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<h3 class="section">1.2 GNU Fortran and GCC</h3>
<p><a name="index-GNU-Compiler-Collection-2"></a><a name="index-GCC-3"></a>
GNU Fortran is a part of GCC, the <dfn>GNU Compiler Collection</dfn>. GCC
consists of a collection of front ends for various languages, which
translate the source code into a language-independent form called
<dfn>GENERIC</dfn>. 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>Functionally, this is implemented with a driver program (<samp><span class="command">gcc</span></samp>)
which provides the command-line interface for the compiler. It calls
the relevant compiler front-end program (e.g., <samp><span class="command">f951</span></samp> 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,
<samp><span class="command">gcc</span></samp> will recognize files with <samp><span class="file">.f</span></samp>, <samp><span class="file">.for</span></samp>, <samp><span class="file">.ftn</span></samp>,
<samp><span class="file">.f90</span></samp>, <samp><span class="file">.f95</span></samp>, <samp><span class="file">.f03</span></samp> and <samp><span class="file">.f08</span></samp> extensions as
Fortran source code, and compile it accordingly. A <samp><span class="command">gfortran</span></samp>
driver program is also provided, which is identical to <samp><span class="command">gcc</span></samp>
except that it automatically links the Fortran runtime libraries into the
compiled program.
<p>Source files with <samp><span class="file">.f</span></samp>, <samp><span class="file">.for</span></samp>, <samp><span class="file">.fpp</span></samp>, <samp><span class="file">.ftn</span></samp>, <samp><span class="file">.F</span></samp>,
<samp><span class="file">.FOR</span></samp>, <samp><span class="file">.FPP</span></samp>, and <samp><span class="file">.FTN</span></samp> extensions are treated as fixed form.
Source files with <samp><span class="file">.f90</span></samp>, <samp><span class="file">.f95</span></samp>, <samp><span class="file">.f03</span></samp>, <samp><span class="file">.f08</span></samp>,
<samp><span class="file">.F90</span></samp>, <samp><span class="file">.F95</span></samp>, <samp><span class="file">.F03</span></samp> and <samp><span class="file">.F08</span></samp> extensions are
treated as free form. The capitalized versions of either form are run
through preprocessing. Source files with the lower case <samp><span class="file">.fpp</span></samp>
extension are also run through preprocessing.
<p>This manual specifically documents the Fortran front end, which handles
the programming language'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="../gcc/index.html#Top">Introduction</a>.
The two manuals together provide a complete reference for the GNU
Fortran compiler.
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