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24<title>The GNU Fortran Compiler: Argument list functions</title>
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71<a name="Argument-list-functions"></a>
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73<p>
74Next: <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> &nbsp; [<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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77<a name="Argument-list-functions-_0025VAL_002c-_0025REF-and-_0025LOC"></a>
78<h4 class="subsection">6.1.18 Argument list functions <code>%VAL</code>, <code>%REF</code> and <code>%LOC</code></h4>
79<a name="index-argument-list-functions"></a>
80<a name="index-_0025VAL"></a>
81<a name="index-_0025REF"></a>
82<a name="index-_0025LOC"></a>
83
84<p>GNU Fortran supports argument list functions <code>%VAL</code>, <code>%REF</code>
85and <code>%LOC</code> statements, for backward compatibility with g77.
86It is recommended that these should be used only for code that is
87accessing facilities outside of GNU Fortran, such as operating system
88or windowing facilities. It is best to constrain such uses to isolated
89portions of a program&ndash;portions that deal specifically and exclusively
90with low-level, system-dependent facilities. Such portions might well
91provide a portable interface for use by the program as a whole, but are
92themselves not portable, and should be thoroughly tested each time they
93are rebuilt using a new compiler or version of a compiler.
94</p>
95<p><code>%VAL</code> passes a scalar argument by value, <code>%REF</code> passes it by
96reference and <code>%LOC</code> passes its memory location. Since gfortran
97already passes scalar arguments by reference, <code>%REF</code> is in effect
98a do-nothing. <code>%LOC</code> has the same effect as a Fortran pointer.
99</p>
100<p>An example of passing an argument by value to a C subroutine foo.:
101</p><div class="smallexample">
102<pre class="smallexample">C
103C prototype void foo_ (float x);
104C
105 external foo
106 real*4 x
107 x = 3.14159
108 call foo (%VAL (x))
109 end
110</pre></div>
111
112<p>For details refer to the g77 manual
113<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>.
114</p>
115<p>Also, <code>c_by_val.f</code> and its partner <code>c_by_val.c</code> of the
116GNU Fortran testsuite are worth a look.
117</p>
118
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