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| <a name="GFORTRAN_005fCONVERT_005fUNIT_002d_002d_002dSet-endianness-for-unformatted-I_002fO"></a> |
| <h3 class="section">3.11 <code>GFORTRAN_CONVERT_UNIT</code>—Set endianness for unformatted I/O</h3> |
| |
| <p>By setting the <code>GFORTRAN_CONVERT_UNIT</code> variable, it is possible |
| to change the representation of data for unformatted files. |
| The syntax for the <code>GFORTRAN_CONVERT_UNIT</code> variable is: |
| </p><div class="smallexample"> |
| <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></div> |
| <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: |
| </p> |
| <ul class="no-bullet"> |
| <li><!-- /@w --> <code>NATIVE</code> Use the native format. This is the default. |
| </li><li><!-- /@w --> <code>SWAP</code> Swap between little- and big-endian. |
| </li><li><!-- /@w --> <code>LITTLE_ENDIAN</code> Use the little-endian format |
| for unformatted files. |
| </li><li><!-- /@w --> <code>BIG_ENDIAN</code> Use the big-endian format for unformatted files. |
| </li></ul> |
| <p>A missing mode for an exception is taken to mean <code>BIG_ENDIAN</code>. |
| Examples of values for <code>GFORTRAN_CONVERT_UNIT</code> are: |
| </p><ul class="no-bullet"> |
| <li><!-- /@w --> <code>'big_endian'</code> Do all unformatted I/O in big_endian mode. |
| </li><li><!-- /@w --> <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><li><!-- /@w --> <code>'10-20'</code> Units 10 to 20 are big-endian, the rest is native. |
| </li></ul> |
| |
| <p>Setting the environment variables should be done on the command |
| line or via the <code>export</code> |
| command for <code>sh</code>-compatible shells and via <code>setenv</code> |
| for <code>csh</code>-compatible shells. |
| </p> |
| <p>Example for <code>sh</code>: |
| </p><div class="smallexample"> |
| <pre class="smallexample">$ gfortran foo.f90 |
| $ GFORTRAN_CONVERT_UNIT='big_endian;native:10-20' ./a.out |
| </pre></div> |
| |
| <p>Example code for <code>csh</code>: |
| </p><div class="smallexample"> |
| <pre class="smallexample">% gfortran foo.f90 |
| % setenv GFORTRAN_CONVERT_UNIT 'big_endian;native:10-20' |
| % ./a.out |
| </pre></div> |
| |
| <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> |
| <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>-fconvert</samp> compile options. |
| </p> |
| <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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