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<h3 class="section">3.12 <samp><span class="env">GFORTRAN_CONVERT_UNIT</span></samp>—Set endianness for unformatted I/O</h3>
<p>By setting the <samp><span class="env">GFORTRAN_CONVERT_UNIT</span></samp> variable, it is possible
to change the representation of data for unformatted files.
The syntax for the <samp><span class="env">GFORTRAN_CONVERT_UNIT</span></samp> variable is:
<pre class="smallexample"> GFORTRAN_CONVERT_UNIT: mode | mode ';' exception | exception ;
mode: 'native' | 'swap' | 'big_endian' | 'little_endian' ;
exception: mode ':' unit_list | unit_list ;
unit_list: unit_spec | unit_list unit_spec ;
unit_spec: INTEGER | INTEGER '-' INTEGER ;
</pre>
<p>The variable consists of an optional default mode, followed by
a list of optional exceptions, which are separated by semicolons
from the preceding default and each other. Each exception consists
of a format and a comma-separated list of units. Valid values for
the modes are the same as for the <code>CONVERT</code> specifier:
<ul>
<li><code>NATIVE</code> Use the native format. This is the default.
<li><code>SWAP</code> Swap between little- and big-endian.
<li><code>LITTLE_ENDIAN</code> Use the little-endian format
for unformatted files.
<li><code>BIG_ENDIAN</code> Use the big-endian format for unformatted files.
</ul>
A missing mode for an exception is taken to mean <code>BIG_ENDIAN</code>.
Examples of values for <samp><span class="env">GFORTRAN_CONVERT_UNIT</span></samp> are:
<ul>
<li><code>'big_endian'</code> Do all unformatted I/O in big_endian mode.
<li><code>'little_endian;native:10-20,25'</code> Do all unformatted I/O
in little_endian mode, except for units 10 to 20 and 25, which are in
native format.
<li><code>'10-20'</code> Units 10 to 20 are big-endian, the rest is native.
</ul>
<p>Setting the environment variables should be done on the command
line or via the <samp><span class="command">export</span></samp>
command for <samp><span class="command">sh</span></samp>-compatible shells and via <samp><span class="command">setenv</span></samp>
for <samp><span class="command">csh</span></samp>-compatible shells.
<p>Example for <samp><span class="command">sh</span></samp>:
<pre class="smallexample"> $ gfortran foo.f90
$ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out
</pre>
<p>Example code for <samp><span class="command">csh</span></samp>:
<pre class="smallexample"> % gfortran foo.f90
% setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20'
% ./a.out
</pre>
<p>Using anything but the native representation for unformatted data
carries a significant speed overhead. If speed in this area matters
to you, it is best if you use this only for data that needs to be
portable.
<p>See <a href="CONVERT-specifier.html#CONVERT-specifier">CONVERT specifier</a>, for an alternative way to specify the
data representation for unformatted files. See <a href="Runtime-Options.html#Runtime-Options">Runtime Options</a>, for
setting a default data representation for the whole program. The
<code>CONVERT</code> specifier overrides the <samp><span class="option">-fconvert</span></samp> compile options.
<p><em>Note that the values specified via the GFORTRAN_CONVERT_UNIT
environment variable will override the CONVERT specifier in the
open statement</em>. This is to give control over data formats to
users who do not have the source code of their program available.
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