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<title>The GNU Fortran Compiler: About GNU Fortran</title>

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<a name="About-GNU-Fortran"></a>
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<p>
Next: <a href="GNU-Fortran-and-GCC.html#GNU-Fortran-and-GCC" accesskey="n" rel="next">GNU Fortran and GCC</a>, Up: <a href="Introduction.html#Introduction" accesskey="u" rel="up">Introduction</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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<a name="About-GNU-Fortran-1"></a>
<h3 class="section">1.1 About GNU Fortran</h3>

<p>The GNU Fortran compiler supports the Fortran 77, 90 and 95 standards
completely, parts of the Fortran 2003 and Fortran 2008 standards, and
several vendor extensions.  The development goal is to provide the
following features:
</p>
<ul>
<li> Read a user&rsquo;s program,
stored in a file and containing instructions written
in Fortran 77, Fortran 90, Fortran 95, Fortran 2003 or Fortran 2008.
This file contains <em>source code</em>.

</li><li> Translate the user&rsquo;s program into instructions a computer
can carry out more quickly than it takes to translate the
instructions in the first
place.  The result after compilation of a program is
<em>machine code</em>,
code designed to be efficiently translated and processed
by a machine such as your computer.
Humans usually are not as good writing machine code
as they are at writing Fortran (or C++, Ada, or Java),
because it is easy to make tiny mistakes writing machine code.

</li><li> Provide the user with information about the reasons why
the compiler is unable to create a binary from the source code.
Usually this will be the case if the source code is flawed.
The Fortran 90 standard requires that the compiler can point out
mistakes to the user.
An incorrect usage of the language causes an <em>error message</em>.

<p>The compiler will also attempt to diagnose cases where the
user&rsquo;s program contains a correct usage of the language,
but instructs the computer to do something questionable.
This kind of diagnostics message is called a <em>warning message</em>.
</p>
</li><li> Provide optional information about the translation passes
from the source code to machine code.
This can help a user of the compiler to find the cause of
certain bugs which may not be obvious in the source code,
but may be more easily found at a lower level compiler output.
It also helps developers to find bugs in the compiler itself.

</li><li> Provide information in the generated machine code that can
make it easier to find bugs in the program (using a debugging tool,
called a <em>debugger</em>, such as the GNU Debugger <code>gdb</code>).

</li><li> Locate and gather machine code already generated to
perform actions requested by statements in the user&rsquo;s program.
This machine code is organized into <em>modules</em> and is located
and <em>linked</em> to the user program.
</li></ul>

<p>The GNU Fortran compiler consists of several components:
</p>
<ul>
<li> A version of the <code>gcc</code> command
(which also might be installed as the system&rsquo;s <code>cc</code> command)
that also understands and accepts Fortran source code.
The <code>gcc</code> command is the <em>driver</em> program for
all the languages in the GNU Compiler Collection (GCC);
With <code>gcc</code>,
you can compile the source code of any language for
which a front end is available in GCC.

</li><li> The <code>gfortran</code> command itself,
which also might be installed as the
system&rsquo;s <code>f95</code> command.
<code>gfortran</code> is just another driver program,
but specifically for the Fortran compiler only.
The difference with <code>gcc</code> is that <code>gfortran</code>
will automatically link the correct libraries to your program.

</li><li> A collection of run-time libraries.
These libraries contain the machine code needed to support
capabilities of the Fortran language that are not directly
provided by the machine code generated by the
<code>gfortran</code> compilation phase,
such as intrinsic functions and subroutines,
and routines for interaction with files and the operating system.

</li><li> The Fortran compiler itself, (<code>f951</code>).
This is the GNU Fortran parser and code generator,
linked to and interfaced with the GCC backend library.
<code>f951</code> &ldquo;translates&rdquo; the source code to
assembler code.  You would typically not use this
program directly;
instead, the <code>gcc</code> or <code>gfortran</code> driver
programs will call it for you.
</li></ul>


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