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

This manual describes how to use `gcj', the GNU compiler for the Java
programming language.  `gcj' can generate both `.class' files and
object files, and it can read both Java source code and `.class' files.

* Menu:

* Copying::             The GNU General Public License
* GNU Free Documentation License::
                        How you can share and copy this manual
* Invoking gcj::        Compiler options supported by `gcj'
* Compatibility::       Compatibility between gcj and other tools for Java
* Invoking jcf-dump::   Print information about class files
* Invoking gij::        Interpreting Java bytecodes
* Invoking gcj-dbtool:: Tool for manipulating class file databases.
* Invoking jv-convert:: Converting from one encoding to another
* Invoking grmic::      Generate stubs for Remote Method Invocation.
* Invoking gc-analyze:: Analyze Garbage Collector (GC) memory dumps.
* Invoking aot-compile:: Compile bytecode to native and generate databases.
* Invoking rebuild-gcj-db:: Merge the per-solib databases made by aot-compile
                            into one system-wide database.
* About CNI::           Description of the Compiled Native Interface
* System properties::   Modifying runtime behavior of the libgcj library
* Resources::           Where to look for more information
* Index::               Index.

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

GNU General Public License
**************************

                        Version 3, 29 June 2007

     Copyright (C) 2007 Free Software Foundation, Inc. `http://fsf.org/'

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

Preamble
========

The GNU General Public License is a free, copyleft license for software
and other kinds of works.

   The licenses for most software and other practical works are designed
to take away your freedom to share and change the works.  By contrast,
the GNU General Public License is intended to guarantee your freedom to
share and change all versions of a program-to make sure it remains free
software for all its users.  We, the Free Software Foundation, use the
GNU General Public License for most of our software; it applies also to
any other work released this way by its authors.  You can apply it to
your programs, too.

   When we speak of free software, we are referring to freedom, not
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   To protect your rights, we need to prevent others from denying you
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have certain responsibilities if you distribute copies of the software,
or if you modify it: responsibilities to respect the freedom of others.

   For example, if you distribute copies of such a program, whether
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   Developers that use the GNU GPL protect your rights with two steps:
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   Some devices are designed to deny users access to install or run
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   The precise terms and conditions for copying, distribution and
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TERMS AND CONDITIONS
====================

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  7. Additional Terms.

     "Additional permissions" are terms that supplement the terms of
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     entire Program remains governed by this License without regard to
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  8. Termination.

     You may not propagate or modify a covered work except as expressly
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     However, if you cease all violation of this License, then your
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  9. Acceptance Not Required for Having Copies.

     You are not required to accept this License in order to receive or
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 10. Automatic Licensing of Downstream Recipients.

     Each time you convey a covered work, the recipient automatically
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     An "entity transaction" is a transaction transferring control of an
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     You may not impose any further restrictions on the exercise of the
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 11. Patents.

     A "contributor" 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's "contributor
     version".

     A contributor's "essential patent claims" are all patent claims
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     contributor version, but do not include claims that would be
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     contributor version.  For purposes of this definition, "control"
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     If you convey a covered work, knowingly relying on a patent
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     A patent license is "discriminatory" if it does not include within
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     was granted, prior to 28 March 2007.

     Nothing in this License shall be construed as excluding or limiting
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 12. No Surrender of Others' Freedom.

     If conditions are imposed on you (whether by court order,
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     License, they do not excuse you from the conditions of this
     License.  If you cannot convey a covered work so as to satisfy
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     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.

 13. Use with the GNU Affero General Public License.

     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.

 14. Revised Versions of this License.

     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
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     Each version is given a distinguishing version number.  If the
     Program specifies that a certain numbered version of the GNU
     General Public License "or any later version" 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
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     any version ever published by the Free Software Foundation.

     If the Program specifies that a proxy can decide which future
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     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
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     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 `http://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 `http://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 `http://www.gnu.org/philosophy/why-not-lgpl.html'.

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

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

                      Version 1.2, November 2002

     Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
     51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA

     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.

     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 for under this License.  Any other
     attempt to copy, modify, sublicense or distribute the Document is
     void, and will automatically terminate your rights under this
     License.  However, parties who have received copies, or rights,
     from you under this License will not have their licenses
     terminated so long as such parties remain in full compliance.

 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
     `http://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.

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 1.2
       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: gcj.info,  Node: Invoking gcj,  Next: Compatibility,  Prev: GNU Free Documentation License,  Up: Top

1 Invoking gcj
**************

As `gcj' is just another front end to `gcc', it supports many of the
same options as gcc.  *Note Option Summary: (gcc)Option Summary.  This
manual only documents the options specific to `gcj'.

* Menu:

* Input and output files::
* Input Options::               How gcj finds files
* Encodings::                   Options controlling source file encoding
* Warnings::                    Options controlling warnings specific to gcj
* Linking::                     Options for making an executable
* Code Generation::             Options controlling the output of gcj
* Configure-time Options::      Options you won't use

File: gcj.info,  Node: Input and output files,  Next: Input Options,  Up: Invoking gcj

1.1 Input and output files
==========================

A `gcj' command is like a `gcc' command, in that it consists of a
number of options and file names.  The following kinds of input file
names are supported:

`FILE.java'
     Java source files.

`FILE.class'
     Java bytecode files.

`FILE.zip'
`FILE.jar'
     An archive containing one or more `.class' files, all of which are
     compiled.  The archive may be compressed.  Files in an archive
     which don't end with `.class' are treated as resource files; they
     are compiled into the resulting object file as `core:' URLs.

`@FILE'
     A file containing a whitespace-separated list of input file names.
     (Currently, these must all be `.java' source files, but that may
     change.)  Each named file is compiled, just as if it had been on
     the command line.

`LIBRARY.a'
`LIBRARY.so'
`-lLIBNAME'
     Libraries to use when linking.  See the `gcc' manual.

   You can specify more than one input file on the `gcj' command line,
in which case they will all be compiled.  If you specify a `-o FILENAME'
option, all the input files will be compiled together, producing a
single output file, named FILENAME.  This is allowed even when using
`-S' or `-c', but not when using `-C' or `--resource'.  (This is an
extension beyond the what plain `gcc' allows.)  (If more than one input
file is specified, all must currently be `.java' files, though we hope
to fix this.)

File: gcj.info,  Node: Input Options,  Next: Encodings,  Prev: Input and output files,  Up: Invoking gcj

1.2 Input Options
=================

`gcj' has options to control where it looks to find files it needs.
For instance, `gcj' might need to load a class that is referenced by
the file it has been asked to compile.  Like other compilers for the
Java language, `gcj' has a notion of a "class path".  There are several
options and environment variables which can be used to manipulate the
class path.  When `gcj' looks for a given class, it searches the class
path looking for matching `.class' or `.java' file.  `gcj' comes with a
built-in class path which points at the installed `libgcj.jar', a file
which contains all the standard classes.

   In the text below, a directory or path component can refer either to
an actual directory on the filesystem, or to a `.zip' or `.jar' file,
which `gcj' will search as if it is a directory.

`-IDIR'
     All directories specified by `-I' are kept in order and prepended
     to the class path constructed from all the other options.  Unless
     compatibility with tools like `javac' is important, we recommend
     always using `-I' instead of the other options for manipulating the
     class path.

`--classpath=PATH'
     This sets the class path to PATH, a colon-separated list of paths
     (on Windows-based systems, a semicolon-separate list of paths).
     This does not override the builtin ("boot") search path.

`--CLASSPATH=PATH'
     Deprecated synonym for `--classpath'.

`--bootclasspath=PATH'
     Where to find the standard builtin classes, such as
     `java.lang.String'.

`--extdirs=PATH'
     For each directory in the PATH, place the contents of that
     directory at the end of the class path.

`CLASSPATH'
     This is an environment variable which holds a list of paths.

   The final class path is constructed like so:

   * First come all directories specified via `-I'.

   * If `--classpath' is specified, its value is appended.  Otherwise,
     if the `CLASSPATH' environment variable is specified, then its
     value is appended.  Otherwise, the current directory (`"."') is
     appended.

   * If `--bootclasspath' was specified, append its value.  Otherwise,
     append the built-in system directory, `libgcj.jar'.

   * Finally, if `--extdirs' was specified, append the contents of the
     specified directories at the end of the class path.  Otherwise,
     append the contents of the built-in extdirs at
     `$(prefix)/share/java/ext'.

   The classfile built by `gcj' for the class `java.lang.Object' (and
placed in `libgcj.jar') contains a special zero length attribute
`gnu.gcj.gcj-compiled'. The compiler looks for this attribute when
loading `java.lang.Object' and will report an error if it isn't found,
unless it compiles to bytecode (the option
`-fforce-classes-archive-check' can be used to override this behavior
in this particular case.)

`-fforce-classes-archive-check'
     This forces the compiler to always check for the special zero
     length attribute `gnu.gcj.gcj-compiled' in `java.lang.Object' and
     issue an error if it isn't found.

`-fsource=VERSION'
     This option is used to choose the source version accepted by
     `gcj'.  The default is `1.5'.

File: gcj.info,  Node: Encodings,  Next: Warnings,  Prev: Input Options,  Up: Invoking gcj

1.3 Encodings
=============

The Java programming language uses Unicode throughout.  In an effort to
integrate well with other locales, `gcj' allows `.java' files to be
written using almost any encoding.  `gcj' knows how to convert these
encodings into its internal encoding at compile time.

   You can use the `--encoding=NAME' option to specify an encoding (of
a particular character set) to use for source files.  If this is not
specified, the default encoding comes from your current locale.  If
your host system has insufficient locale support, then `gcj' assumes
the default encoding to be the `UTF-8' encoding of Unicode.

   To implement `--encoding', `gcj' simply uses the host platform's
`iconv' conversion routine.  This means that in practice `gcj' is
limited by the capabilities of the host platform.

   The names allowed for the argument `--encoding' vary from platform
to platform (since they are not standardized anywhere).  However, `gcj'
implements the encoding named `UTF-8' internally, so if you choose to
use this for your source files you can be assured that it will work on
every host.

File: gcj.info,  Node: Warnings,  Next: Linking,  Prev: Encodings,  Up: Invoking gcj

1.4 Warnings
============

`gcj' implements several warnings.  As with other generic `gcc'
warnings, if an option of the form `-Wfoo' enables a warning, then
`-Wno-foo' will disable it.  Here we've chosen to document the form of
the warning which will have an effect - the default being the opposite
of what is listed.

`-Wredundant-modifiers'
     With this flag, `gcj' will warn about redundant modifiers.  For
     instance, it will warn if an interface method is declared `public'.

`-Wextraneous-semicolon'
     This causes `gcj' to warn about empty statements.  Empty statements
     have been deprecated.

`-Wno-out-of-date'
     This option will cause `gcj' not to warn when a source file is
     newer than its matching class file.  By default `gcj' will warn
     about this.

`-Wno-deprecated'
     Warn if a deprecated class, method, or field is referred to.

`-Wunused'
     This is the same as `gcc''s `-Wunused'.

`-Wall'
     This is the same as `-Wredundant-modifiers -Wextraneous-semicolon
     -Wunused'.

File: gcj.info,  Node: Linking,  Next: Code Generation,  Prev: Warnings,  Up: Invoking gcj

1.5 Linking
===========

To turn a Java application into an executable program, you need to link
it with the needed libraries, just as for C or C++.  The linker by
default looks for a global function named `main'.  Since Java does not
have global functions, and a collection of Java classes may have more
than one class with a `main' method, you need to let the linker know
which of those `main' methods it should invoke when starting the
application.  You can do that in any of these ways:

   * Specify the class containing the desired `main' method when you
     link the application, using the `--main' flag, described below.

   * Link the Java package(s) into a shared library (dll) rather than an
     executable.  Then invoke the application using the `gij' program,
     making sure that `gij' can find the libraries it needs.

   * Link the Java packages(s) with the flag `-lgij', which links in
     the `main' routine from the `gij' command.  This allows you to
     select the class whose `main' method you want to run when you run
     the application.  You can also use other `gij' flags, such as `-D'
     flags to set properties.  Using the `-lgij' library (rather than
     the `gij' program of the previous mechanism) has some advantages:
     it is compatible with static linking, and does not require
     configuring or installing libraries.

   These `gij' options relate to linking an executable:

`--main=CLASSNAME'
     This option is used when linking to specify the name of the class
     whose `main' method should be invoked when the resulting
     executable is run.

`-DNAME[=VALUE]'
     This option can only be used with `--main'.  It defines a system
     property named NAME with value VALUE.  If VALUE is not specified
     then it defaults to the empty string.  These system properties are
     initialized at the program's startup and can be retrieved at
     runtime using the `java.lang.System.getProperty' method.

`-lgij'
     Create an application whose command-line processing is that of the
     `gij' command.

     This option is an alternative to using `--main'; you cannot use
     both.

`-static-libgcj'
     This option causes linking to be done against a static version of
     the libgcj runtime library.  This option is only available if
     corresponding linker support exists.

     *Caution:* Static linking of libgcj may cause essential parts of
     libgcj to be omitted.  Some parts of libgcj use reflection to load
     classes at runtime.  Since the linker does not see these
     references at link time, it can omit the referred to classes.  The
     result is usually (but not always) a `ClassNotFoundException'
     being thrown at runtime. Caution must be used when using this
     option.  For more details see:
     `http://gcc.gnu.org/wiki/Statically%20linking%20libgcj'

File: gcj.info,  Node: Code Generation,  Next: Configure-time Options,  Prev: Linking,  Up: Invoking gcj

1.6 Code Generation
===================

In addition to the many `gcc' options controlling code generation,
`gcj' has several options specific to itself.

`-C'
     This option is used to tell `gcj' to generate bytecode (`.class'
     files) rather than object code.

`--resource RESOURCE-NAME'
     This option is used to tell `gcj' to compile the contents of a
     given file to object code so it may be accessed at runtime with
     the core protocol handler as `core:/RESOURCE-NAME'.  Note that
     RESOURCE-NAME is the name of the resource as found at runtime; for
     instance, it could be used in a call to `ResourceBundle.getBundle'.
     The actual file name to be compiled this way must be specified
     separately.

`-ftarget=VERSION'
     This can be used with `-C' to choose the version of bytecode
     emitted by `gcj'.  The default is `1.5'.  When not generating
     bytecode, this option has no effect.

`-d DIRECTORY'
     When used with `-C', this causes all generated `.class' files to
     be put in the appropriate subdirectory of DIRECTORY.  By default
     they will be put in subdirectories of the current working
     directory.

`-fno-bounds-check'
     By default, `gcj' generates code which checks the bounds of all
     array indexing operations.  With this option, these checks are
     omitted, which can improve performance for code that uses arrays
     extensively.  Note that this can result in unpredictable behavior
     if the code in question actually does violate array bounds
     constraints.  It is safe to use this option if you are sure that
     your code will never throw an `ArrayIndexOutOfBoundsException'.

`-fno-store-check'
     Don't generate array store checks.  When storing objects into
     arrays, a runtime check is normally generated in order to ensure
     that the object is assignment compatible with the component type
     of the array (which may not be known at compile-time).  With this
     option, these checks are omitted.  This can improve performance
     for code which stores objects into arrays frequently.  It is safe
     to use this option if you are sure your code will never throw an
     `ArrayStoreException'.

`-fjni'
     With `gcj' there are two options for writing native methods: CNI
     and JNI.  By default `gcj' assumes you are using CNI.  If you are
     compiling a class with native methods, and these methods are
     implemented using JNI, then you must use `-fjni'.  This option
     causes `gcj' to generate stubs which will invoke the underlying JNI
     methods.

`-fno-assert'
     Don't recognize the `assert' keyword.  This is for compatibility
     with older versions of the language specification.

`-fno-optimize-static-class-initialization'
     When the optimization level is greater or equal to `-O2', `gcj'
     will try to optimize the way calls into the runtime are made to
     initialize static classes upon their first use (this optimization
     isn't carried out if `-C' was specified.) When compiling to native
     code, `-fno-optimize-static-class-initialization' will turn this
     optimization off, regardless of the optimization level in use.

`--disable-assertions[=CLASS-OR-PACKAGE]'
     Don't include code for checking assertions in the compiled code.
     If `=CLASS-OR-PACKAGE' is missing disables assertion code
     generation for all classes, unless overridden by a more specific
     `--enable-assertions' flag.  If CLASS-OR-PACKAGE is a class name,
     only disables generating assertion checks within the named class
     or its inner classes.  If CLASS-OR-PACKAGE is a package name,
     disables generating assertion checks within the named package or a
     subpackage.

     By default, assertions are enabled when generating class files or
     when not optimizing, and disabled when generating optimized
     binaries.

`--enable-assertions[=CLASS-OR-PACKAGE]'
     Generates code to check assertions.  The option is perhaps
     misnamed, as you still need to turn on assertion checking at
     run-time, and we don't support any easy way to do that.  So this
     flag isn't very useful yet, except to partially override
     `--disable-assertions'.

`-findirect-dispatch'
     `gcj' has a special binary compatibility ABI, which is enabled by
     the `-findirect-dispatch' option.  In this mode, the code
     generated by `gcj' honors the binary compatibility guarantees in
     the Java Language Specification, and the resulting object files do
     not need to be directly linked against their dependencies.
     Instead, all dependencies are looked up at runtime.  This allows
     free mixing of interpreted and compiled code.

     Note that, at present, `-findirect-dispatch' can only be used when
     compiling `.class' files.  It will not work when compiling from
     source.  CNI also does not yet work with the binary compatibility
     ABI.  These restrictions will be lifted in some future release.

     However, if you compile CNI code with the standard ABI, you can
     call it from code built with the binary compatibility ABI.

`-fbootstrap-classes'
     This option can be use to tell `libgcj' that the compiled classes
     should be loaded by the bootstrap loader, not the system class
     loader.  By default, if you compile a class and link it into an
     executable, it will be treated as if it was loaded using the
     system class loader.  This is convenient, as it means that things
     like `Class.forName()' will search `CLASSPATH' to find the desired
     class.

`-freduced-reflection'
     This option causes the code generated by `gcj' to contain a
     reduced amount of the class meta-data used to support runtime
     reflection. The cost of this savings is the loss of the ability to
     use certain reflection capabilities of the standard Java runtime
     environment. When set all meta-data except for that which is
     needed to obtain correct runtime semantics is eliminated.

     For code that does not use reflection (i.e. serialization, RMI,
     CORBA or call methods in the `java.lang.reflect' package),
     `-freduced-reflection' will result in proper operation with a
     savings in executable code size.

     JNI (`-fjni') and the binary compatibility ABI
     (`-findirect-dispatch') do not work properly without full
     reflection meta-data.  Because of this, it is an error to use
     these options with `-freduced-reflection'.

     *Caution:* If there is no reflection meta-data, code that uses a
     `SecurityManager' may not work properly.  Also calling
     `Class.forName()' may fail if the calling method has no reflection
     meta-data.


File: gcj.info,  Node: Configure-time Options,  Prev: Code Generation,  Up: Invoking gcj

1.7 Configure-time Options
==========================

Some `gcj' code generations options affect the resulting ABI, and so
can only be meaningfully given when `libgcj', the runtime package, is
configured.  `libgcj' puts the appropriate options from this group into
a `spec' file which is read by `gcj'.  These options are listed here
for completeness; if you are using `libgcj' then you won't want to
touch these options.

`-fuse-boehm-gc'
     This enables the use of the Boehm GC bitmap marking code.  In
     particular this causes `gcj' to put an object marking descriptor
     into each vtable.

`-fhash-synchronization'
     By default, synchronization data (the data used for `synchronize',
     `wait', and `notify') is pointed to by a word in each object.
     With this option `gcj' assumes that this information is stored in a
     hash table and not in the object itself.

`-fuse-divide-subroutine'
     On some systems, a library routine is called to perform integer
     division.  This is required to get exception handling correct when
     dividing by zero.

`-fcheck-references'
     On some systems it's necessary to insert inline checks whenever
     accessing an object via a reference.  On other systems you won't
     need this because null pointer accesses are caught automatically
     by the processor.

File: gcj.info,  Node: Compatibility,  Next: Invoking jcf-dump,  Prev: Invoking gcj,  Up: Top

2 Compatibility with the Java Platform
**************************************

As we believe it is important that the Java platform not be fragmented,
`gcj' and `libgcj' try to conform to the relevant Java specifications.
However, limited manpower and incomplete and unclear documentation work
against us.  So, there are caveats to using `gcj'.

* Menu:

* Limitations::
* Extensions::

File: gcj.info,  Node: Limitations,  Next: Extensions,  Up: Compatibility

2.1 Standard features not yet supported
=======================================

This list of compatibility issues is by no means complete.

   * `gcj' implements the JDK 1.2 language.  It supports inner classes
     and the new 1.4 `assert' keyword.  It does not yet support the
     Java 2 `strictfp' keyword (it recognizes the keyword but ignores
     it).

   * `libgcj' is largely compatible with the JDK 1.2 libraries.
     However, `libgcj' is missing many packages, most notably
     `java.awt'.  There are also individual missing classes and methods.
     We currently do not have a list showing differences between
     `libgcj' and the Java 2 platform.

   * Sometimes the `libgcj' implementation of a method or class differs
     from the JDK implementation.  This is not always a bug.  Still, if
     it affects you, it probably makes sense to report it so that we
     can discuss the appropriate response.

   * `gcj' does not currently allow for piecemeal replacement of
     components within `libgcj'. Unfortunately, programmers often want
     to use newer versions of certain packages, such as those provided
     by the Apache Software Foundation's Jakarta project.  This has
     forced us to place the `org.w3c.dom' and `org.xml.sax' packages
     into their own libraries, separate from `libgcj'.  If you intend to
     use these classes, you must link them explicitly with
     `-l-org-w3c-dom' and `-l-org-xml-sax'.  Future versions of `gcj'
     may not have this restriction.

File: gcj.info,  Node: Extensions,  Prev: Limitations,  Up: Compatibility

2.2 Extra features unique to gcj
================================

The main feature of `gcj' is that it can compile programs written in
the Java programming language to native code.  Most extensions that
have been added are to facilitate this functionality.

   * `gcj' makes it easy and efficient to mix code written in Java and
     C++.  *Note About CNI::, for more info on how to use this in your
     programs.

   * When you compile your classes into a shared library using
     `-findirect-dispatch' then add them to the system-wide classmap.db
     file using `gcj-dbtool', they will be automatically loaded by the
     `libgcj' system classloader.  This is the new, preferred
     classname-to-library resolution mechanism.  *Note Invoking
     gcj-dbtool::, for more information on using the classmap database.

   * The old classname-to-library lookup mechanism is still supported
     through the `gnu.gcj.runtime.VMClassLoader.library_control'
     property, but it is deprecated and will likely be removed in some
     future release.  When trying to load a class `gnu.pkg.SomeClass'
     the system classloader will first try to load the shared library
     `lib-gnu-pkg-SomeClass.so', if that fails to load the class then
     it will try to load `lib-gnu-pkg.so' and finally when the class is
     still not loaded it will try to load `lib-gnu.so'.  Note that all
     `.'s will be transformed into `-'s and that searching for inner
     classes starts with their outermost outer class.  If the class
     cannot be found this way the system classloader tries to use the
     `libgcj' bytecode interpreter to load the class from the standard
     classpath.  This process can be controlled to some degree via the
     `gnu.gcj.runtime.VMClassLoader.library_control' property; *Note
     libgcj Runtime Properties::.

   * `libgcj' includes a special `gcjlib' URL type.  A URL of this form
     is like a `jar' URL, and looks like
     `gcjlib:/path/to/shared/library.so!/path/to/resource'.  An access
     to one of these URLs causes the shared library to be `dlopen()'d,
     and then the resource is looked for in that library.  These URLs
     are most useful when used in conjunction with
     `java.net.URLClassLoader'.  Note that, due to implementation
     limitations, currently any such URL can be accessed by only one
     class loader, and libraries are never unloaded.  This means some
     care must be exercised to make sure that a `gcjlib' URL is not
     accessed by more than one class loader at once.  In a future
     release this limitation will be lifted, and such libraries will be
     mapped privately.

   * A program compiled by `gcj' will examine the `GCJ_PROPERTIES'
     environment variable and change its behavior in some ways.  In
     particular `GCJ_PROPERTIES' holds a list of assignments to global
     properties, such as would be set with the `-D' option to `java'.
     For instance, `java.compiler=gcj' is a valid (but currently
     meaningless) setting.


File: gcj.info,  Node: Invoking jcf-dump,  Next: Invoking gij,  Prev: Compatibility,  Up: Top

3 Invoking jcf-dump
*******************

This is a class file examiner, similar to `javap'.  It will print
information about a number of classes, which are specified by class name
or file name.

`-c'
     Disassemble method bodies.  By default method bodies are not
     printed.

`--print-constants'
     Print the constant pool.  When printing a reference to a constant
     also print its index in the constant pool.

`--javap'
     Generate output in `javap' format.  The implementation of this
     feature is very incomplete.

`--classpath=PATH'
`--CLASSPATH=PATH'
`-IDIRECTORY'
`-o FILE'
     These options as the same as the corresponding `gcj' options.

`--help'
     Print help, then exit.

`--version'
     Print version number, then exit.

`-v, --verbose'
     Print extra information while running.  Implies
     `--print-constants'.

File: gcj.info,  Node: Invoking gij,  Next: Invoking gcj-dbtool,  Prev: Invoking jcf-dump,  Up: Top

4 Invoking gij
**************

`gij' is a Java bytecode interpreter included with `libgcj'.  `gij' is
not available on every platform; porting it requires a small amount of
assembly programming which has not been done for all the targets
supported by `gcj'.

   The primary argument to `gij' is the name of a class or, with
`-jar', a jar file.  Options before this argument are interpreted by
`gij'; remaining options are passed to the interpreted program.

   If a class name is specified and this class does not have a `main'
method with the appropriate signature (a `static void' method with a
`String[]' as its sole argument), then `gij' will print an error and
exit.

   If a jar file is specified then `gij' will use information in it to
determine which class' `main' method will be invoked.

   `gij' will invoke the `main' method with all the remaining
command-line options.

   Note that `gij' is not limited to interpreting code.  Because
`libgcj' includes a class loader which can dynamically load shared
objects, it is possible to give `gij' the name of a class which has
been compiled and put into a shared library on the class path.

`-cp PATH'
`-classpath PATH'
     Set the initial class path.  The class path is used for finding
     class and resource files.  If specified, this option overrides the
     `CLASSPATH' environment variable.  Note that this option is
     ignored if `-jar' is used.

`-DNAME[=VALUE]'
     This defines a system property named NAME with value VALUE.  If
     VALUE is not specified then it defaults to the empty string.
     These system properties are initialized at the program's startup
     and can be retrieved at runtime using the
     `java.lang.System.getProperty' method.

`-ms=NUMBER'
     Equivalent to `-Xms'.

`-mx=NUMBER'
     Equivalent to `-Xmx'.

`-noverify'
     Do not verify compliance of bytecode with the VM specification. In
     addition, this option disables type verification which is
     otherwise performed on BC-ABI compiled code.

`-X'
`-XARGUMENT'
     Supplying `-X' by itself will cause `gij' to list all the
     supported `-X' options.  Currently these options are supported:

    `-XmsSIZE'
          Set the initial heap size.

    `-XmxSIZE'
          Set the maximum heap size.

    `-XssSIZE'
          Set the thread stack size.

     Unrecognized `-X' options are ignored, for compatibility with
     other runtimes.

`-jar'
     This indicates that the name passed to `gij' should be interpreted
     as the name of a jar file, not a class.

`--help'
`-?'
     Print help, then exit.

`--showversion'
     Print version number and continue.

`--fullversion'
     Print detailed version information, then exit.

`--version'
     Print version number, then exit.

`-verbose'
`-verbose:class'
     Each time a class is initialized, print a short message on
     standard error.

   `gij' also recognizes and ignores the following options, for
compatibility with existing application launch scripts: `-client',
`-server', `-hotspot', `-jrockit', `-agentlib', `-agentpath', `-debug',
`-d32', `-d64', `-javaagent', `-noclassgc', `-verify', and
`-verifyremote'.

File: gcj.info,  Node: Invoking gcj-dbtool,  Next: Invoking jv-convert,  Prev: Invoking gij,  Up: Top

5 Invoking gcj-dbtool.
**********************

`gcj-dbtool' is a tool for creating and manipulating class file mapping
databases.  `libgcj' can use these databases to find a shared library
corresponding to the bytecode representation of a class.  This
functionality is useful for ahead-of-time compilation of a program that
has no knowledge of `gcj'.

   `gcj-dbtool' works best if all the jar files added to it are
compiled using `-findirect-dispatch'.

   Note that `gcj-dbtool' is currently available as "preview
technology".  We believe it is a reasonable way to allow
application-transparent ahead-of-time compilation, but this is an
unexplored area.  We welcome your comments.

`-n DBFILE [SIZE]'
     This creates a new database.  Currently, databases cannot be
     resized; you can choose a larger initial size if desired.  The
     default size is 32,749.

`-a DBFILE JARFILE LIB'
`-f DBFILE JARFILE LIB'
     This adds a jar file to the database.  For each class file in the
     jar, a cryptographic signature of the bytecode representation of
     the class is recorded in the database.  At runtime, a class is
     looked up by its signature and the compiled form of the class is
     looked for in the corresponding shared library.  The `-a' option
     will verify that LIB exists before adding it to the database; `-f'
     skips this check.

`[`-'][`-0'] -m DBFILE DBFILE,[DBFILE]'
     Merge a number of databases.  The output database overwrites any
     existing database.  To add databases into an existing database,
     include the destination in the list of sources.

     If `-' or `-0' are used, the list of files to read is taken from
     standard input instead of the command line.  For `-0', Input
     filenames are terminated by a null character instead of by
     whitespace.  Useful when arguments might contain white space.  The
     GNU find -print0 option produces input suitable for this mode.

`-t DBFILE'
     Test a database.

`-l DBFILE'
     List the contents of a database.

`-p'
     Print the name of the default database.  If there is no default
     database, this prints a blank line.  If LIBDIR is specified, use
     it instead of the default library directory component of the
     database name.

`--help'
     Print a help message, then exit.

`--version'
`-v'
     Print version information, then exit.


File: gcj.info,  Node: Invoking jv-convert,  Next: Invoking grmic,  Prev: Invoking gcj-dbtool,  Up: Top

6 Invoking jv-convert
*********************

`jv-convert' [`OPTION'] ... [INPUTFILE [OUTPUTFILE]]

   `jv-convert' is a utility included with `libgcj' which converts a
file from one encoding to another.  It is similar to the Unix `iconv'
utility.

   The encodings supported by `jv-convert' are platform-dependent.
Currently there is no way to get a list of all supported encodings.

`--encoding NAME'
`--from NAME'
     Use NAME as the input encoding.  The default is the current
     locale's encoding.

`--to NAME'
     Use NAME as the output encoding.  The default is the `JavaSrc'
     encoding; this is ASCII with `\u' escapes for non-ASCII characters.

`-i FILE'
     Read from FILE.  The default is to read from standard input.

`-o FILE'
     Write to FILE.  The default is to write to standard output.

`--reverse'
     Swap the input and output encodings.

`--help'
     Print a help message, then exit.

`--version'
     Print version information, then exit.

File: gcj.info,  Node: Invoking grmic,  Next: Invoking gc-analyze,  Prev: Invoking jv-convert,  Up: Top

7 Invoking grmic
****************

`grmic' [`OPTION'] ... CLASS ...

   `grmic' is a utility included with `libgcj' which generates stubs
for remote objects.

   Note that this program isn't yet fully compatible with the JDK
`grmic'.  Some options, such as `-classpath', are recognized but
currently ignored.  We have left these options undocumented for now.

   Long options can also be given with a GNU-style leading `--'.  For
instance, `--help' is accepted.

`-keep'
`-keepgenerated'
     By default, `grmic' deletes intermediate files.  Either of these
     options causes it not to delete such files.

`-v1.1'
     Cause `grmic' to create stubs and skeletons for the 1.1 protocol
     version.

`-vcompat'
     Cause `grmic' to create stubs and skeletons compatible with both
     the 1.1 and 1.2 protocol versions.  This is the default.

`-v1.2'
     Cause `grmic' to create stubs and skeletons for the 1.2 protocol
     version.

`-nocompile'
     Don't compile the generated files.

`-verbose'
     Print information about what `grmic' is doing.

`-d DIRECTORY'
     Put output files in DIRECTORY.  By default the files are put in
     the current working directory.

`-help'
     Print a help message, then exit.

`-version'
     Print version information, then exit.

File: gcj.info,  Node: Invoking gc-analyze,  Next: Invoking aot-compile,  Prev: Invoking grmic,  Up: Top

8 Invoking gc-analyze
*********************

`gc-analyze' [`OPTION'] ... [FILE]

   `gc-analyze' prints an analysis of a GC memory dump to standard out.

   The memory dumps may be created by calling
`gnu.gcj.util.GCInfo.enumerate(String namePrefix)' from java code.  A
memory dump will be created on an out of memory condition if
`gnu.gcj.util.GCInfo.setOOMDump(String namePrefix)' is called before
the out of memory occurs.

   Running this program will create two files: `TestDump001' and
`TestDump001.bytes'.

     import gnu.gcj.util.*;
     import java.util.*;

     public class GCDumpTest
     {
         static public void main(String args[])
         {
             ArrayList<String> l = new ArrayList<String>(1000);

             for (int i = 1; i < 1500; i++) {
                 l.add("This is string #" + i);
             }
             GCInfo.enumerate("TestDump");
         }
     }

   The memory dump may then be displayed by running:

     gc-analyze -v TestDump001

`--verbose'
`-v'
     Verbose output.

`-p TOOL-PREFIX'
     Prefix added to the names of the `nm' and `readelf' commands.

`-d DIRECTORY'
     Directory that contains the executable and shared libraries used
     when the dump was generated.

`--help'
     Print a help message, then exit.

`--version'
     Print version information, then exit.

File: gcj.info,  Node: Invoking aot-compile,  Next: Invoking rebuild-gcj-db,  Prev: Invoking gc-analyze,  Up: Top

9 Invoking aot-compile
**********************

`aot-compile' is a script that searches a directory for Java bytecode
(as class files, or in jars) and uses `gcj' to compile it to native
code and generate the databases from it.

`-M, --make=PATH'
     Specify the path to the `make' executable to use.

`-C, --gcj=PATH'
     Specify the path to the `gcj' executable to use.

`-D, --dbtool=PATH'
     Specify the path to the `gcj-dbtool' executable to use.

`-m, --makeflags=FLAGS'
     Specify flags to pass to `make' during the build.

`-c, --gcjflags=FLAGS'
     Specify flags to pass to `gcj' during compilation, in addition to
     '-fPIC -findirect-dispatch -fjni'.

`-l, --ldflags=FLAGS'
     Specify flags to pass to `gcj' during linking, in addition to
     '-Wl,-Bsymbolic'.

`-e, --exclude=PATH'
     Do not compile PATH.


File: gcj.info,  Node: Invoking rebuild-gcj-db,  Next: About CNI,  Prev: Invoking aot-compile,  Up: Top

10 Invoking rebuild-gcj-db
**************************

`rebuild-gcj-db' is a script that merges the per-solib databases made by
`aot-compile' into one system-wide database so `gij' can find the
solibs.

File: gcj.info,  Node: About CNI,  Next: System properties,  Prev: Invoking rebuild-gcj-db,  Up: Top

11 About CNI
************

This documents CNI, the Compiled Native Interface, which is is a
convenient way to write Java native methods using C++.  This is a more
efficient, more convenient, but less portable alternative to the
standard JNI (Java Native Interface).

* Menu:

* Basic concepts::              Introduction to using CNI.
* Packages::                    How packages are mapped to C++.
* Primitive types::             Handling primitive Java types in C++.
* Reference types::             Handling Java reference types in C++.
* Interfaces::                  How Java interfaces map to C++.
* Objects and Classes::         C++ and Java classes.
* Class Initialization::        How objects are initialized.
* Object allocation::           How to create Java objects in C++.
* Memory allocation::           How to allocate and free memory.
* Arrays::                      Dealing with Java arrays in C++.
* Methods::                     Java methods in C++.
* Strings::                     Information about Java Strings.
* Mixing with C++::             How CNI can interoperate with C++.
* Exception Handling::          How exceptions are handled.
* Synchronization::             Synchronizing between Java and C++.
* Invocation::                  Starting the Java runtime from C++.
* Reflection::                  Using reflection from C++.

File: gcj.info,  Node: Basic concepts,  Next: Packages,  Up: About CNI

11.1 Basic concepts
===================

In terms of languages features, Java is mostly a subset of C++.  Java
has a few important extensions, plus a powerful standard class library,
but on the whole that does not change the basic similarity.  Java is a
hybrid object-oriented language, with a few native types, in addition
to class types.  It is class-based, where a class may have static as
well as per-object fields, and static as well as instance methods.
Non-static methods may be virtual, and may be overloaded.  Overloading
is resolved at compile time by matching the actual argument types
against the parameter types.  Virtual methods are implemented using
indirect calls through a dispatch table (virtual function table).
Objects are allocated on the heap, and initialized using a constructor
method.  Classes are organized in a package hierarchy.

   All of the listed attributes are also true of C++, though C++ has
extra features (for example in C++ objects may be allocated not just on
the heap, but also statically or in a local stack frame).  Because
`gcj' uses the same compiler technology as G++ (the GNU C++ compiler),
it is possible to make the intersection of the two languages use the
same ABI (object representation and calling conventions).  The key idea
in CNI is that Java objects are C++ objects, and all Java classes are
C++ classes (but not the other way around).  So the most important task
in integrating Java and C++ is to remove gratuitous incompatibilities.

   You write CNI code as a regular C++ source file.  (You do have to use
a Java/CNI-aware C++ compiler, specifically a recent version of G++.)

A CNI C++ source file must have:

     #include <gcj/cni.h>

and then must include one header file for each Java class it uses, e.g.:

     #include <java/lang/Character.h>
     #include <java/util/Date.h>
     #include <java/lang/IndexOutOfBoundsException.h>

These header files are automatically generated by `gcjh'.

   CNI provides some functions and macros to make using Java objects and
primitive types from C++ easier.  In general, these CNI functions and
macros start with the `Jv' prefix, for example the function
`JvNewObjectArray'.  This convention is used to avoid conflicts with
other libraries.  Internal functions in CNI start with the prefix
`_Jv_'.  You should not call these; if you find a need to, let us know
and we will try to come up with an alternate solution.

11.1.1 Limitations
------------------

Whilst a Java class is just a C++ class that doesn't mean that you are
freed from the shackles of Java, a CNI C++ class must adhere to the
rules of the Java programming language.

   For example: it is not possible to declare a method in a CNI class
that will take a C string (`char*') as an argument, or to declare a
member variable of some non-Java datatype.

File: gcj.info,  Node: Packages,  Next: Primitive types,  Prev: Basic concepts,  Up: About CNI

11.2 Packages
=============

The only global names in Java are class names, and packages.  A
"package" can contain zero or more classes, and also zero or more
sub-packages.  Every class belongs to either an unnamed package or a
package that has a hierarchical and globally unique name.

   A Java package is mapped to a C++ "namespace".  The Java class
`java.lang.String' is in the package `java.lang', which is a
sub-package of `java'.  The C++ equivalent is the class
`java::lang::String', which is in the namespace `java::lang' which is
in the namespace `java'.

Here is how you could express this:

     (// Declare the class(es), possibly in a header file:
     namespace java {
       namespace lang {
         class Object;
         class String;
         ...
       }
     }

     class java::lang::String : public java::lang::Object
     {
       ...
     };

The `gcjh' tool automatically generates the necessary namespace
declarations.

11.2.1 Leaving out package names
--------------------------------

Always using the fully-qualified name of a java class can be tiresomely
verbose.  Using the full qualified name also ties the code to a single
package making code changes necessary should the class move from one
package to another.  The Java `package' declaration specifies that the
following class declarations are in the named package, without having
to explicitly name the full package qualifiers.  The `package'
declaration can be followed by zero or more `import' declarations, which
allows either a single class or all the classes in a package to be
named by a simple identifier.  C++ provides something similar with the
`using' declaration and directive.

In Java:

     import PACKAGE-NAME.CLASS-NAME;

allows the program text to refer to CLASS-NAME as a shorthand for the
fully qualified name: `PACKAGE-NAME.CLASS-NAME'.

To achieve the same effect C++, you have to do this:

     using PACKAGE-NAME::CLASS-NAME;

Java can also cause imports on demand, like this:

     import PACKAGE-NAME.*;

Doing this allows any class from the package PACKAGE-NAME to be
referred to only by its class-name within the program text.

The same effect can be achieved in C++ like this:

     using namespace PACKAGE-NAME;

File: gcj.info,  Node: Primitive types,  Next: Reference types,  Prev: Packages,  Up: About CNI

11.3 Primitive types
====================

Java provides 8 "primitives" types which represent integers, floats,
characters and booleans (and also the void type).  C++ has its own very
similar concrete types.  Such types in C++ however are not always
implemented in the same way (an int might be 16, 32 or 64 bits for
example) so CNI provides a special C++ type for each primitive Java
type:

*Java type*    *C/C++ typename*   *Description*
`char'         `jchar'            16 bit Unicode character
`boolean'      `jboolean'         logical (true or false) values
`byte'         `jbyte'            8-bit signed integer
`short'        `jshort'           16 bit signed integer
`int'          `jint'             32 bit signed integer
`long'         `jlong'            64 bit signed integer
`float'        `jfloat'           32 bit IEEE floating point number
`double'       `jdouble'          64 bit IEEE floating point number
`void'         `void'             no value

   When referring to a Java type You should always use these C++
typenames (e.g.: `jint') to avoid disappointment.

11.3.1 Reference types associated with primitive types
------------------------------------------------------

In Java each primitive type has an associated reference type, e.g.:
`boolean' has an associated `java.lang.Boolean.TYPE' class.  In order
to make working with such classes easier GCJ provides the macro
`JvPrimClass':

 -- macro: JvPrimClass type
     Return a pointer to the `Class' object corresponding to the type
     supplied.

          JvPrimClass(void) => java.lang.Void.TYPE


File: gcj.info,  Node: Reference types,  Next: Interfaces,  Prev: Primitive types,  Up: About CNI

11.4 Reference types
====================

A Java reference type is treated as a class in C++.  Classes and
interfaces are handled this way.  A Java reference is translated to a
C++ pointer, so for instance a Java `java.lang.String' becomes, in C++,
`java::lang::String *'.

   CNI provides a few built-in typedefs for the most common classes:
*Java type*            *C++ typename*     *Description*
`java.lang.Object'     `jobject'          Object type
`java.lang.String'     `jstring'          String type
`java.lang.Class'      `jclass'           Class type

   Every Java class or interface has a corresponding `Class' instance.
These can be accessed in CNI via the static `class$' field of a class.
The `class$' field is of type `Class' (and not `Class *'), so you will
typically take the address of it.

   Here is how you can refer to the class of `String', which in Java
would be written `String.class':

     using namespace java::lang;
     doSomething (&String::class$);

File: gcj.info,  Node: Interfaces,  Next: Objects and Classes,  Prev: Reference types,  Up: About CNI

11.5 Interfaces
===============

A Java class can "implement" zero or more "interfaces", in addition to
inheriting from a single base class.

   CNI allows CNI code to implement methods of interfaces.  You can
also call methods through interface references, with some limitations.

   CNI doesn't understand interface inheritance at all yet.  So, you
can only call an interface method when the declared type of the field
being called matches the interface which declares that method.  The
workaround is to cast the interface reference to the right
superinterface.

   For example if you have:

     interface A
     {
       void a();
     }

     interface B extends A
     {
       void b();
     }

   and declare a variable of type `B' in C++, you can't call `a()'
unless you cast it to an `A' first.

File: gcj.info,  Node: Objects and Classes,  Next: Class Initialization,  Prev: Interfaces,  Up: About CNI

11.6 Objects and Classes
========================

11.6.1 Classes
--------------

All Java classes are derived from `java.lang.Object'.  C++ does not
have a unique root class, but we use the C++ class `java::lang::Object'
as the C++ version of the `java.lang.Object' Java class.  All other
Java classes are mapped into corresponding C++ classes derived from
`java::lang::Object'.

   Interface inheritance (the `implements' keyword) is currently not
reflected in the C++ mapping.

11.6.2 Object fields
--------------------

Each object contains an object header, followed by the instance fields
of the class, in order.  The object header consists of a single pointer
to a dispatch or virtual function table.  (There may be extra fields
_in front of_ the object, for example for memory management, but this
is invisible to the application, and the reference to the object points
to the dispatch table pointer.)

   The fields are laid out in the same order, alignment, and size as in
C++.  Specifically, 8-bit and 16-bit native types (`byte', `short',
`char', and `boolean') are _not_ widened to 32 bits.  Note that the
Java VM does extend 8-bit and 16-bit types to 32 bits when on the VM
stack or temporary registers.

   If you include the `gcjh'-generated header for a class, you can
access fields of Java classes in the _natural_ way.  For example, given
the following Java class:

     public class Int
     {
       public int i;
       public Int (int i) { this.i = i; }
       public static Int zero = new Int(0);
     }

   you can write:

     #include <gcj/cni.h>;
     #include <Int>;

     Int*
     mult (Int *p, jint k)
     {
       if (k == 0)
         return Int::zero;  // Static member access.
       return new Int(p->i * k);
     }

11.6.3 Access specifiers
------------------------

CNI does not strictly enforce the Java access specifiers, because Java
permissions cannot be directly mapped into C++ permission.  Private
Java fields and methods are mapped to private C++ fields and methods,
but other fields and methods are mapped to public fields and methods.

File: gcj.info,  Node: Class Initialization,  Next: Object allocation,  Prev: Objects and Classes,  Up: About CNI

11.7 Class Initialization
=========================

Java requires that each class be automatically initialized at the time
of the first active use.  Initializing a class involves initializing
the static fields, running code in class initializer methods, and
initializing base classes.  There may also be some implementation
specific actions, such as allocating `String' objects corresponding to
string literals in the code.

   The GCJ compiler inserts calls to `JvInitClass' at appropriate
places to ensure that a class is initialized when required.  The C++
compiler does not insert these calls automatically--it is the
programmer's responsibility to make sure classes are initialized.
However, this is fairly painless because of the conventions assumed by
the Java system.

   First, `libgcj' will make sure a class is initialized before an
instance of that object is created.  This is one of the
responsibilities of the `new' operation.  This is taken care of both in
Java code, and in C++ code.  When G++ sees a `new' of a Java class, it
will call a routine in `libgcj' to allocate the object, and that
routine will take care of initializing the class.  Note however that
this does not happen for Java arrays; you must allocate those using the
appropriate CNI function.  It follows that you can access an instance
field, or call an instance (non-static) method and be safe in the
knowledge that the class and all of its base classes have been
initialized.

   Invoking a static method is also safe.  This is because the Java
compiler adds code to the start of a static method to make sure the
class is initialized.  However, the C++ compiler does not add this
extra code.  Hence, if you write a native static method using CNI, you
are responsible for calling `JvInitClass' before doing anything else in
the method (unless you are sure it is safe to leave it out).

   Accessing a static field also requires the class of the field to be
initialized.  The Java compiler will generate code to call
`JvInitClass' before getting or setting the field.  However, the C++
compiler will not generate this extra code, so it is your
responsibility to make sure the class is initialized before you access
a static field from C++.

File: gcj.info,  Node: Object allocation,  Next: Memory allocation,  Prev: Class Initialization,  Up: About CNI

11.8 Object allocation
======================

New Java objects are allocated using a "class instance creation
expression", e.g.:

     new TYPE ( ... )

   The same syntax is used in C++.  The main difference is that C++
objects have to be explicitly deleted; in Java they are automatically
deleted by the garbage collector.  Using CNI, you can allocate a new
Java object using standard C++ syntax and the C++ compiler will allocate
memory from the garbage collector.  If you have overloaded
constructors, the compiler will choose the correct one using standard
C++ overload resolution rules.

For example:

     java::util::Hashtable *ht = new java::util::Hashtable(120);

File: gcj.info,  Node: Memory allocation,  Next: Arrays,  Prev: Object allocation,  Up: About CNI

11.9 Memory allocation
======================

When allocating memory in CNI methods it is best to handle
out-of-memory conditions by throwing a Java exception.  These functions
are provided for that purpose:

 -- Function: void* JvMalloc (jsize SIZE)
     Calls malloc.  Throws `java.lang.OutOfMemoryError' if allocation
     fails.

 -- Function: void* JvRealloc (void* PTR, jsize SIZE)
     Calls realloc.  Throws `java.lang.OutOfMemoryError' if
     reallocation fails.

 -- Function: void JvFree (void* PTR)
     Calls free.

File: gcj.info,  Node: Arrays,  Next: Methods,  Prev: Memory allocation,  Up: About CNI

11.10 Arrays
============

While in many ways Java is similar to C and C++, it is quite different
in its treatment of arrays.  C arrays are based on the idea of pointer
arithmetic, which would be incompatible with Java's security
requirements.  Java arrays are true objects (array types inherit from
`java.lang.Object').  An array-valued variable is one that contains a
reference (pointer) to an array object.

   Referencing a Java array in C++ code is done using the `JArray'
template, which as defined as follows:

     class __JArray : public java::lang::Object
     {
     public:
       int length;
     };

     template<class T>
     class JArray : public __JArray
     {
       T data[0];
     public:
       T& operator[](jint i) { return data[i]; }
     };

   There are a number of `typedef's which correspond to `typedef's from
the JNI.  Each is the type of an array holding objects of the relevant
type:

     typedef __JArray *jarray;
     typedef JArray<jobject> *jobjectArray;
     typedef JArray<jboolean> *jbooleanArray;
     typedef JArray<jbyte> *jbyteArray;
     typedef JArray<jchar> *jcharArray;
     typedef JArray<jshort> *jshortArray;
     typedef JArray<jint> *jintArray;
     typedef JArray<jlong> *jlongArray;
     typedef JArray<jfloat> *jfloatArray;
     typedef JArray<jdouble> *jdoubleArray;

 -- Method on template<class T>: T* elements (JArray<T> ARRAY)
     This template function can be used to get a pointer to the
     elements of the `array'.  For instance, you can fetch a pointer to
     the integers that make up an `int[]' like so:

          extern jintArray foo;
          jint *intp = elements (foo);

     The name of this function may change in the future.

 -- Function: jobjectArray JvNewObjectArray (jsize LENGTH, jclass
          KLASS, jobject INIT)
     This creates a new array whose elements have reference type.
     `klass' is the type of elements of the array and `init' is the
     initial value put into every slot in the array.

     using namespace java::lang;
     JArray<String *> *array
       = (JArray<String *> *) JvNewObjectArray(length, &String::class$, NULL);

11.10.1 Creating arrays
-----------------------

For each primitive type there is a function which can be used to create
a new array of that type.  The name of the function is of the form:

     JvNewTYPEArray

For example:

     JvNewBooleanArray

can be used to create an array of Java primitive boolean types.

The following function definition is the template for all such
functions:

 -- Function: jbooleanArray JvNewBooleanArray (jint LENGTH)
     Creates an array LENGTH indices long.

 -- Function: jsize JvGetArrayLength (jarray ARRAY)
     Returns the length of the ARRAY.

File: gcj.info,  Node: Methods,  Next: Strings,  Prev: Arrays,  Up: About CNI

11.11 Methods
=============

Java methods are mapped directly into C++ methods.  The header files
generated by `gcjh' include the appropriate method definitions.
Basically, the generated methods have the same names and
_corresponding_ types as the Java methods, and are called in the
natural manner.

11.11.1 Overloading
-------------------

Both Java and C++ provide method overloading, where multiple methods in
a class have the same name, and the correct one is chosen (at compile
time) depending on the argument types.  The rules for choosing the
correct method are (as expected) more complicated in C++ than in Java,
but given a set of overloaded methods generated by `gcjh' the C++
compiler will choose the expected one.

   Common assemblers and linkers are not aware of C++ overloading, so
the standard implementation strategy is to encode the parameter types
of a method into its assembly-level name.  This encoding is called
"mangling", and the encoded name is the "mangled name".  The same
mechanism is used to implement Java overloading.  For C++/Java
interoperability, it is important that both the Java and C++ compilers
use the _same_ encoding scheme.

11.11.2 Static methods
----------------------

Static Java methods are invoked in CNI using the standard C++ syntax,
using the `::' operator rather than the `.' operator.

For example:

     jint i = java::lang::Math::round((jfloat) 2.3);

C++ method definition syntax is used to define a static native method.
For example:

     #include <java/lang/Integer>
     java::lang::Integer*
     java::lang::Integer::getInteger(jstring str)
     {
       ...
     }

11.11.3 Object Constructors
---------------------------

Constructors are called implicitly as part of object allocation using
the `new' operator.

For example:

     java::lang::Integer *x = new java::lang::Integer(234);

   Java does not allow a constructor to be a native method.  This
limitation can be coded round however because a constructor can _call_
a native method.

11.11.4 Instance methods
------------------------

Calling a Java instance method from a C++ CNI method is done using the
standard C++ syntax, e.g.:

     // First create the Java object.
     java::lang::Integer *x = new java::lang::Integer(234);
     // Now call a method.
     jint prim_value = x->intValue();
     if (x->longValue == 0)
       ...

Defining a Java native instance method is also done the natural way:

     #include <java/lang/Integer.h>

     jdouble
     java::lang:Integer::doubleValue()
     {
       return (jdouble) value;
     }

11.11.5 Interface methods
-------------------------

In Java you can call a method using an interface reference.  This is
supported, but not completely.  *Note Interfaces::.

File: gcj.info,  Node: Strings,  Next: Mixing with C++,  Prev: Methods,  Up: About CNI

11.12 Strings
=============

CNI provides a number of utility functions for working with Java Java
`String' objects.  The names and interfaces are analogous to those of
JNI.

 -- Function: jstring JvNewString (const jchar* CHARS, jsize LEN)
     Returns a Java `String' object with characters from the array of
     Unicode characters CHARS up to the index LEN in that array.

 -- Function: jstring JvNewStringLatin1 (const char* BYTES, jsize LEN)
     Returns a Java `String' made up of LEN bytes from BYTES.

 -- Function: jstring JvNewStringLatin1 (const char* BYTES)
     As above but the length of the `String' is `strlen(BYTES)'.

 -- Function: jstring JvNewStringUTF (const char* BYTES)
     Returns a `String' which is made up of the UTF encoded characters
     present in the C string BYTES.

 -- Function: jchar* JvGetStringChars (jstring STR)
     Returns a pointer to an array of characters making up the `String'
     STR.

 -- Function: int JvGetStringUTFLength (jstring STR)
     Returns the number of bytes required to encode the contents of the
     `String' STR in UTF-8.

 -- Function: jsize JvGetStringUTFRegion (jstring STR, jsize START,
          jsize LEN, char* BUF)
     Puts the UTF-8 encoding of a region of the `String' STR into the
     buffer `buf'.  The region to fetch is marked by START and LEN.

     Note that BUF is a buffer, not a C string.  It is _not_ null
     terminated.

File: gcj.info,  Node: Mixing with C++,  Next: Exception Handling,  Prev: Strings,  Up: About CNI

11.13 Interoperating with C/C++
===============================

Because CNI is designed to represent Java classes and methods it cannot
be mixed readily with C/C++ types.

   One important restriction is that Java classes cannot have non-Java
type instance or static variables and cannot have methods which take
non-Java types as arguments or return non-Java types.

None of the following is possible with CNI:


     class ::MyClass : public java::lang::Object
     {
        char* variable;  // char* is not a valid Java type.
     }


     uint
     ::SomeClass::someMethod (char *arg)
     {
       .
       .
       .
     }   // `uint' is not a valid Java type, neither is `char*'

Of course, it is ok to use C/C++ types within the scope of a method:

     jint
     ::SomeClass::otherMethod (jstring str)
     {
        char *arg = ...
        .
        .
        .
     }

11.13.1 RawData
---------------

The above restriction can be problematic, so CNI includes the
`gnu.gcj.RawData' class.  The `RawData' class is a "non-scanned
reference" type.  In other words variables declared of type `RawData'
can contain any data and are not checked by the compiler or memory
manager in any way.

   This means that you can put C/C++ data structures (including classes)
in your CNI classes, as long as you use the appropriate cast.

Here are some examples:


     class ::MyClass : public java::lang::Object
     {
        gnu.gcj.RawData string;

        MyClass ();
        gnu.gcj.RawData getText ();
        void printText ();
     }

     ::MyClass::MyClass ()
     {
        char* text = ...
        string = text;
     }

     gnu.gcj.RawData
     ::MyClass::getText ()
     {
        return string;
     }

     void
     ::MyClass::printText ()
     {
       printf("%s\n", (char*) string);
     }

11.13.2 RawDataManaged
----------------------

`gnu.gcj.RawDataManaged' is another type used to indicate special data
used by native code. Unlike the `RawData' type, fields declared as
`RawDataManaged' will be "marked" by the memory manager and considered
for garbage collection.

   Native data which is allocated using CNI's `JvAllocBytes()' function
and stored in a `RawDataManaged' will be automatically freed when the
Java object it is associated with becomes unreachable.

11.13.3 Native memory allocation
--------------------------------

 -- Function: void* JvAllocBytes (jsize SIZE)
     Allocates SIZE bytes from the heap.  The memory returned is zeroed.
     This memory is not scanned for pointers by the garbage collector,
     but will be freed if no references to it are discovered.

     This function can be useful if you need to associate some native
     data with a Java object. Using a CNI's special `RawDataManaged'
     type, native data allocated with `JvAllocBytes' will be
     automatically freed when the Java object itself becomes
     unreachable.

11.13.4 Posix signals
---------------------

On Posix based systems the `libgcj' library uses several signals
internally.  CNI code should not attempt to use the same signals as
doing so may cause `libgcj' and/or the CNI code to fail.

   SIGSEGV is used on many systems to generate `NullPointerExceptions'.
SIGCHLD is used internally by `Runtime.exec()'.  Several other signals
(that vary from platform to platform) can be used by the memory manager
and by `Thread.interrupt()'.

File: gcj.info,  Node: Exception Handling,  Next: Synchronization,  Prev: Mixing with C++,  Up: About CNI

11.14 Exception Handling
========================

While C++ and Java share a common exception handling framework, things
are not yet perfectly integrated.  The main issue is that the run-time
type information facilities of the two languages are not integrated.

   Still, things work fairly well.  You can throw a Java exception from
C++ using the ordinary `throw' construct, and this exception can be
caught by Java code.  Similarly, you can catch an exception thrown from
Java using the C++ `catch' construct.

Here is an example:

     if (i >= count)
        throw new java::lang::IndexOutOfBoundsException();

   Normally, G++ will automatically detect when you are writing C++
code that uses Java exceptions, and handle them appropriately.
However, if C++ code only needs to execute destructors when Java
exceptions are thrown through it, GCC will guess incorrectly.  Sample
problematic code:

     struct S { ~S(); };

     extern void bar();    // Is implemented in Java and may throw exceptions.

     void foo()
     {
       S s;
       bar();
     }

   The usual effect of an incorrect guess is a link failure,
complaining of a missing routine called `__gxx_personality_v0'.

   You can inform the compiler that Java exceptions are to be used in a
translation unit, irrespective of what it might think, by writing
`#pragma GCC java_exceptions' at the head of the file.  This `#pragma'
must appear before any functions that throw or catch exceptions, or run
destructors when exceptions are thrown through them.

File: gcj.info,  Node: Synchronization,  Next: Invocation,  Prev: Exception Handling,  Up: About CNI

11.15 Synchronization
=====================

Each Java object has an implicit monitor.  The Java VM uses the
instruction `monitorenter' to acquire and lock a monitor, and
`monitorexit' to release it.

   The corresponding CNI macros are `JvMonitorEnter' and
`JvMonitorExit' (JNI has similar  methods `MonitorEnter' and
`MonitorExit').

   The Java source language does not provide direct access to these
primitives.  Instead, there is a `synchronized' statement that does an
implicit `monitorenter' before entry to the block, and does a
`monitorexit' on exit from the block.  Note that the lock has to be
released even when the block is abnormally terminated by an exception,
which means there is an implicit `try finally' surrounding
synchronization locks.

   From C++, it makes sense to use a destructor to release a lock.  CNI
defines the following utility class:

     class JvSynchronize() {
       jobject obj;
       JvSynchronize(jobject o) { obj = o; JvMonitorEnter(o); }
       ~JvSynchronize() { JvMonitorExit(obj); }
     };

   So this Java code:

     synchronized (OBJ)
     {
        CODE
     }

might become this C++ code:

     {
        JvSynchronize dummy (OBJ);
        CODE;
     }

   Java also has methods with the `synchronized' attribute.  This is
equivalent to wrapping the entire method body in a `synchronized'
statement.  (Alternatively, an implementation could require the caller
to do the synchronization.  This is not practical for a compiler,
because each virtual method call would have to test at run-time if
synchronization is needed.)  Since in `gcj' the `synchronized'
attribute is handled by the method implementation, it is up to the
programmer of a synchronized native method to handle the synchronization
(in the C++ implementation of the method).  In other words, you need to
manually add `JvSynchronize' in a `native synchronized' method.

File: gcj.info,  Node: Invocation,  Next: Reflection,  Prev: Synchronization,  Up: About CNI

11.16 Invocation
================

CNI permits C++ applications to make calls into Java classes, in
addition to allowing Java code to call into C++. Several functions,
known as the "invocation API", are provided to support this.

 -- Function: jint JvCreateJavaVM (JvVMInitArgs* VM_ARGS)
     Initializes the Java runtime. This function performs essential
     initialization of the threads interface, garbage collector,
     exception handling and other key aspects of the runtime. It must
     be called once by an application with a non-Java `main()'
     function, before any other Java or CNI calls are made.  It is
     safe, but not recommended, to call `JvCreateJavaVM()' more than
     once provided it is only called from a single thread.  The VMARGS
     parameter can be used to specify initialization parameters for the
     Java runtime. It may be `NULL'.

     JvVMInitArgs represents a list of virtual machine initialization
     arguments. `JvCreateJavaVM()' ignores the version field.

          typedef struct JvVMOption
          {
            // a VM initialization option
            char* optionString;
            // extra information associated with this option
            void* extraInfo;
          } JvVMOption;

          typedef struct JvVMInitArgs
          {
            // for compatibility with JavaVMInitArgs
            jint version;

            // number of VM initialization options
            jint nOptions;

            // an array of VM initialization options
            JvVMOption* options;

            // true if the option parser should ignore unrecognized options
            jboolean ignoreUnrecognized;
          } JvVMInitArgs;

     `JvCreateJavaVM()' returns `0' upon success, or `-1' if the
     runtime is already initialized.

     _Note:_ In GCJ 3.1, the `vm_args' parameter is ignored. It is
     recognized and used as of release 4.0.

 -- Function: java::lang::Thread* JvAttachCurrentThread (jstring NAME,
          java::lang::ThreadGroup* GROUP)
     Registers an existing thread with the Java runtime.  This must be
     called once from each thread, before that thread makes any other
     Java or CNI calls. It must be called after `JvCreateJavaVM'.  NAME
     specifies a name for the thread. It may be `NULL', in which case a
     name will be generated.  GROUP is the ThreadGroup in which this
     thread will be a member. If it is `NULL', the thread will be a
     member of the main thread group.  The return value is the Java
     `Thread' object that represents the thread.  It is safe to call
     `JvAttachCurrentThread()' more than once from the same thread. If
     the thread is already attached, the call is ignored and the current
     thread object is returned.

 -- Function: jint JvDetachCurrentThread ()
     Unregisters a thread from the Java runtime. This should be called
     by threads that were attached using `JvAttachCurrentThread()',
     after they have finished making calls to Java code. This ensures
     that any resources associated with the thread become eligible for
     garbage collection.  This function returns `0' upon success, or
     `-1' if the current thread is not attached.

11.16.1 Handling uncaught exceptions
------------------------------------

If an exception is thrown from Java code called using the invocation
API, and no handler for the exception can be found, the runtime will
abort the application. In order to make the application more robust, it
is recommended that code which uses the invocation API be wrapped by a
top-level try/catch block that catches all Java exceptions.

11.16.2 Example
---------------

The following code demonstrates the use of the invocation API. In this
example, the C++ application initializes the Java runtime and attaches
itself. The `java.lang.System' class is initialized in order to access
its `out' field, and a Java string is printed. Finally, the thread is
detached from the runtime once it has finished making Java calls.
Everything is wrapped with a try/catch block to provide a default
handler for any uncaught exceptions.

   The example can be compiled with `c++ -c test.cc; gcj test.o'.

     // test.cc
     #include <gcj/cni.h>
     #include <java/lang/System.h>
     #include <java/io/PrintStream.h>
     #include <java/lang/Throwable.h>

     int main(int argc, char *argv[])
     {
       using namespace java::lang;

       try
       {
         JvCreateJavaVM(NULL);
         JvAttachCurrentThread(NULL, NULL);

         String *message = JvNewStringLatin1("Hello from C++");
         JvInitClass(&System::class$);
         System::out->println(message);

         JvDetachCurrentThread();
       }
       catch (Throwable *t)
       {
         System::err->println(JvNewStringLatin1("Unhandled Java exception:"));
         t->printStackTrace();
       }
     }

File: gcj.info,  Node: Reflection,  Prev: Invocation,  Up: About CNI

11.17 Reflection
================

Reflection is possible with CNI code, it functions similarly to how it
functions with JNI.

   The types `jfieldID' and `jmethodID' are as in JNI.

The functions:

   * `JvFromReflectedField',

   * `JvFromReflectedMethod',

   * `JvToReflectedField'

   * `JvToFromReflectedMethod'

will be added shortly, as will other functions corresponding to JNI.

File: gcj.info,  Node: System properties,  Next: Resources,  Prev: About CNI,  Up: Top

12 System properties
********************

The runtime behavior of the `libgcj' library can be modified by setting
certain system properties.  These properties can be compiled into the
program using the `-DNAME[=VALUE]' option to `gcj' or by setting them
explicitly in the program by calling the
`java.lang.System.setProperty()' method.  Some system properties are
only used for informational purposes (like giving a version number or a
user name).  A program can inspect the current value of a property by
calling the `java.lang.System.getProperty()' method.

* Menu:

* Standard Properties::         Standard properties supported by `libgcj'
* GNU Classpath Properties::    Properties found in Classpath based libraries
* libgcj Runtime Properties::   Properties specific to `libgcj'

File: gcj.info,  Node: Standard Properties,  Next: GNU Classpath Properties,  Up: System properties

12.1 Standard Properties
========================

The following properties are normally found in all implementations of
the core libraries for the Java language.

`java.version'
     The `libgcj' version number.

`java.vendor'
     Set to `The Free Software Foundation, Inc.'

`java.vendor.url'
     Set to `http://gcc.gnu.org/java/'.

`java.home'
     The directory where `gcj' was installed.  Taken from the `--prefix'
     option given to `configure'.

`java.class.version'
     The class format version number supported by the libgcj byte code
     interpreter.  (Currently `46.0')

`java.vm.specification.version'
     The Virtual Machine Specification version implemented by `libgcj'.
     (Currently `1.0')

`java.vm.specification.vendor'
     The name of the Virtual Machine specification designer.

`java.vm.specification.name'
     The name of the Virtual Machine specification (Set to `Java
     Virtual Machine Specification').

`java.vm.version'
     The `gcj' version number.

`java.vm.vendor'
     Set to `The Free Software Foundation, Inc.'

`java.vm.name'
     Set to `GNU libgcj'.

`java.specification.version'
     The Runtime Environment specification version implemented by
     `libgcj'.  (Currently set to `1.3')

`java.specification.vendor'
     The Runtime Environment specification designer.

`java.specification.name'
     The name of the Runtime Environment specification (Set to `Java
     Platform API Specification').

`java.class.path'
     The paths (jar files, zip files and directories) used for finding
     class files.

`java.library.path'
     Directory path used for finding native libraries.

`java.io.tmpdir'
     The directory used to put temporary files in.

`java.compiler'
     Name of the Just In Time compiler to use by the byte code
     interpreter.  Currently not used in `libgcj'.

`java.ext.dirs'
     Directories containing jar files with extra libraries.  Will be
     used when resolving classes.

`java.protocol.handler.pkgs'
     A `|' separated list of package names that is used to find classes
     that implement handlers for `java.net.URL'.

`java.rmi.server.codebase'
     A list of URLs that is used by the `java.rmi.server.RMIClassLoader'
     to load classes from.

`jdbc.drivers'
     A list of class names that will be loaded by the
     `java.sql.DriverManager' when it starts up.

`file.separator'
     The separator used in when directories are included in a filename
     (normally `/' or `\' ).

`file.encoding'
     The default character encoding used when converting platform
     native files to Unicode (usually set to `8859_1').

`path.separator'
     The standard separator used when a string contains multiple paths
     (normally `:' or `;'), the string is usually not a valid character
     to use in normal directory names.)

`line.separator'
     The default line separator used on the platform (normally `\n',
     `\r' or a combination of those two characters).

`policy.provider'
     The class name used for the default policy provider returned by
     `java.security.Policy.getPolicy'.

`user.name'
     The name of the user running the program.  Can be the full name,
     the login name or empty if unknown.

`user.home'
     The default directory to put user specific files in.

`user.dir'
     The current working directory from which the program was started.

`user.language'
     The default language as used by the `java.util.Locale' class.

`user.region'
     The default region as used by the `java.util.Local' class.

`user.variant'
     The default variant of the language and region local used.

`user.timezone'
     The default timezone as used by the `java.util.TimeZone' class.

`os.name'
     The operating system/kernel name that the program runs on.

`os.arch'
     The hardware that we are running on.

`os.version'
     The version number of the operating system/kernel.

`awt.appletWarning'
     The string to display when an untrusted applet is displayed.
     Returned by `java.awt.Window.getWarningString()' when the window is
     "insecure".

`awt.toolkit'
     The class name used for initializing the default
     `java.awt.Toolkit'.  Defaults to `gnu.awt.gtk.GtkToolkit'.

`http.proxyHost'
     Name of proxy host for http connections.

`http.proxyPort'
     Port number to use when a proxy host is in use.


File: gcj.info,  Node: GNU Classpath Properties,  Next: libgcj Runtime Properties,  Prev: Standard Properties,  Up: System properties

12.2 GNU Classpath Properties
=============================

`libgcj' is based on the GNU Classpath (Essential Libraries for Java) a
GNU project to create free core class libraries for use with virtual
machines and compilers for the Java language.  The following properties
are common to libraries based on GNU Classpath.

`gcj.dumpobject'
     Enables printing serialization debugging by the
     `java.io.ObjectInput' and `java.io.ObjectOutput' classes when set
     to something else then the empty string.  Only used when running a
     debug build of the library.

`gnu.classpath.vm.shortname'
     This is a succinct name of the virtual machine.  For `libgcj',
     this will always be `libgcj'.

`gnu.classpath.home.url'
     A base URL used for finding system property files (e.g.,
     `classpath.security').  By default this is a `file:' URL pointing
     to the `lib' directory under `java.home'.


File: gcj.info,  Node: libgcj Runtime Properties,  Prev: GNU Classpath Properties,  Up: System properties

12.3 libgcj Runtime Properties
==============================

The following properties are specific to the `libgcj' runtime and will
normally not be found in other core libraries for the java language.

`java.fullversion'
     The combination of `java.vm.name' and `java.vm.version'.

`java.vm.info'
     Same as `java.fullversion'.

`impl.prefix'
     Used by the `java.net.DatagramSocket' class when set to something
     else then the empty string.  When set all newly created
     `DatagramSocket's will try to load a class
     `java.net.[impl.prefix]DatagramSocketImpl' instead of the normal
     `java.net.PlainDatagramSocketImpl'.

`gnu.gcj.progname'
     The class or binary name that was used to invoke the program. This
     will be the name of the "main" class in the case where the `gij'
     front end is used, or the program binary name in the case where an
     application is compiled to a native binary.

`gnu.gcj.user.realname'
     The real name of the user, as taken from the password file.  This
     may not always hold only the user's name (as some sites put extra
     information in this field).  Also, this property is not available
     on all platforms.

`gnu.gcj.runtime.NameFinder.use_addr2line'
     Whether an external process, `addr2line', should be used to
     determine line number information when tracing the stack. Setting
     this to `false' may suppress line numbers when printing stack
     traces and when using the java.util.logging infrastructure.
     However, performance may improve significantly for applications
     that print stack traces or make logging calls frequently.

`gnu.gcj.runtime.NameFinder.show_raw'
     Whether the address of a stack frame should be printed when the
     line number is unavailable. Setting this to `true' will cause the
     name of the object and the offset within that object to be printed
     when no line number is available.  This allows for off-line
     decoding of stack traces if necessary debug information is
     available.  The default is `false', no raw addresses are printed.

`gnu.gcj.runtime.NameFinder.remove_unknown'
     Whether stack frames for non-java code should be included in a
     stack trace.  The default value is `true', stack frames for
     non-java code are suppressed.  Setting this to `false' will cause
     any non-java stack frames to be printed in addition to frames for
     the java code.

`gnu.gcj.runtime.VMClassLoader.library_control'
     This controls how shared libraries are automatically loaded by the
     built-in class loader.  If this property is set to `full', a full
     search is done for each requested class.  If this property is set
     to `cache', then any failed lookups are cached and not tried again.
     If this property is set to `never' (the default), then lookups are
     never done.  For more information, *Note Extensions::.

`gnu.gcj.runtime.endorsed.dirs'
     This is like the standard `java.endorsed.dirs', property, but
     specifies some extra directories which are searched after the
     standard endorsed directories.  This is primarily useful for
     telling `libgcj' about additional libraries which are ordinarily
     incorporated into the JDK, and which should be loaded by the
     bootstrap class loader, but which are not yet part of `libgcj'
     itself for some reason.

`gnu.gcj.jit.compiler'
     This is the full path to `gcj' executable which should be used to
     compile classes just-in-time when `ClassLoader.defineClass' is
     called.  If not set, `gcj' will not be invoked by the runtime;
     this can also be controlled via `Compiler.disable'.

`gnu.gcj.jit.options'
     This is a space-separated string of options which should be passed
     to `gcj' when in JIT mode.  If not set, a sensible default is
     chosen.

`gnu.gcj.jit.cachedir'
     This is the directory where cached shared library files are
     stored.  If not set, JIT compilation is disabled.  This should
     never be set to a directory that is writable by any other user.

`gnu.gcj.precompiled.db.path'
     This is a sequence of file names, each referring to a file created
     by `gcj-dbtool'.  These files will be used by `libgcj' to find
     shared libraries corresponding to classes that are loaded from
     bytecode.  `libgcj' often has a built-in default database; it can
     be queried using `gcj-dbtool -p'.


File: gcj.info,  Node: Resources,  Next: Index,  Prev: System properties,  Up: Top

13 Resources
************

While writing `gcj' and `libgcj' we have, of course, relied heavily on
documentation from Sun Microsystems.  In particular we have used The
Java Language Specification (both first and second editions), the Java
Class Libraries (volumes one and two), and the Java Virtual Machine
Specification.  In addition we've used the online documentation at
`http://java.sun.com/'.

   The current `gcj' home page is `http://gcc.gnu.org/java/'.

   For more information on gcc, see `http://gcc.gnu.org/'.

   Some `libgcj' testing is done using the Mauve test suite.  This is a
free software Java class library test suite which is being written
because the JCK is not free.  See `http://sources.redhat.com/mauve/'
for more information.

File: gcj.info,  Node: Index,  Prev: Resources,  Up: Top

Index
*****

[index]
* Menu:

* class path:                            Input Options.        (line  6)
* class$:                                Reference types.      (line 20)
* elements on template<class T>:         Arrays.               (line 46)
* FDL, GNU Free Documentation License:   GNU Free Documentation License.
                                                               (line  6)
* GCJ_PROPERTIES:                        Extensions.           (line 56)
* jclass:                                Reference types.      (line 16)
* jobject:                               Reference types.      (line 16)
* jstring:                               Reference types.      (line 16)
* JvAllocBytes:                          Mixing with C++.      (line 99)
* JvAttachCurrentThread:                 Invocation.           (line 55)
* JvCreateJavaVM:                        Invocation.           (line 11)
* JvDetachCurrentThread:                 Invocation.           (line 68)
* JvFree:                                Memory allocation.    (line 19)
* JvGetArrayLength:                      Arrays.               (line 86)
* JvGetStringChars:                      Strings.              (line 25)
* JvGetStringUTFLength:                  Strings.              (line 29)
* JvGetStringUTFRegion:                  Strings.              (line 34)
* JvMalloc:                              Memory allocation.    (line 11)
* JvNewBooleanArray:                     Arrays.               (line 83)
* JvNewObjectArray:                      Arrays.               (line 57)
* JvNewString:                           Strings.              (line 11)
* JvNewStringLatin1:                     Strings.              (line 15)
* JvNewStringUTF:                        Strings.              (line 21)
* JvPrimClass:                           Primitive types.      (line 36)
* JvRealloc:                             Memory allocation.    (line 15)