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<title>The GNU Fortran Compiler: GFORTRAN_CONVERT_UNIT</title>
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Next: <a href="GFORTRAN_005fERROR_005fBACKTRACE.html#GFORTRAN_005fERROR_005fBACKTRACE" accesskey="n" rel="next">GFORTRAN_ERROR_BACKTRACE</a>, Previous: <a href="GFORTRAN_005fLIST_005fSEPARATOR.html#GFORTRAN_005fLIST_005fSEPARATOR" accesskey="p" rel="prev">GFORTRAN_LIST_SEPARATOR</a>, Up: <a href="Runtime.html#Runtime" accesskey="u" rel="up">Runtime</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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<a name="GFORTRAN_005fCONVERT_005fUNIT_002d_002d_002dSet-endianness-for-unformatted-I_002fO"></a>
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<h3 class="section">3.11 <code>GFORTRAN_CONVERT_UNIT</code>—Set endianness for unformatted I/O</h3>
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<p>By setting the <code>GFORTRAN_CONVERT_UNIT</code> variable, it is possible
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to change the representation of data for unformatted files.
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The syntax for the <code>GFORTRAN_CONVERT_UNIT</code> variable is:
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</p><div class="smallexample">
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<pre class="smallexample">GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
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mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
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exception: mode ':' unit_list | unit_list ;
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unit_list: unit_spec | unit_list unit_spec ;
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unit_spec: INTEGER | INTEGER '-' INTEGER ;
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</pre></div>
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<p>The variable consists of an optional default mode, followed by
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a list of optional exceptions, which are separated by semicolons
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from the preceding default and each other. Each exception consists
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of a format and a comma-separated list of units. Valid values for
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the modes are the same as for the <code>CONVERT</code> specifier:
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</p>
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<ul class="no-bullet">
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<li><!-- /@w --> <code>NATIVE</code> Use the native format. This is the default.
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</li><li><!-- /@w --> <code>SWAP</code> Swap between little- and big-endian.
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</li><li><!-- /@w --> <code>LITTLE_ENDIAN</code> Use the little-endian format
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for unformatted files.
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</li><li><!-- /@w --> <code>BIG_ENDIAN</code> Use the big-endian format for unformatted files.
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</li></ul>
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<p>A missing mode for an exception is taken to mean <code>BIG_ENDIAN</code>.
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Examples of values for <code>GFORTRAN_CONVERT_UNIT</code> are:
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</p><ul class="no-bullet">
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<li><!-- /@w --> <code>'big_endian'</code> Do all unformatted I/O in big_endian mode.
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</li><li><!-- /@w --> <code>'little_endian;native:10-20,25'</code> Do all unformatted I/O
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in little_endian mode, except for units 10 to 20 and 25, which are in
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native format.
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</li><li><!-- /@w --> <code>'10-20'</code> Units 10 to 20 are big-endian, the rest is native.
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</li></ul>
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<p>Setting the environment variables should be done on the command
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line or via the <code>export</code>
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command for <code>sh</code>-compatible shells and via <code>setenv</code>
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for <code>csh</code>-compatible shells.
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</p>
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<p>Example for <code>sh</code>:
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</p><div class="smallexample">
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<pre class="smallexample">$ gfortran foo.f90
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$ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
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</pre></div>
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<p>Example code for <code>csh</code>:
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</p><div class="smallexample">
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<pre class="smallexample">% gfortran foo.f90
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% setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
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% ./a.out
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</pre></div>
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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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<p>See <a href="CONVERT-specifier.html#CONVERT-specifier">CONVERT specifier</a>, for an alternative way to specify the
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data representation for unformatted files. See <a href="Runtime-Options.html#Runtime-Options">Runtime Options</a>, for
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setting a default data representation for the whole program. The
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<code>CONVERT</code> specifier overrides the <samp>-fconvert</samp> compile options.
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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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