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<title>The GNU Fortran Compiler: Argument list functions</title>
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<a name="Argument-list-functions"></a>
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Next: <a href="Read_002fWrite-after-EOF-marker.html#Read_002fWrite-after-EOF-marker" accesskey="n" rel="next">Read/Write after EOF marker</a>, Previous: <a href="OpenACC.html#OpenACC" accesskey="p" rel="prev">OpenACC</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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<a name="Argument-list-functions-_0025VAL_002c-_0025REF-and-_0025LOC"></a>
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<h4 class="subsection">6.1.18 Argument list functions <code>%VAL</code>, <code>%REF</code> and <code>%LOC</code></h4>
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<a name="index-argument-list-functions"></a>
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<a name="index-_0025VAL"></a>
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<a name="index-_0025REF"></a>
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<a name="index-_0025LOC"></a>
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<p>GNU Fortran supports argument list functions <code>%VAL</code>, <code>%REF</code>
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and <code>%LOC</code> statements, for backward compatibility with g77.
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It is recommended that these should be used only for code that is
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accessing facilities outside of GNU Fortran, such as operating system
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or windowing facilities. It is best to constrain such uses to isolated
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portions of a program–portions that deal specifically and exclusively
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with low-level, system-dependent facilities. Such portions might well
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provide a portable interface for use by the program as a whole, but are
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themselves not portable, and should be thoroughly tested each time they
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are rebuilt using a new compiler or version of a compiler.
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</p>
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<p><code>%VAL</code> passes a scalar argument by value, <code>%REF</code> passes it by
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reference and <code>%LOC</code> passes its memory location. Since gfortran
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already passes scalar arguments by reference, <code>%REF</code> is in effect
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a do-nothing. <code>%LOC</code> has the same effect as a Fortran pointer.
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</p>
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<p>An example of passing an argument by value to a C subroutine foo.:
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</p><div class="smallexample">
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<pre class="smallexample">C
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C prototype void foo_ (float x);
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C
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external foo
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real*4 x
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x = 3.14159
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call foo (%VAL (x))
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end
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</pre></div>
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<p>For details refer to the g77 manual
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<a href="https://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/index.html#Top">https://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/index.html#Top</a>.
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</p>
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<p>Also, <code>c_by_val.f</code> and its partner <code>c_by_val.c</code> of the
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GNU Fortran testsuite are worth a look.
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