONVERT</code> specifier</h4>
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<a name="index-CONVERT-specifier"></a>
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<p>GNU Fortran allows the conversion of unformatted data between little-
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and big-endian representation to facilitate moving of data
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between different systems. The conversion can be indicated with
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the <code>CONVERT</code> specifier on the <code>OPEN</code> statement.
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See <a href="GFORTRAN_005fCONVERT_005fUNIT.html#GFORTRAN_005fCONVERT_005fUNIT">GFORTRAN_CONVERT_UNIT</a>, for an alternative way of specifying
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the data format via an environment variable.
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</p>
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<p>Valid values for <code>CONVERT</code> are:
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</p><ul class="no-bullet">
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<li><!-- /@w --> <code>CONVERT='NATIVE'</code> Use the native format. This is the default.
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</li><li><!-- /@w --> <code>CONVERT='SWAP'</code> Swap between little- and big-endian.
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</li><li><!-- /@w --> <code>CONVERT='LITTLE_ENDIAN'</code> Use the little-endian representation
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for unformatted files.
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</li><li><!-- /@w --> <code>CONVERT='BIG_ENDIAN'</code> Use the big-endian representation for
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unformatted files.
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</li></ul>
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<p>Using the option could look like this:
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</p><div class="smallexample">
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<pre class="smallexample"> open(file='big.dat',form='unformatted',access='sequential', &
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convert='big_endian')
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</pre></div>
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<p>The value of the conversion can be queried by using
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<code>INQUIRE(CONVERT=ch)</code>. The values returned are
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<code>'BIG_ENDIAN'</code> and <code>'LITTLE_ENDIAN'</code>.
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</p>
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<p><code>CONVERT</code> works between big- and little-endian for
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<code>INTEGER</code> values of all supported kinds and for <code>REAL</code>
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on IEEE systems of kinds 4 and 8. Conversion between different
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“extended double” types on different architectures such as
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m68k and x86_64, which GNU Fortran
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supports as <code>REAL(KIND=10)</code> and <code>REAL(KIND=16)</code>, will
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probably not work.
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</p>
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<p><em>Note that the values specified via the GFORTRAN_CONVERT_UNIT
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environment variable will override the CONVERT specifier in the
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open statement</em>. This is to give control over data formats to
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users who do not have the source code of their program available.
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</p>
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<p>Using anything but the native representation for unformatted data
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carries a significant speed overhead. If speed in this area matters
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to you, it is best if you use this only for data that needs to be
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portable.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="OpenMP.html#OpenMP" accesskey="n" rel="next">OpenMP</a>, Previous: <a href="Cray-pointers.html#Cray-pointers" accesskey="p" rel="prev">Cray pointers</a>, Up: <a href="Extensions-implemented-in-GNU-Fortran.html#Extensions-implemented-in-GNU-Fortran" accesskey="u" rel="up">Extensions implemented in GNU Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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