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<a name="GNU-Fortran-and-GCC"></a>
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<a name="GNU-Fortran-and-GCC-1"></a>
<h3 class="section">1.2 GNU Fortran and GCC</h3>
<a name="index-GNU-Compiler-Collection"></a>
<a name="index-GCC"></a>

<p>GNU Fortran is a part of GCC, the <em>GNU Compiler Collection</em>.  GCC
consists of a collection of front ends for various languages, which
translate the source code into a language-independent form called
<em>GENERIC</em>.  This is then processed by a common middle end which
provides optimization, and then passed to one of a collection of back
ends which generate code for different computer architectures and
operating systems.
</p>
<p>Functionally, this is implemented with a driver program (<code>gcc</code>)
which provides the command-line interface for the compiler.  It calls
the relevant compiler front-end program (e.g., <code>f951</code> for
Fortran) for each file in the source code, and then calls the assembler
and linker as appropriate to produce the compiled output.  In a copy of
GCC which has been compiled with Fortran language support enabled,
<code>gcc</code> will recognize files with <samp>.f</samp>, <samp>.for</samp>, <samp>.ftn</samp>,
<samp>.f90</samp>, <samp>.f95</samp>, <samp>.f03</samp> and <samp>.f08</samp> extensions as
Fortran source code, and compile it accordingly.  A <code>gfortran</code>
driver program is also provided, which is identical to <code>gcc</code>
except that it automatically links the Fortran runtime libraries into the
compiled program.
</p>
<p>Source files with <samp>.f</samp>, <samp>.for</samp>, <samp>.fpp</samp>, <samp>.ftn</samp>, <samp>.F</samp>,
<samp>.FOR</samp>, <samp>.FPP</samp>, and <samp>.FTN</samp> extensions are treated as fixed form.
Source files with <samp>.f90</samp>, <samp>.f95</samp>, <samp>.f03</samp>, <samp>.f08</samp>,
<samp>.F90</samp>, <samp>.F95</samp>, <samp>.F03</samp> and <samp>.F08</samp> extensions are
treated as free form.  The capitalized versions of either form are run
through preprocessing.  Source files with the lower case <samp>.fpp</samp>
extension are also run through preprocessing.
</p>
<p>This manual specifically documents the Fortran front end, which handles
the programming language&rsquo;s syntax and semantics.  The aspects of GCC
which relate to the optimization passes and the back-end code generation
are documented in the GCC manual; see 
<a href="http://gcc.gnu.org/onlinedocs/gcc/index.html#Top">Introduction</a> in <cite>Using the GNU Compiler Collection (GCC)</cite>.
The two manuals together provide a complete reference for the GNU
Fortran compiler.
</p>


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