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This is gccgo.info, produced by makeinfo version 6.5 from gccgo.texi.

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   A GNU Manual

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INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Gccgo: (gccgo).           A GCC-based compiler for the Go language
END-INFO-DIR-ENTRY


   Copyright (C) 2010-2022 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
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with the Back-Cover Texts being (b) (see below).  A copy of the license
is included in the section entitled "GNU Free Documentation License".

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
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File: gccgo.info,  Node: Top,  Next: Copying,  Up: (dir)

Introduction
************

This manual describes how to use 'gccgo', the GNU compiler for the Go
programming language.  This manual is specifically about 'gccgo'.  For
more information about the Go programming language in general, including
language specifications and standard package documentation, see
<https://golang.org/>.

* Menu:

* Copying::                     The GNU General Public License.
* GNU Free Documentation License::
                                How you can share and copy this manual.
* Invoking gccgo::              How to run gccgo.
* Import and Export::           Importing and exporting package data.
* Compiler Directives::         Comments to control compilation.
* C Interoperability::          Calling C from Go and vice-versa.
* Index::                       Index.


File: gccgo.info,  Node: Copying,  Next: GNU Free Documentation License,  Prev: Top,  Up: Top

GNU General Public License
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                        Version 3, 29 June 2007

     Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>

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Preamble
========

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     proxy's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Program.

     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.

  15. Disclaimer of Warranty.

     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 "AS IS"
     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.

  16. Limitation of Liability.

     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.

  17. Interpretation of Sections 15 and 16.

     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.

END OF TERMS AND CONDITIONS
===========================

How to Apply These Terms to Your New Programs
=============================================

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.

   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
"copyright" line and a pointer to where the full notice is found.

     ONE LINE TO GIVE THE PROGRAM'S NAME AND A BRIEF IDEA OF WHAT IT DOES.
     Copyright (C) YEAR NAME OF AUTHOR

     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 <https://www.gnu.org/licenses/>.

   Also add information on how to contact you by electronic and paper
mail.

   If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:

     PROGRAM Copyright (C) YEAR NAME OF AUTHOR
     This program comes with ABSOLUTELY NO WARRANTY; for details type 'show w'.
     This is free software, and you are welcome to redistribute it
     under certain conditions; type 'show c' for details.

   The hypothetical commands 'show w' and 'show c' should show the
appropriate parts of the General Public License.  Of course, your
program's commands might be different; for a GUI interface, you would
use an "about box".

   You should also get your employer (if you work as a programmer) or
school, if any, to sign a "copyright disclaimer" for the program, if
necessary.  For more information on this, and how to apply and follow
the GNU GPL, see <https://www.gnu.org/licenses/>.

   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 <https://www.gnu.org/licenses/why-not-lgpl.html>.


File: gccgo.info,  Node: GNU Free Documentation License,  Next: Invoking gccgo,  Prev: Copying,  Up: Top

GNU Free Documentation License
******************************

                     Version 1.3, 3 November 2008

     Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     <https://fsf.org/>

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

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" 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.

     This License is a kind of "copyleft", 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.

     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.

  1. APPLICABILITY AND DEFINITIONS

     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
     "Document", below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as "you".  You accept
     the license if you copy, modify or distribute the work in a way
     requiring permission under copyright law.

     A "Modified Version" 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.

     A "Secondary Section" 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'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.

     The "Invariant Sections" 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.

     The "Cover Texts" 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.

     A "Transparent" 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
     "Transparent" is called "Opaque".

     Examples of suitable formats for Transparent copies include plain
     ASCII without markup, Texinfo input format, LaTeX input format,
     SGML or XML using a publicly available DTD, and standard-conforming
     simple HTML, PostScript or PDF designed for human modification.
     Examples of transparent image formats include PNG, XCF and JPG.
     Opaque formats include proprietary formats that can be read and
     edited only by proprietary word processors, SGML or XML for which
     the DTD and/or processing tools are not generally available, and
     the machine-generated HTML, PostScript or PDF produced by some word
     processors for output purposes only.

     The "Title Page" 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, "Title
     Page" means the text near the most prominent appearance of the
     work's title, preceding the beginning of the body of the text.

     The "publisher" means any person or entity that distributes copies
     of the Document to the public.

     A section "Entitled XYZ" 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
     "Acknowledgements", "Dedications", "Endorsements", or "History".)
     To "Preserve the Title" of such a section when you modify the
     Document means that it remains a section "Entitled XYZ" according
     to this definition.

     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.

  2. VERBATIM COPYING

     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.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document'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.

     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.

     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.

     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.

  4. MODIFICATIONS

     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:

       A. 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.

       B. 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.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. 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.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document's
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", 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 "History" 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.

       J. 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
          "History" 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.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          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.

       L. 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.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     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's
     license notice.  These titles must be distinct from any other
     section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     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.

     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.

  5. COMBINING DOCUMENTS

     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.

     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.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     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.

     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.

  7. AGGREGATION WITH INDEPENDENT WORKS

     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 "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation'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.

     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'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.

  8. TRANSLATION

     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.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     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.

     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.

     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.

     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.

  10. FUTURE REVISIONS OF THIS LICENSE

     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
     <https://www.gnu.org/copyleft/>.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" 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's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Document.

  11. RELICENSING

     "Massive Multiauthor Collaboration Site" (or "MMC Site") 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 "Massive Multiauthor Collaboration" (or "MMC") contained in the
     site means any set of copyrightable works thus published on the MMC
     site.

     "CC-BY-SA" 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.

     "Incorporate" means to publish or republish a Document, in whole or
     in part, as part of another Document.

     An MMC is "eligible for relicensing" 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.

     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.

ADDENDUM: How to use this License for your documents
====================================================

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:

       Copyright (C)  YEAR  YOUR NAME.
       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''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts."  line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   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.


File: gccgo.info,  Node: Invoking gccgo,  Next: Import and Export,  Prev: GNU Free Documentation License,  Up: Top

1 Invoking gccgo
****************

The 'gccgo' command is a frontend to 'gcc' and supports many of the same
options.  *Note Option Summary: (gcc)Option Summary.  This manual only
documents the options specific to 'gccgo'.

   The 'gccgo' 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.

   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 'gccgo'.  A
single invocation of 'gccgo' may only compile a single package.

   One Go package may 'import' a different Go package.  The imported
package must have already been compiled; 'gccgo' 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.

   Go programs must generally be compiled with debugging information,
and '-g1' is the default as described below.  Stripping a Go program
will generally cause it to misbehave or fail.

'-IDIR'
     Specify a directory to use when searching for an import package at
     compile time.

'-LDIR'
     When linking, specify a library search directory, as with 'gcc'.

'-fgo-pkgpath=STRING'
     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 '-I' option.

'-fgo-prefix=STRING'
     An alternative to '-fgo-pkgpath'.  The argument will be combined
     with the package name from the source file to produce the package
     path.  If '-fgo-pkgpath' is used, '-fgo-prefix' will be ignored.

     Go permits a single program to include more than one package with
     the same name in the 'package' clause in the source file, though
     obviously the two packages must be imported using different
     pathnames.  In order for this to work with 'gccgo', either
     '-fgo-pkgpath' or '-fgo-prefix' must be specified when compiling a
     package.

     Using either '-fgo-pkgpath' or '-fgo-prefix' disables the special
     treatment of the 'main' package and permits that package to be
     imported like any other.

'-fgo-relative-import-path=DIR'
     A relative import is an import that starts with './' or '../'.  If
     this option is used, 'gccgo' will use DIR as a prefix for the
     relative import when searching for it.

'-frequire-return-statement'
'-fno-require-return-statement'
     By default 'gccgo' will warn about functions which have one or more
     return parameters but lack an explicit 'return' statement.  This
     warning may be disabled using '-fno-require-return-statement'.

'-fgo-check-divide-zero'
     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 'SIGFPE' 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 '-fno-go-check-divide-zero'.  This option is currently
     on by default, but in the future may be off by default on systems
     that do not require it.

'-fgo-check-divide-overflow'
     Add explicit checks for division overflow.  For example, division
     overflow occurs when computing 'INT_MIN / -1'.  In Go this should
     be wrapped, to produce 'INT_MIN'.  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
     '-fno-go-check-divide-overflow'.  This option is currently on by
     default, but in the future may be off by default on systems that do
     not require it.

'-fno-go-optimize-allocs'
     Disable escape analysis, which tries to allocate objects on the
     stack rather than the heap.

'-fgo-debug-escapeN'
     Output escape analysis debugging information.  Larger values of N
     generate more information.

'-fgo-debug-escape-hash=N'
     A hash value to debug escape analysis.  N is a binary string.  This
     runs escape analysis only on functions whose names hash to values
     that match the given suffix N.  This can be used to binary search
     across functions to uncover escape analysis bugs.

'-fgo-debug-optimization'
     Output optimization diagnostics.

'-fgo-c-header=FILE'
     Write top-level named Go struct definitions to FILE as C code.
     This is used when compiling the runtime package.

'-fgo-compiling-runtime'
     Apply special rules for compiling the runtime package.  Implicit
     memory allocation is forbidden.  Some additional compiler
     directives are supported.

'-fgo-embedcfg=FILE'
     Identify a JSON file used to map patterns used with special
     '//go:embed' comments to the files named by the patterns.  The JSON
     file should have two components: 'Patterns' maps each pattern to a
     list of file names, and 'Files' maps each file name to a full path
     to the file.  This option is intended for use by the 'go' command
     to implement '//go:embed'.

'-g'
     This is the standard 'gcc' option (*note Debugging Options:
     (gcc)Debugging Options.).  It is mentioned here because by default
     'gccgo' turns on debugging information generation with the
     equivalent of the standard option '-g1'.  This is because Go
     programs require debugging information to be available in order to
     get backtrace information.  An explicit '-g0' may be used to
     disable the generation of debugging information, in which case
     certain standard library functions, such as 'runtime.Callers', will
     not operate correctly.


File: gccgo.info,  Node: Import and Export,  Next: Compiler Directives,  Prev: Invoking gccgo,  Up: Top

2 Import and Export
*******************

When 'gccgo' compiles a package which exports anything, the export
information will be stored directly in the object file.  When a package
is imported, 'gccgo' must be able to find the file.

   When Go code imports the package 'GOPACKAGE', 'gccgo' will look for
the import data using the following filenames, using the first one that
it finds.

'GOPACKAGE.gox'
'libGOPACKAGE.so'
'libGOPACKAGE.a'
'GOPACKAGE.o'

   The compiler will search for these files in the directories named by
any '-I' 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 '-I.').

   The compiler will extract the export information directly from the
compiled object file.  The file 'GOPACKAGE.gox' will typically contain
nothing but export data.  This can be generated from 'GOPACKAGE.o' via

     objcopy -j .go_export GOPACKAGE.o GOPACKAGE.gox

   For example, it may be desirable to extract the export information
from several different packages into their independent 'GOPACKAGE.gox'
files, and then to combine the different package object files together
into a single shared library or archive.

   At link time you must explicitly tell 'gccgo' which files to link
together into the executable, as is usual with 'gcc'.  This is different
from the behavior of other Go compilers.


File: gccgo.info,  Node: Compiler Directives,  Next: C Interoperability,  Prev: Import and Export,  Up: Top

3 Compiler Directives
*********************

The Go compiler supports a few compiler directives.  A compiler
directive uses a '//' comment at the start of a line.  There must be no
space between the '//' and the name of the directive.

'//line FILE:LINE'
     The '//line' 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.

'//extern EXTERN_NAME'
     The 'extern' directive sets the externally visible name of the next
     function declaration.  See *note Function Names::.

'//go:compile GO_NAME EXTERN_NAME'
     The 'go:compile' directives sets the externally visible name of a
     function definition or declaration.  See *note Function Names::.

'//go:noescape'
     The '//go:noescape' 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's escape analysis of Go code calling the
     function.

'//go:nosplit'
     The '//go:nosplit' 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.

'//go:noinline'
     The '//go:noinline' directive specifies that the next function
     defined in the file may not be inlined.


File: gccgo.info,  Node: C Interoperability,  Next: Index,  Prev: Compiler Directives,  Up: Top

4 C Interoperability
********************

When using 'gccgo' there is limited interoperability with C, or with C++
code compiled using 'extern "C"'.

   This information is provided largely for documentation purposes.  For
ordinary use it is best to build programs with the go tool and then use
'import "C"', as described at <https://golang.org/cmd/cgo>.

* Menu:

* C Type Interoperability::     How C and Go types match up.
* Function Names::              How Go functions are named.


File: gccgo.info,  Node: C Type Interoperability,  Next: Function Names,  Up: C Interoperability

4.1 C Type Interoperability
===========================

Basic types map directly: an 'int' in Go is an 'int' in C, etc.  Go
'byte' is equivalent to C 'unsigned char'.  Pointers in Go are pointers
in C. A Go 'struct' is the same as C 'struct' with the same field names
and types.

   The Go 'string' type is currently defined as a two-element structure:

     struct __go_string {
       const unsigned char *__data;
       int __length;
     };

   You can'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 '*[10]int' is equivalent to C 'int*',
assuming that the C pointer does point to 10 elements.

   A slice in Go is a structure.  The current definition is:

     struct __go_slice {
       void *__values;
       int __count;
       int __capacity;
     };

   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:

     func GoFunction(int) (int, float)
     struct { int i; float f; } CFunction(int)

   A pointer to a Go function is equivalent to a pointer to a C function
when the functions have equivalent types.

   Go 'interface', 'channel', and 'map' types have no corresponding C
type ('interface' is a two-element struct and 'channel' and 'map' are
pointers to structs in C, but the structs are deliberately
undocumented).  C 'enum' types correspond to some integer type, but
precisely which one is difficult to predict in general; use a cast.  C
'union' types have no corresponding Go type.  C 'struct' types
containing bitfields have no corresponding Go type.  C++ 'class' types
have no corresponding Go type.

   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.


File: gccgo.info,  Node: Function Names,  Prev: C Type Interoperability,  Up: C Interoperability

4.2 Function Names
==================

Go code can call C functions directly using the '//extern' or
'//go:linkname' compiler directives.  An '//extern' directive must be at
the beginning of the line and must start with '//extern'.  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 'open' can be declared in Go:

     //extern open
     func c_open(name *byte, mode int, perm int) int

   You can do the same thing using the '//go:linkname' compiler
directive.  The '//go:linkname' 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 '//extern',
'//go:linkname' does not need to appear immediately adjacent to the
function definition or declaration.

     //go:linkname c_open open
     func c_open(name *byte, mode int, perm int) int

   The C function naturally expects a nul terminated string, which in Go
is equivalent to a pointer to an array (not a slice!)  of 'byte' with a
terminating zero byte.  So a sample call from Go would look like (after
importing the 'os' package):

     var name = [4]byte{'f', 'o', 'o', 0};
     i := c_open(&name[0], os.O_RDONLY, 0);

   Note that this serves as an example only.  To open a file in Go
please use Go's 'os.Open' function instead.

   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
'pkgpath.Functionname'.  The PKGPATH is set by the '-fgo-pkgpath' option
used when the package is compiled; if the option is not used, the
default is 'go.PACKAGENAME'.  To call the function from C you must set
the name using the 'gcc' '__asm__' extension.

     extern int go_function(int) __asm__ ("mypkgpath.Function");


File: gccgo.info,  Node: Index,  Prev: C Interoperability,  Up: Top

Index
*****

[index]
* Menu:

* -fgo-c-header:                         Invoking gccgo.      (line 110)
* -fgo-check-divide-overflow:            Invoking gccgo.      (line  83)
* -fgo-check-divide-zero:                Invoking gccgo.      (line  72)
* -fgo-compiling-runtime:                Invoking gccgo.      (line 114)
* -fgo-debug-escape:                     Invoking gccgo.      (line  97)
* -fgo-debug-escape-hash:                Invoking gccgo.      (line 101)
* -fgo-debug-optimization:               Invoking gccgo.      (line 107)
* -fgo-embedcfg:                         Invoking gccgo.      (line 119)
* -fgo-pkgpath:                          Invoking gccgo.      (line  37)
* -fgo-prefix:                           Invoking gccgo.      (line  45)
* -fgo-relative-import-path:             Invoking gccgo.      (line  61)
* -fno-go-check-divide-overflow:         Invoking gccgo.      (line  83)
* -fno-go-check-divide-zero:             Invoking gccgo.      (line  72)
* -fno-go-debug-optimization:            Invoking gccgo.      (line 107)
* -fno-go-optimize-allocs:               Invoking gccgo.      (line  93)
* -fno-require-return-statement:         Invoking gccgo.      (line  67)
* -frequire-return-statement:            Invoking gccgo.      (line  67)
* -g for gccgo:                          Invoking gccgo.      (line 127)
* -I:                                    Invoking gccgo.      (line  30)
* -L:                                    Invoking gccgo.      (line  34)
* .gox:                                  Import and Export.   (line  10)
* extern:                                Function Names.      (line   6)
* external names:                        Function Names.      (line   6)
* FDL, GNU Free Documentation License:   GNU Free Documentation License.
                                                              (line   6)
* slice in C:                            C Type Interoperability.
                                                              (line  23)
* string in C:                           C Type Interoperability.
                                                              (line  11)



Tag Table:
Node: Top1688
Node: Copying2548
Node: GNU Free Documentation License40084
Node: Invoking gccgo65214
Node: Import and Export71394
Node: Compiler Directives72990
Node: C Interoperability74974
Node: C Type Interoperability75561
Node: Function Names78120
Node: Index80124

End Tag Table