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GCJ(1)                                GNU                               GCJ(1)



NAME
       gcj - Ahead-of-time compiler for the Java language

SYNOPSIS
       gcj [-Idir...] [-d dir...]
           [--CLASSPATH=path] [--classpath=path]
           [-foption...] [--encoding=name]
           [--main=classname] [-Dname[=value]...]
           [-C] [--resource resource-name] [-d directory]
           [-Wwarn...]
           sourcefile...

DESCRIPTION
       As gcj is just another front end to gcc, it supports many of the same options as gcc.    This manual only
       documents the options specific to gcj.

OPTIONS
   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.)

   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.

   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.

   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".

   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>

   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.

   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.

SEE ALSO
       gcc(1), gcjh(1), gjnih(1), gij(1), jcf-dump(1), gfdl(7), and the Info entries for gcj and gcc.

COPYRIGHT
       Copyright (c) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free Software Foundation, Inc.

       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, the Front-Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see
       below).  A copy of the license is included in the man page gfdl(7).

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

            A GNU Manual

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

            You have freedom to copy and modify this GNU Manual, like GNU
            software.  Copies published by the Free Software Foundation raise
            funds for GNU development.



gcc-4.4.7                         2012-03-13                            GCJ(1)