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<title>The GNU Fortran Compiler: GNU Fortran Compiler Directives</title>
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<a name="GNU-Fortran-Compiler-Directives"></a>
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<p>
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Next: <a href="Non_002dFortran-Main-Program.html#Non_002dFortran-Main-Program" accesskey="n" rel="next">Non-Fortran Main Program</a>, Previous: <a href="Interoperability-with-C.html#Interoperability-with-C" accesskey="p" rel="prev">Interoperability with C</a>, Up: <a href="Mixed_002dLanguage-Programming.html#Mixed_002dLanguage-Programming" accesskey="u" rel="up">Mixed-Language Programming</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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<hr>
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<a name="GNU-Fortran-Compiler-Directives-1"></a>
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<h3 class="section">7.2 GNU Fortran Compiler Directives</h3>
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<p>The Fortran standard describes how a conforming program shall
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behave; however, the exact implementation is not standardized. In order
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to allow the user to choose specific implementation details, compiler
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directives can be used to set attributes of variables and procedures
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which are not part of the standard. Whether a given attribute is
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supported and its exact effects depend on both the operating system and
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on the processor; see
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<a href="http://gcc.gnu.org/onlinedocs/gcc/index.html#Top">C Extensions</a> in <cite>Using the GNU Compiler Collection (GCC)</cite>
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for details.
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</p>
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<p>For procedures and procedure pointers, the following attributes can
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be used to change the calling convention:
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</p>
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<ul>
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<li> <code>CDECL</code> – standard C calling convention
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</li><li> <code>STDCALL</code> – convention where the called procedure pops the stack
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</li><li> <code>FASTCALL</code> – part of the arguments are passed via registers
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instead using the stack
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</li></ul>
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<p>Besides changing the calling convention, the attributes also influence
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the decoration of the symbol name, e.g., by a leading underscore or by
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a trailing at-sign followed by the number of bytes on the stack. When
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assigning a procedure to a procedure pointer, both should use the same
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calling convention.
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</p>
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<p>On some systems, procedures and global variables (module variables and
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<code>COMMON</code> blocks) need special handling to be accessible when they
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are in a shared library. The following attributes are available:
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</p>
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<ul>
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<li> <code>DLLEXPORT</code> – provide a global pointer to a pointer in the DLL
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</li><li> <code>DLLIMPORT</code> – reference the function or variable using a
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global pointer
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</li></ul>
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<p>For dummy arguments, the <code>NO_ARG_CHECK</code> attribute can be used; in
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other compilers, it is also known as <code>IGNORE_TKR</code>. For dummy arguments
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with this attribute actual arguments of any type and kind (similar to
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<code>TYPE(*)</code>), scalars and arrays of any rank (no equivalent
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in Fortran standard) are accepted. As with <code>TYPE(*)</code>, the argument
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is unlimited polymorphic and no type information is available.
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Additionally, the argument may only be passed to dummy arguments
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with the <code>NO_ARG_CHECK</code> attribute and as argument to the
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<code>PRESENT</code> intrinsic function and to <code>C_LOC</code> of the
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<code>ISO_C_BINDING</code> module.
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</p>
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<p>Variables with <code>NO_ARG_CHECK</code> attribute shall be of assumed-type
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(<code>TYPE(*)</code>; recommended) or of type <code>INTEGER</code>, <code>LOGICAL</code>,
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<code>REAL</code> or <code>COMPLEX</code>. They shall not have the <code>ALLOCATE</code>,
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<code>CODIMENSION</code>, <code>INTENT(OUT)</code>, <code>POINTER</code> or <code>VALUE</code>
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attribute; furthermore, they shall be either scalar or of assumed-size
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(<code>dimension(*)</code>). As <code>TYPE(*)</code>, the <code>NO_ARG_CHECK</code> attribute
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requires an explicit interface.
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</p>
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<ul>
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<li> <code>NO_ARG_CHECK</code> – disable the type, kind and rank checking
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</li></ul>
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<p>The attributes are specified using the syntax
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</p>
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<p><code>!GCC$ ATTRIBUTES</code> <var>attribute-list</var> <code>::</code> <var>variable-list</var>
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</p>
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<p>where in free-form source code only whitespace is allowed before <code>!GCC$</code>
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and in fixed-form source code <code>!GCC$</code>, <code>cGCC$</code> or <code>*GCC$</code> shall
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start in the first column.
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</p>
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<p>For procedures, the compiler directives shall be placed into the body
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of the procedure; for variables and procedure pointers, they shall be in
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the same declaration part as the variable or procedure pointer.
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</p>
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<div class="header">
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<p>
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Next: <a href="Non_002dFortran-Main-Program.html#Non_002dFortran-Main-Program" accesskey="n" rel="next">Non-Fortran Main Program</a>, Previous: <a href="Interoperability-with-C.html#Interoperability-with-C" accesskey="p" rel="prev">Interoperability with C</a>, Up: <a href="Mixed_002dLanguage-Programming.html#Mixed_002dLanguage-Programming" accesskey="u" rel="up">Mixed-Language Programming</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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