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<h1 class="settitle" align="center">The GNU Go Compiler</h1>












<a name="SEC_Contents"></a>
<h2 class="contents-heading">Table of Contents</h2>

<div class="contents">

<ul class="no-bullet">
  <li><a name="toc-GNU-General-Public-License" href="#Copying">GNU General Public License</a></li>
  <li><a name="toc-GNU-Free-Documentation-License-1" href="#GNU-Free-Documentation-License">GNU Free Documentation License</a>
  <ul class="no-bullet">
    <li><a name="toc-ADDENDUM_003a-How-to-use-this-License-for-your-documents" href="#ADDENDUM_003a-How-to-use-this-License-for-your-documents">ADDENDUM: How to use this License for your documents</a></li>
  </ul></li>
  <li><a name="toc-Invoking-gccgo-1" href="#Invoking-gccgo">1 Invoking gccgo</a></li>
  <li><a name="toc-Import-and-Export-1" href="#Import-and-Export">2 Import and Export</a></li>
  <li><a name="toc-Compiler-Directives-1" href="#Compiler-Directives">3 Compiler Directives</a></li>
  <li><a name="toc-C-Interoperability-1" href="#C-Interoperability">4 C Interoperability</a>
  <ul class="no-bullet">
    <li><a name="toc-C-Type-Interoperability-1" href="#C-Type-Interoperability">4.1 C Type Interoperability</a></li>
    <li><a name="toc-Function-Names-1" href="#Function-Names">4.2 Function Names</a></li>
  </ul></li>
  <li><a name="toc-Index-1" href="#Index">Index</a></li>
</ul>
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<p>
Next: <a href="#Copying" accesskey="n" rel="next">Copying</a>, Up: <a href="dir.html#Top" accesskey="u" rel="up">(dir)</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Introduction"></a>
<h1 class="top">Introduction</h1>

<p>This manual describes how to use <code>gccgo</code>, the GNU compiler for
the Go programming language.  This manual is specifically about
<code>gccgo</code>.  For more information about the Go programming
language in general, including language specifications and standard
package documentation, see <a href="http://golang.org/">http://golang.org/</a>.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="#Copying" accesskey="1">Copying</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">The GNU General Public License.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#GNU-Free-Documentation-License" accesskey="2">GNU Free Documentation License</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
                                How you can share and copy this manual.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Invoking-gccgo" accesskey="3">Invoking gccgo</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to run gccgo.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Import-and-Export" accesskey="4">Import and Export</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Importing and exporting package data.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Compiler-Directives" accesskey="5">Compiler Directives</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Comments to control compilation.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#C-Interoperability" accesskey="6">C Interoperability</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Calling C from Go and vice-versa.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Index" accesskey="7">Index</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Index.
</td></tr>
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<a name="GNU-General-Public-License"></a>
<h2 class="unnumbered">GNU General Public License</h2>
<div align="center">Version 3, 29 June 2007
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<pre class="display">Copyright &copy; 2007 Free Software Foundation, Inc. <a href="http://fsf.org/">http://fsf.org/</a>

Everyone is permitted to copy and distribute verbatim copies of this
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this License.  If your rights have been terminated and not permanently
reinstated, you do not qualify to receive new licenses for the same
material under section 10.
</p>
</li><li> Acceptance Not Required for Having Copies.

<p>You are not required to accept this License in order to receive or run
a copy of the Program.  Ancillary propagation of a covered work
occurring solely as a consequence of using peer-to-peer transmission
to receive a copy likewise does not require acceptance.  However,
nothing other than this License grants you permission to propagate or
modify any covered work.  These actions infringe copyright if you do
not accept this License.  Therefore, by modifying or propagating a
covered work, you indicate your acceptance of this License to do so.
</p>
</li><li> Automatic Licensing of Downstream Recipients.

<p>Each time you convey a covered work, the recipient automatically
receives a license from the original licensors, to run, modify and
propagate that work, subject to this License.  You are not responsible
for enforcing compliance by third parties with this License.
</p>
<p>An &ldquo;entity transaction&rdquo; is a transaction transferring control of an
organization, or substantially all assets of one, or subdividing an
organization, or merging organizations.  If propagation of a covered
work results from an entity transaction, each party to that
transaction who receives a copy of the work also receives whatever
licenses to the work the party&rsquo;s predecessor in interest had or could
give under the previous paragraph, plus a right to possession of the
Corresponding Source of the work from the predecessor in interest, if
the predecessor has it or can get it with reasonable efforts.
</p>
<p>You may not impose any further restrictions on the exercise of the
rights granted or affirmed under this License.  For example, you may
not impose a license fee, royalty, or other charge for exercise of
rights granted under this License, and you may not initiate litigation
(including a cross-claim or counterclaim in a lawsuit) alleging that
any patent claim is infringed by making, using, selling, offering for
sale, or importing the Program or any portion of it.
</p>
</li><li> Patents.

<p>A &ldquo;contributor&rdquo; is a copyright holder who authorizes use under this
License of the Program or a work on which the Program is based.  The
work thus licensed is called the contributor&rsquo;s &ldquo;contributor version&rdquo;.
</p>
<p>A contributor&rsquo;s &ldquo;essential patent claims&rdquo; are all patent claims owned
or controlled by the contributor, whether already acquired or
hereafter acquired, that would be infringed by some manner, permitted
by this License, of making, using, or selling its contributor version,
but do not include claims that would be infringed only as a
consequence of further modification of the contributor version.  For
purposes of this definition, &ldquo;control&rdquo; includes the right to grant
patent sublicenses in a manner consistent with the requirements of
this License.
</p>
<p>Each contributor grants you a non-exclusive, worldwide, royalty-free
patent license under the contributor&rsquo;s essential patent claims, to
make, use, sell, offer for sale, import and otherwise run, modify and
propagate the contents of its contributor version.
</p>
<p>In the following three paragraphs, a &ldquo;patent license&rdquo; is any express
agreement or commitment, however denominated, not to enforce a patent
(such as an express permission to practice a patent or covenant not to
sue for patent infringement).  To &ldquo;grant&rdquo; such a patent license to a
party means to make such an agreement or commitment not to enforce a
patent against the party.
</p>
<p>If you convey a covered work, knowingly relying on a patent license,
and the Corresponding Source of the work is not available for anyone
to copy, free of charge and under the terms of this License, through a
publicly available network server or other readily accessible means,
then you must either (1) cause the Corresponding Source to be so
available, or (2) arrange to deprive yourself of the benefit of the
patent license for this particular work, or (3) arrange, in a manner
consistent with the requirements of this License, to extend the patent
license to downstream recipients.  &ldquo;Knowingly relying&rdquo; means you have
actual knowledge that, but for the patent license, your conveying the
covered work in a country, or your recipient&rsquo;s use of the covered work
in a country, would infringe one or more identifiable patents in that
country that you have reason to believe are valid.
</p>
<p>If, pursuant to or in connection with a single transaction or
arrangement, you convey, or propagate by procuring conveyance of, a
covered work, and grant a patent license to some of the parties
receiving the covered work authorizing them to use, propagate, modify
or convey a specific copy of the covered work, then the patent license
you grant is automatically extended to all recipients of the covered
work and works based on it.
</p>
<p>A patent license is &ldquo;discriminatory&rdquo; if it does not include within the
scope of its coverage, prohibits the exercise of, or is conditioned on
the non-exercise of one or more of the rights that are specifically
granted under this License.  You may not convey a covered work if you
are a party to an arrangement with a third party that is in the
business of distributing software, under which you make payment to the
third party based on the extent of your activity of conveying the
work, and under which the third party grants, to any of the parties
who would receive the covered work from you, a discriminatory patent
license (a) in connection with copies of the covered work conveyed by
you (or copies made from those copies), or (b) primarily for and in
connection with specific products or compilations that contain the
covered work, unless you entered into that arrangement, or that patent
license was granted, prior to 28 March 2007.
</p>
<p>Nothing in this License shall be construed as excluding or limiting
any implied license or other defenses to infringement that may
otherwise be available to you under applicable patent law.
</p>
</li><li> No Surrender of Others&rsquo; Freedom.

<p>If conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License.  If you cannot convey
a covered work so as to satisfy simultaneously your obligations under
this License and any other pertinent obligations, then as a
consequence you may not convey it at all.  For example, if you agree
to terms that obligate you to collect a royalty for further conveying
from those to whom you convey the Program, the only way you could
satisfy both those terms and this License would be to refrain entirely
from conveying the Program.
</p>
</li><li> Use with the GNU Affero General Public License.

<p>Notwithstanding any other provision of this License, you have
permission to link or combine any covered work with a work licensed
under version 3 of the GNU Affero General Public License into a single
combined work, and to convey the resulting work.  The terms of this
License will continue to apply to the part which is the covered work,
but the special requirements of the GNU Affero General Public License,
section 13, concerning interaction through a network will apply to the
combination as such.
</p>
</li><li> Revised Versions of this License.

<p>The Free Software Foundation may publish revised and/or new versions
of the GNU General Public License from time to time.  Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns.
</p>
<p>Each version is given a distinguishing version number.  If the Program
specifies that a certain numbered version of the GNU General Public
License &ldquo;or any later version&rdquo; applies to it, you have the option of
following the terms and conditions either of that numbered version or
of any later version published by the Free Software Foundation.  If
the Program does not specify a version number of the GNU General
Public License, you may choose any version ever published by the Free
Software Foundation.
</p>
<p>If the Program specifies that a proxy can decide which future versions
of the GNU General Public License can be used, that proxy&rsquo;s public
statement of acceptance of a version permanently authorizes you to
choose that version for the Program.
</p>
<p>Later license versions may give you additional or different
permissions.  However, no additional obligations are imposed on any
author or copyright holder as a result of your choosing to follow a
later version.
</p>
</li><li> Disclaimer of Warranty.

<p>THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM &ldquo;AS IS&rdquo; WITHOUT
WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND
PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE PROGRAM PROVE
DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR
CORRECTION.
</p>
</li><li> Limitation of Liability.

<p>IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR
CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES
ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT
NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR
LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM
TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER
PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
</p>
</li><li> Interpretation of Sections 15 and 16.

<p>If the disclaimer of warranty and limitation of liability provided
above cannot be given local legal effect according to their terms,
reviewing courts shall apply local law that most closely approximates
an absolute waiver of all civil liability in connection with the
Program, unless a warranty or assumption of liability accompanies a
copy of the Program in return for a fee.
</p>
</li></ol>

<a name="END-OF-TERMS-AND-CONDITIONS"></a>
<h3 class="heading">END OF TERMS AND CONDITIONS</h3>

<a name="How-to-Apply-These-Terms-to-Your-New-Programs"></a>
<h3 class="heading">How to Apply These Terms to Your New Programs</h3>

<p>If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these
terms.
</p>
<p>To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
state the exclusion of warranty; and each file should have at least
the &ldquo;copyright&rdquo; line and a pointer to where the full notice is found.
</p>
<div class="smallexample">
<pre class="smallexample"><var>one line to give the program's name and a brief idea of what it does.</var>  
Copyright (C) <var>year</var> <var>name of author</var>

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or (at
your option) any later version.

This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <a href="http://www.gnu.org/licenses/">http://www.gnu.org/licenses/</a>.
</pre></div>

<p>Also add information on how to contact you by electronic and paper mail.
</p>
<p>If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:
</p>
<div class="smallexample">
<pre class="smallexample"><var>program</var> Copyright (C) <var>year</var> <var>name of author</var> 
This program comes with ABSOLUTELY NO WARRANTY; for details type &lsquo;<samp>show w</samp>&rsquo;.
This is free software, and you are welcome to redistribute it
under certain conditions; type &lsquo;<samp>show c</samp>&rsquo; for details.
</pre></div>

<p>The hypothetical commands &lsquo;<samp>show w</samp>&rsquo; and &lsquo;<samp>show c</samp>&rsquo; should show
the appropriate parts of the General Public License.  Of course, your
program&rsquo;s commands might be different; for a GUI interface, you would
use an &ldquo;about box&rdquo;.
</p>
<p>You should also get your employer (if you work as a programmer) or school,
if any, to sign a &ldquo;copyright disclaimer&rdquo; for the program, if necessary.
For more information on this, and how to apply and follow the GNU GPL, see
<a href="http://www.gnu.org/licenses/">http://www.gnu.org/licenses/</a>.
</p>
<p>The GNU General Public License does not permit incorporating your
program into proprietary programs.  If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library.  If this is what you want to do, use
the GNU Lesser General Public License instead of this License.  But
first, please read <a href="http://www.gnu.org/philosophy/why-not-lgpl.html">http://www.gnu.org/philosophy/why-not-lgpl.html</a>.
</p>
<hr>
<a name="GNU-Free-Documentation-License"></a>
<div class="header">
<p>
Next: <a href="#Invoking-gccgo" accesskey="n" rel="next">Invoking gccgo</a>, Previous: <a href="#Copying" accesskey="p" rel="prev">Copying</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="GNU-Free-Documentation-License-1"></a>
<h2 class="unnumbered">GNU Free Documentation License</h2>

<a name="index-FDL_002c-GNU-Free-Documentation-License"></a>
<div align="center">Version 1.3, 3 November 2008
</div>
<div class="display">
<pre class="display">Copyright &copy; 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
<a href="http://fsf.org/">http://fsf.org/</a>

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
</pre></div>

<ol start="0">
<li> PREAMBLE

<p>The purpose of this License is to make a manual, textbook, or other
functional and useful document <em>free</em> in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or noncommercially.
Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible
for modifications made by others.
</p>
<p>This License is a kind of &ldquo;copyleft&rdquo;, which means that derivative
works of the document must themselves be free in the same sense.  It
complements the GNU General Public License, which is a copyleft
license designed for free software.
</p>
<p>We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free
program should come with manuals providing the same freedoms that the
software does.  But this License is not limited to software manuals;
it can be used for any textual work, regardless of subject matter or
whether it is published as a printed book.  We recommend this License
principally for works whose purpose is instruction or reference.
</p>
</li><li> APPLICABILITY AND DEFINITIONS

<p>This License applies to any manual or other work, in any medium, that
contains a notice placed by the copyright holder saying it can be
distributed under the terms of this License.  Such a notice grants a
world-wide, royalty-free license, unlimited in duration, to use that
work under the conditions stated herein.  The &ldquo;Document&rdquo;, below,
refers to any such manual or work.  Any member of the public is a
licensee, and is addressed as &ldquo;you&rdquo;.  You accept the license if you
copy, modify or distribute the work in a way requiring permission
under copyright law.
</p>
<p>A &ldquo;Modified Version&rdquo; of the Document means any work containing the
Document or a portion of it, either copied verbatim, or with
modifications and/or translated into another language.
</p>
<p>A &ldquo;Secondary Section&rdquo; is a named appendix or a front-matter section
of the Document that deals exclusively with the relationship of the
publishers or authors of the Document to the Document&rsquo;s overall
subject (or to related matters) and contains nothing that could fall
directly within that overall subject.  (Thus, if the Document is in
part a textbook of mathematics, a Secondary Section may not explain
any mathematics.)  The relationship could be a matter of historical
connection with the subject or with related matters, or of legal,
commercial, philosophical, ethical or political position regarding
them.
</p>
<p>The &ldquo;Invariant Sections&rdquo; are certain Secondary Sections whose titles
are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License.  If a
section does not fit the above definition of Secondary then it is not
allowed to be designated as Invariant.  The Document may contain zero
Invariant Sections.  If the Document does not identify any Invariant
Sections then there are none.
</p>
<p>The &ldquo;Cover Texts&rdquo; are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License.  A Front-Cover Text may
be at most 5 words, and a Back-Cover Text may be at most 25 words.
</p>
<p>A &ldquo;Transparent&rdquo; copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, that is suitable for revising the document
straightforwardly with generic text editors or (for images composed of
pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or
for automatic translation to a variety of formats suitable for input
to text formatters.  A copy made in an otherwise Transparent file
format whose markup, or absence of markup, has been arranged to thwart
or discourage subsequent modification by readers is not Transparent.
An image format is not Transparent if used for any substantial amount
of text.  A copy that is not &ldquo;Transparent&rdquo; is called &ldquo;Opaque&rdquo;.
</p>
<p>Examples of suitable formats for Transparent copies include plain
<small>ASCII</small> without markup, Texinfo input format, LaTeX input
format, <acronym>SGML</acronym> or <acronym>XML</acronym> using a publicly available
<acronym>DTD</acronym>, and standard-conforming simple <acronym>HTML</acronym>,
PostScript or <acronym>PDF</acronym> designed for human modification.  Examples
of transparent image formats include <acronym>PNG</acronym>, <acronym>XCF</acronym> and
<acronym>JPG</acronym>.  Opaque formats include proprietary formats that can be
read and edited only by proprietary word processors, <acronym>SGML</acronym> or
<acronym>XML</acronym> for which the <acronym>DTD</acronym> and/or processing tools are
not generally available, and the machine-generated <acronym>HTML</acronym>,
PostScript or <acronym>PDF</acronym> produced by some word processors for
output purposes only.
</p>
<p>The &ldquo;Title Page&rdquo; means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page.  For works in
formats which do not have any title page as such, &ldquo;Title Page&rdquo; means
the text near the most prominent appearance of the work&rsquo;s title,
preceding the beginning of the body of the text.
</p>
<p>The &ldquo;publisher&rdquo; means any person or entity that distributes copies
of the Document to the public.
</p>
<p>A section &ldquo;Entitled XYZ&rdquo; means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following
text that translates XYZ in another language.  (Here XYZ stands for a
specific section name mentioned below, such as &ldquo;Acknowledgements&rdquo;,
&ldquo;Dedications&rdquo;, &ldquo;Endorsements&rdquo;, or &ldquo;History&rdquo;.)  To &ldquo;Preserve the Title&rdquo;
of such a section when you modify the Document means that it remains a
section &ldquo;Entitled XYZ&rdquo; according to this definition.
</p>
<p>The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document.  These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.
</p>
</li><li> VERBATIM COPYING

<p>You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License.  You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute.  However, you may accept
compensation in exchange for copies.  If you distribute a large enough
number of copies you must also follow the conditions in section 3.
</p>
<p>You may also lend copies, under the same conditions stated above, and
you may publicly display copies.
</p>
</li><li> COPYING IN QUANTITY

<p>If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document&rsquo;s license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover.  Both covers must also clearly and legibly identify
you as the publisher of these copies.  The front cover must present
the full title with all words of the title equally prominent and
visible.  You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve
the title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.
</p>
<p>If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.
</p>
<p>If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.
</p>
<p>It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.
</p>
</li><li> MODIFICATIONS

<p>You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it.  In addition, you must do these things in the Modified Version:
</p>
<ol type="A" start="1">
<li> Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document).  You may use the same title as a previous version
if the original publisher of that version gives permission.

</li><li> List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.

</li><li> State on the Title page the name of the publisher of the
Modified Version, as the publisher.

</li><li> Preserve all the copyright notices of the Document.

</li><li> Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

</li><li> Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

</li><li> Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document&rsquo;s license notice.

</li><li> Include an unaltered copy of this License.

</li><li> Preserve the section Entitled &ldquo;History&rdquo;, Preserve its Title, and add
to it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page.  If
there is no section Entitled &ldquo;History&rdquo; in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.

</li><li> Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on.  These may be placed in the &ldquo;History&rdquo; section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

</li><li> For any section Entitled &ldquo;Acknowledgements&rdquo; or &ldquo;Dedications&rdquo;, Preserve
the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.

</li><li> Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles.  Section numbers
or the equivalent are not considered part of the section titles.

</li><li> Delete any section Entitled &ldquo;Endorsements&rdquo;.  Such a section
may not be included in the Modified Version.

</li><li> Do not retitle any existing section to be Entitled &ldquo;Endorsements&rdquo; or
to conflict in title with any Invariant Section.

</li><li> Preserve any Warranty Disclaimers.
</li></ol>

<p>If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant.  To do this, add their titles to the
list of Invariant Sections in the Modified Version&rsquo;s license notice.
These titles must be distinct from any other section titles.
</p>
<p>You may add a section Entitled &ldquo;Endorsements&rdquo;, provided it contains
nothing but endorsements of your Modified Version by various
parties&mdash;for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.
</p>
<p>You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version.  Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity.  If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.
</p>
<p>The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.
</p>
</li><li> COMBINING DOCUMENTS

<p>You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.
</p>
<p>The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy.  If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.
</p>
<p>In the combination, you must combine any sections Entitled &ldquo;History&rdquo;
in the various original documents, forming one section Entitled
&ldquo;History&rdquo;; likewise combine any sections Entitled &ldquo;Acknowledgements&rdquo;,
and any sections Entitled &ldquo;Dedications&rdquo;.  You must delete all
sections Entitled &ldquo;Endorsements.&rdquo;
</p>
</li><li> COLLECTIONS OF DOCUMENTS

<p>You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.
</p>
<p>You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.
</p>
</li><li> AGGREGATION WITH INDEPENDENT WORKS

<p>A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an &ldquo;aggregate&rdquo; if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation&rsquo;s users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.
</p>
<p>If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document&rsquo;s Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.
</p>
</li><li> TRANSLATION

<p>Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections.  You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers.  In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.
</p>
<p>If a section in the Document is Entitled &ldquo;Acknowledgements&rdquo;,
&ldquo;Dedications&rdquo;, or &ldquo;History&rdquo;, the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.
</p>
</li><li> TERMINATION

<p>You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License.  Any attempt
otherwise to copy, modify, sublicense, or distribute it is void, and
will automatically terminate your rights under this License.
</p>
<p>However, if you cease all violation of this License, then your license
from a particular copyright holder is reinstated (a) provisionally,
unless and until the copyright holder explicitly and finally
terminates your license, and (b) permanently, if the copyright holder
fails to notify you of the violation by some reasonable means prior to
60 days after the cessation.
</p>
<p>Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.
</p>
<p>Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License.  If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.
</p>
</li><li> FUTURE REVISIONS OF THIS LICENSE

<p>The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time.  Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns.  See
<a href="http://www.gnu.org/copyleft/">http://www.gnu.org/copyleft/</a>.
</p>
<p>Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License &ldquo;or any later version&rdquo; applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation.  If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.  If the Document
specifies that a proxy can decide which future versions of this
License can be used, that proxy&rsquo;s public statement of acceptance of a
version permanently authorizes you to choose that version for the
Document.
</p>
</li><li> RELICENSING

<p>&ldquo;Massive Multiauthor Collaboration Site&rdquo; (or &ldquo;MMC Site&rdquo;) means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works.  A
public wiki that anybody can edit is an example of such a server.  A
&ldquo;Massive Multiauthor Collaboration&rdquo; (or &ldquo;MMC&rdquo;) contained in the
site means any set of copyrightable works thus published on the MMC
site.
</p>
<p>&ldquo;CC-BY-SA&rdquo; means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.
</p>
<p>&ldquo;Incorporate&rdquo; means to publish or republish a Document, in whole or
in part, as part of another Document.
</p>
<p>An MMC is &ldquo;eligible for relicensing&rdquo; if it is licensed under this
License, and if all works that were first published under this License
somewhere other than this MMC, and subsequently incorporated in whole
or in part into the MMC, (1) had no cover texts or invariant sections,
and (2) were thus incorporated prior to November 1, 2008.
</p>
<p>The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.
</p>
</li></ol>

<a name="ADDENDUM_003a-How-to-use-this-License-for-your-documents"></a>
<h3 class="unnumberedsec">ADDENDUM: How to use this License for your documents</h3>

<p>To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:
</p>
<div class="smallexample">
<pre class="smallexample">  Copyright (C)  <var>year</var>  <var>your name</var>.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.3
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
  Texts.  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.
</pre></div>

<p>If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the &ldquo;with...Texts.&rdquo; line with this:
</p>
<div class="smallexample">
<pre class="smallexample">    with the Invariant Sections being <var>list their titles</var>, with
    the Front-Cover Texts being <var>list</var>, and with the Back-Cover Texts
    being <var>list</var>.
</pre></div>

<p>If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.
</p>
<p>If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.
</p>



<hr>
<a name="Invoking-gccgo"></a>
<div class="header">
<p>
Next: <a href="#Import-and-Export" accesskey="n" rel="next">Import and Export</a>, Previous: <a href="#GNU-Free-Documentation-License" accesskey="p" rel="prev">GNU Free Documentation License</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Invoking-gccgo-1"></a>
<h2 class="chapter">1 Invoking gccgo</h2>




<p>The <code>gccgo</code> command is a frontend to <code>gcc</code> and
supports many of the same options.  See <a href="x86_64-linux-gnu-gcc-8.html#Option-Summary">Option
Summary</a> in <cite>Using the GNU Compiler Collection (GCC)</cite>.  This manual
only documents the options specific to <code>gccgo</code>.
</p>
<p>The <code>gccgo</code> command may be used to compile Go source code into
an object file, link a collection of object files together, or do both
in sequence.
</p>
<p>Go source code is compiled as packages.  A package consists of one or
more Go source files.  All the files in a single package must be
compiled together, by passing all the files as arguments to
<code>gccgo</code>.  A single invocation of <code>gccgo</code> may only
compile a single package.
</p>
<p>One Go package may <code>import</code> a different Go package.  The imported
package must have already been compiled; <code>gccgo</code> will read
the import data directly from the compiled package.  When this package
is later linked, the compiled form of the package must be included in
the link command.
</p>
<p>Go programs must generally be compiled with debugging information, and
<samp>-g1</samp> is the default as described below.  Stripping a Go
program will generally cause it to misbehave or fail.
</p>


<dl compact="compact">
<dt><code>-I<var>dir</var></code></dt>
<dd><a name="index-_002dI"></a>
<p>Specify a directory to use when searching for an import package at
compile time.
</p>
</dd>
<dt><code>-L<var>dir</var></code></dt>
<dd><a name="index-_002dL"></a>
<p>When linking, specify a library search directory, as with
<code>gcc</code>.
</p>
</dd>
<dt><code>-fgo-pkgpath=<var>string</var></code></dt>
<dd><a name="index-_002dfgo_002dpkgpath"></a>
<p>Set the package path to use.  This sets the value returned by the
PkgPath method of reflect.Type objects.  It is also used for the names
of globally visible symbols.  The argument to this option should
normally be the string that will be used to import this package after
it has been installed; in other words, a pathname within the
directories specified by the <samp>-I</samp> option.
</p>
</dd>
<dt><code>-fgo-prefix=<var>string</var></code></dt>
<dd><a name="index-_002dfgo_002dprefix"></a>
<p>An alternative to <samp>-fgo-pkgpath</samp>.  The argument will be
combined with the package name from the source file to produce the
package path.  If <samp>-fgo-pkgpath</samp> is used, <samp>-fgo-prefix</samp>
will be ignored.
</p>
<p>Go permits a single program to include more than one package with the
same name in the <code>package</code> clause in the source file, though
obviously the two packages must be imported using different pathnames.
In order for this to work with <code>gccgo</code>, either
<samp>-fgo-pkgpath</samp> or <samp>-fgo-prefix</samp> must be specified when
compiling a package.
</p>
<p>Using either <samp>-fgo-pkgpath</samp> or <samp>-fgo-prefix</samp> disables
the special treatment of the <code>main</code> package and permits that
package to be imported like any other.
</p>
</dd>
<dt><code>-fgo-relative-import-path=<var>dir</var></code></dt>
<dd><a name="index-_002dfgo_002drelative_002dimport_002dpath"></a>
<p>A relative import is an import that starts with <samp>./</samp> or
<samp>../</samp>.  If this option is used, <code>gccgo</code> will use
<var>dir</var> as a prefix for the relative import when searching for it.
</p>
</dd>
<dt><code>-frequire-return-statement</code></dt>
<dt><code>-fno-require-return-statement</code></dt>
<dd><a name="index-_002dfrequire_002dreturn_002dstatement"></a>
<a name="index-_002dfno_002drequire_002dreturn_002dstatement"></a>
<p>By default <code>gccgo</code> will warn about functions which have one or
more return parameters but lack an explicit <code>return</code> statement.
This warning may be disabled using
<samp>-fno-require-return-statement</samp>.
</p>
</dd>
<dt><code>-fgo-check-divide-zero</code></dt>
<dd><a name="index-_002dfgo_002dcheck_002ddivide_002dzero"></a>
<a name="index-_002dfno_002dgo_002dcheck_002ddivide_002dzero"></a>
<p>Add explicit checks for division by zero.  In Go a division (or
modulos) by zero causes a panic.  On Unix systems this is detected in
the runtime by catching the <code>SIGFPE</code> signal.  Some processors,
such as PowerPC, do not generate a SIGFPE on division by zero.  Some
runtimes do not generate a signal that can be caught.  On those
systems, this option may be used.  Or the checks may be removed via
<samp>-fno-go-check-divide-zero</samp>.  This option is currently on by
default, but in the future may be off by default on systems that do
not require it.
</p>
</dd>
<dt><code>-fgo-check-divide-overflow</code></dt>
<dd><a name="index-_002dfgo_002dcheck_002ddivide_002doverflow"></a>
<a name="index-_002dfno_002dgo_002dcheck_002ddivide_002doverflow"></a>
<p>Add explicit checks for division overflow.  For example, division
overflow occurs when computing <code>INT_MIN / -1</code>.  In Go this should
be wrapped, to produce <code>INT_MIN</code>.  Some processors, such as x86,
generate a trap on division overflow.  On those systems, this option
may be used.  Or the checks may be removed via
<samp>-fno-go-check-divide-overflow</samp>.  This option is currently on
by default, but in the future may be off by default on systems that do
not require it.
</p>
</dd>
<dt><code>-fno-go-optimize-allocs</code></dt>
<dd><a name="index-_002dfno_002dgo_002doptimize_002dallocs"></a>
<p>Disable escape analysis, which tries to allocate objects on the stack
rather than the heap.
</p>
</dd>
<dt><code>-fgo-debug-escape<var>n</var></code></dt>
<dd><a name="index-_002dfgo_002ddebug_002descape"></a>
<p>Output escape analysis debugging information.  Larger values of
<var>n</var> generate more information.
</p>
</dd>
<dt><code>-fgo-debug-escape-hash=<var>n</var></code></dt>
<dd><a name="index-_002dfgo_002ddebug_002descape_002dhash"></a>
<p>A hash value to debug escape analysis.  <var>n</var> is a binary string.
This runs escape analysis only on functions whose names hash to values
that match the given suffix <var>n</var>.  This can be used to binary
search across functions to uncover escape analysis bugs.
</p>
</dd>
<dt><code>-fgo-c-header=<var>file</var></code></dt>
<dd><a name="index-_002dfgo_002dc_002dheader"></a>
<p>Write top-level named Go struct definitions to <var>file</var> as C code.
This is used when compiling the runtime package.
</p>
</dd>
<dt><code>-fgo-compiling-runtime</code></dt>
<dd><a name="index-_002dfgo_002dcompiling_002druntime"></a>
<p>Apply special rules for compiling the runtime package.  Implicit
memory allocation is forbidden.  Some additional compiler directives
are supported.
</p>
</dd>
<dt><code>-g</code></dt>
<dd><a name="index-_002dg-for-gccgo"></a>
<p>This is the standard <code>gcc</code> option (see <a href="http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#Debugging-Options">Debugging Options</a> in <cite>Using the GNU Compiler Collection (GCC)</cite>).  It
is mentioned here because by default <code>gccgo</code> turns on
debugging information generation with the equivalent of the standard
option <samp>-g1</samp>.  This is because Go programs require debugging
information to be available in order to get backtrace information.  An
explicit <samp>-g0</samp> may be used to disable the generation of
debugging information, in which case certain standard library
functions, such as <code>runtime.Callers</code>, will not operate correctly.
</p></dd>
</dl>


<hr>
<a name="Import-and-Export"></a>
<div class="header">
<p>
Next: <a href="#Compiler-Directives" accesskey="n" rel="next">Compiler Directives</a>, Previous: <a href="#Invoking-gccgo" accesskey="p" rel="prev">Invoking gccgo</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Import-and-Export-1"></a>
<h2 class="chapter">2 Import and Export</h2>

<p>When <code>gccgo</code> compiles a package which exports anything, the
export information will be stored directly in the object file.  When a
package is imported, <code>gccgo</code> must be able to find the file.
</p>
<a name="index-_002egox"></a>
<p>When Go code imports the package <samp><var>gopackage</var></samp>, <code>gccgo</code>
will look for the import data using the following filenames, using the
first one that it finds.
</p>
<dl compact="compact">
<dt><samp><var>gopackage</var>.gox</samp></dt>
<dt><samp>lib<var>gopackage</var>.so</samp></dt>
<dt><samp>lib<var>gopackage</var>.a</samp></dt>
<dt><samp><var>gopackage</var>.o</samp></dt>
</dl>

<p>The compiler will search for these files in the directories named by
any <samp>-I</samp> options, in order in which the directories appear on
the command line.  The compiler will then search several standard
system directories.  Finally the compiler will search the current
directory (to search the current directory earlier, use &lsquo;<samp>-I.</samp>&rsquo;).
</p>
<p>The compiler will extract the export information directly from the
compiled object file.  The file <samp><var>gopackage</var>.gox</samp> will
typically contain nothing but export data.  This can be generated from
<samp><var>gopackage</var>.o</samp> via
</p>
<div class="smallexample">
<pre class="smallexample">objcopy -j .go_export <var>gopackage</var>.o <var>gopackage</var>.gox
</pre></div>

<p>For example, it may be desirable to extract the export information
from several different packages into their independent
<samp><var>gopackage</var>.gox</samp> files, and then to combine the different
package object files together into a single shared library or archive.
</p>
<p>At link time you must explicitly tell <code>gccgo</code> which files to
link together into the executable, as is usual with <code>gcc</code>.
This is different from the behavior of other Go compilers.
</p>
<hr>
<a name="Compiler-Directives"></a>
<div class="header">
<p>
Next: <a href="#C-Interoperability" accesskey="n" rel="next">C Interoperability</a>, Previous: <a href="#Import-and-Export" accesskey="p" rel="prev">Import and Export</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Compiler-Directives-1"></a>
<h2 class="chapter">3 Compiler Directives</h2>

<p>The Go compiler supports a few compiler directives.  A compiler
directive uses a <code>//</code> comment at the start of a line.  There must
be no space between the <code>//</code> and the name of the directive.
</p>
<dl compact="compact">
<dt><code>//line <var>file</var>:<var>line</var></code></dt>
<dd><p>The <code>//line</code> directive specifies that the source line that
follows should be recorded as having come from the given file path and
line number.  Successive lines are recorded using increasing line
numbers, until the next directive.  This directive typically appears
in machine-generated code, so that compilers and debuggers will show
lines in the original input to the generator.
</p>
</dd>
<dt><code>//extern <var>extern_name</var></code></dt>
<dd><p>The <code>extern</code> directive sets the externally visible name of the
next function declaration.  See <a href="#Function-Names">Function Names</a>.
</p>
</dd>
<dt><code>//go:compile <var>go_name</var> <var>extern_name</var></code></dt>
<dd><p>The <code>go:compile</code> directives sets the externally visible name of a
function definition or declaration.  See <a href="#Function-Names">Function Names</a>.
</p>
</dd>
<dt><code>//go:noescape</code></dt>
<dd><p>The <code>//go:noescape</code> directive specifies that the next declaration
in the file, which must be a func without a body (meaning that it has
an implementation not written in Go) does not allow any of the
pointers passed as arguments to escape into the heap or into the
values returned from the function. This information can be used during
the compiler&rsquo;s escape analysis of Go code calling the function.
</p>
</dd>
<dt><code>//go:nosplit</code></dt>
<dd><p>The <code>//go:nosplit</code> directive specifies that the next function
declared in the file must not include a stack overflow check. This is
most commonly used by low-level runtime sources invoked at times when
it is unsafe for the calling goroutine to be preempted.
</p>
</dd>
<dt><code>//go:noinline</code></dt>
<dd><p>The <code>//go:noinline</code> directive specifies that the next function
defined in the file may not be inlined.
</p>
</dd>
</dl>

<hr>
<a name="C-Interoperability"></a>
<div class="header">
<p>
Next: <a href="#Index" accesskey="n" rel="next">Index</a>, Previous: <a href="#Compiler-Directives" accesskey="p" rel="prev">Compiler Directives</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="C-Interoperability-1"></a>
<h2 class="chapter">4 C Interoperability</h2>

<p>When using <code>gccgo</code> there is limited interoperability with C,
or with C++ code compiled using <code>extern &quot;C&quot;</code>.
</p>
<p>This information is provided largely for documentation purposes.  For
ordinary use it is best to build programs with the go tool and then
use <code>import &quot;C&quot;</code>, as described at
<a href="http://golang.org/cmd/cgo">http://golang.org/cmd/cgo</a>.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="#C-Type-Interoperability" accesskey="1">C Type Interoperability</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How C and Go types match up.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Function-Names" accesskey="2">Function Names</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How Go functions are named.
</td></tr>
</table>

<hr>
<a name="C-Type-Interoperability"></a>
<div class="header">
<p>
Next: <a href="#Function-Names" accesskey="n" rel="next">Function Names</a>, Up: <a href="#C-Interoperability" accesskey="u" rel="up">C Interoperability</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="C-Type-Interoperability-1"></a>
<h3 class="section">4.1 C Type Interoperability</h3>

<p>Basic types map directly: an <code>int</code> in Go is an <code>int</code> in C,
etc.  Go <code>byte</code> is equivalent to C <code>unsigned char</code>.
Pointers in Go are pointers in C.  A Go <code>struct</code> is the same as C
<code>struct</code> with the same field names and types.
</p>
<a name="index-string-in-C"></a>
<p>The Go <code>string</code> type is currently defined as a two-element
structure:
</p>
<div class="smallexample">
<pre class="smallexample">struct __go_string {
  const unsigned char *__data;
  int __length;
};
</pre></div>

<p>You can&rsquo;t pass arrays between C and Go.  However, a pointer to an
array in Go is equivalent to a C pointer to the equivalent of the
element type.  For example, Go <code>*[10]int</code> is equivalent to C
<code>int*</code>, assuming that the C pointer does point to 10 elements.
</p>
<a name="index-slice-in-C"></a>
<p>A slice in Go is a structure.  The current definition is:
</p>
<div class="smallexample">
<pre class="smallexample">struct __go_slice {
  void *__values;
  int __count;
  int __capacity;
};
</pre></div>

<p>The type of a Go function with no receiver is equivalent to a C
function whose parameter types are equivalent.  When a Go function
returns more than one value, the C function returns a struct.  For
example, these functions have equivalent types:
</p>
<div class="smallexample">
<pre class="smallexample">func GoFunction(int) (int, float)
struct { int i; float f; } CFunction(int)
</pre></div>

<p>A pointer to a Go function is equivalent to a pointer to a C function
when the functions have equivalent types.
</p>
<p>Go <code>interface</code>, <code>channel</code>, and <code>map</code> types have no
corresponding C type (<code>interface</code> is a two-element struct and
<code>channel</code> and <code>map</code> are pointers to structs in C, but the
structs are deliberately undocumented).  C <code>enum</code> types
correspond to some integer type, but precisely which one is difficult
to predict in general; use a cast.  C <code>union</code> types have no
corresponding Go type.  C <code>struct</code> types containing bitfields
have no corresponding Go type.  C++ <code>class</code> types have no
corresponding Go type.
</p>
<p>Memory allocation is completely different between C and Go, as Go uses
garbage collection.  The exact guidelines in this area are
undetermined, but it is likely that it will be permitted to pass a
pointer to allocated memory from C to Go.  The responsibility of
eventually freeing the pointer will remain with C side, and of course
if the C side frees the pointer while the Go side still has a copy the
program will fail.  When passing a pointer from Go to C, the Go
function must retain a visible copy of it in some Go variable.
Otherwise the Go garbage collector may delete the pointer while the C
function is still using it.
</p>
<hr>
<a name="Function-Names"></a>
<div class="header">
<p>
Previous: <a href="#C-Type-Interoperability" accesskey="p" rel="prev">C Type Interoperability</a>, Up: <a href="#C-Interoperability" accesskey="u" rel="up">C Interoperability</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Function-Names-1"></a>
<h3 class="section">4.2 Function Names</h3>

<a name="index-extern"></a>
<a name="index-external-names"></a>
<p>Go code can call C functions directly using the <code>//extern</code> or
<code>//go:linkname</code> compiler directives.  An <code>//extern</code>
directive must be at the beginning of the line and must start with
<code>//extern</code>.  This must be followed by a space and then the
external name of the function.  The function declaration must be on
the line immediately after the comment.  For example, here is how the
C function <code>open</code> can be declared in Go:
</p>
<div class="smallexample">
<pre class="smallexample">//extern open
func c_open(name *byte, mode int, perm int) int
</pre></div>

<p>You can do the same thing using the <code>//go:linkname</code> compiler
directive.  The <code>//go:linkname</code> directive must be at the start of
the line.  It is followed by whitespace, the name of the Go function,
more whitespace, and the external name of the function.  Unlike
<code>//extern</code>, <code>//go:linkname</code> does not need to appear
immediately adjacent to the function definition or declaration.
</p>
<div class="smallexample">
<pre class="smallexample">//go:linkname c_open open
func c_open(name *byte, mode int, perm int) int
</pre></div>

<p>The C function naturally expects a nul terminated string, which in Go
is equivalent to a pointer to an array (not a slice!) of <code>byte</code>
with a terminating zero byte.  So a sample call from Go would look
like (after importing the <code>os</code> package):
</p>
<div class="smallexample">
<pre class="smallexample">var name = [4]byte{'f', 'o', 'o', 0};
i := c_open(&amp;amp;name[0], os.O_RDONLY, 0);
</pre></div>

<p>Note that this serves as an example only.  To open a file in Go please
use Go&rsquo;s <code>os.Open</code> function instead.
</p>
<p>The name of Go functions accessed from C is subject to change.  At
present the name of a Go function that does not have a receiver is
<code>pkgpath.Functionname</code>.  The <var>pkgpath</var> is set by the
<samp>-fgo-pkgpath</samp> option used when the package is compiled; if the
option is not used, the default is <code>go.<var>packagename</var></code>.  To
call the function from C you must set the name using the <code>gcc</code>
<code>__asm__</code> extension.
</p>
<div class="smallexample">
<pre class="smallexample">extern int go_function(int) __asm__ (&quot;mypkgpath.Function&quot;);
</pre></div>

<hr>
<a name="Index"></a>
<div class="header">
<p>
Previous: <a href="#C-Interoperability" accesskey="p" rel="prev">C Interoperability</a>, Up: <a href="#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="#Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Index-1"></a>
<h2 class="unnumbered">Index</h2>

<table><tr><th valign="top">Jump to: &nbsp; </th><td><a class="summary-letter" href="#Index_cp_symbol-1"><b>-</b></a>
 &nbsp; 
<a class="summary-letter" href="#Index_cp_symbol-2"><b>.</b></a>
 &nbsp; 
<br>
<a class="summary-letter" href="#Index_cp_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter" href="#Index_cp_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter" href="#Index_cp_letter-S"><b>S</b></a>
 &nbsp; 
</td></tr></table>
<table class="index-cp" border="0">
<tr><td></td><th align="left">Index Entry</th><td>&nbsp;</td><th align="left"> Section</th></tr>
<tr><td colspan="4"> <hr></td></tr>
<tr><th><a name="Index_cp_symbol-1">-</a></th><td></td><td></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfgo_002dc_002dheader"><samp>-fgo-c-header</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfgo_002dcheck_002ddivide_002doverflow"><samp>-fgo-check-divide-overflow</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfgo_002dcheck_002ddivide_002dzero"><samp>-fgo-check-divide-zero</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfgo_002dcompiling_002druntime"><samp>-fgo-compiling-runtime</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfgo_002ddebug_002descape"><samp>-fgo-debug-escape</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfgo_002ddebug_002descape_002dhash"><samp>-fgo-debug-escape-hash</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfgo_002dpkgpath"><samp>-fgo-pkgpath</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfgo_002dprefix"><samp>-fgo-prefix</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfgo_002drelative_002dimport_002dpath"><samp>-fgo-relative-import-path</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfno_002dgo_002dcheck_002ddivide_002doverflow"><samp>-fno-go-check-divide-overflow</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfno_002dgo_002dcheck_002ddivide_002dzero"><samp>-fno-go-check-divide-zero</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfno_002dgo_002doptimize_002dallocs"><samp>-fno-go-optimize-allocs</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfno_002drequire_002dreturn_002dstatement"><samp>-fno-require-return-statement</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dfrequire_002dreturn_002dstatement"><samp>-frequire-return-statement</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dg-for-gccgo"><samp>-g for gccgo</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dI"><samp>-I</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002dL"><samp>-L</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Invoking-gccgo">Invoking gccgo</a></td></tr>
<tr><td colspan="4"> <hr></td></tr>
<tr><th><a name="Index_cp_symbol-2">.</a></th><td></td><td></td></tr>
<tr><td></td><td valign="top"><a href="#index-_002egox"><samp>.gox</samp></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Import-and-Export">Import and Export</a></td></tr>
<tr><td colspan="4"> <hr></td></tr>
<tr><th><a name="Index_cp_letter-E">E</a></th><td></td><td></td></tr>
<tr><td></td><td valign="top"><a href="#index-extern"><code>extern</code></a>:</td><td>&nbsp;</td><td valign="top"><a href="#Function-Names">Function Names</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-external-names">external names</a>:</td><td>&nbsp;</td><td valign="top"><a href="#Function-Names">Function Names</a></td></tr>
<tr><td colspan="4"> <hr></td></tr>
<tr><th><a name="Index_cp_letter-F">F</a></th><td></td><td></td></tr>
<tr><td></td><td valign="top"><a href="#index-FDL_002c-GNU-Free-Documentation-License">FDL, GNU Free Documentation License</a>:</td><td>&nbsp;</td><td valign="top"><a href="#GNU-Free-Documentation-License">GNU Free Documentation License</a></td></tr>
<tr><td colspan="4"> <hr></td></tr>
<tr><th><a name="Index_cp_letter-S">S</a></th><td></td><td></td></tr>
<tr><td></td><td valign="top"><a href="#index-slice-in-C"><code>slice</code> in C</a>:</td><td>&nbsp;</td><td valign="top"><a href="#C-Type-Interoperability">C Type Interoperability</a></td></tr>
<tr><td></td><td valign="top"><a href="#index-string-in-C"><code>string</code> in C</a>:</td><td>&nbsp;</td><td valign="top"><a href="#C-Type-Interoperability">C Type Interoperability</a></td></tr>
<tr><td colspan="4"> <hr></td></tr>
</table>
<table><tr><th valign="top">Jump to: &nbsp; </th><td><a class="summary-letter" href="#Index_cp_symbol-1"><b>-</b></a>
 &nbsp; 
<a class="summary-letter" href="#Index_cp_symbol-2"><b>.</b></a>
 &nbsp; 
<br>
<a class="summary-letter" href="#Index_cp_letter-E"><b>E</b></a>
 &nbsp; 
<a class="summary-letter" href="#Index_cp_letter-F"><b>F</b></a>
 &nbsp; 
<a class="summary-letter" href="#Index_cp_letter-S"><b>S</b></a>
 &nbsp; 
</td></tr></table>

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