GCJ(1) GNU GCJ(1)
gcj - Ahead-of-time compiler for the Java language
gcj [-Idir...] [-d dir...]
[-C] [--resource resource-name] [-d directory]
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.
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:
Java source files.
Java bytecode files.
An archive containing one or more ".class" files, all of which are compiled. The ar-
chive may be compressed.
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.
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 com-
piled 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.)
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 sev-
eral 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 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.
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 imported, we recommend always using "-I" instead of the other options for manipu-
lating the class 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.
Deprecated synonym for "--classpath".
Where to find the standard builtin classes, such as "java.lang.String".
For each directory in the path, place the contents of that directory at the end of the
This is an environment variable which holds a list of paths.
The final class path is constructed like so:
o First come all directories specified via "-I".
o 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.
o If "--bootclasspath" was specified, append its value. Otherwise, append the built-in
system directory, libgcj.jar.
o Finaly, 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
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.)
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.
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 inter-
nally, so if you choose to use this for your source files you can be assured that it will
work on every host.
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 oppo-
site of what is listed.
With this flag, gcj will warn about redundant modifiers. For instance, it will warn
if an interface method is declared "public".
This causes gcj to warn about empty statements. Empty statements have been depre-
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.
This is the same as gcc's "-Wunused".
This is the same as "-Wredundant-modifiers -Wextraneous-semicolon -Wunused".
In addition to the many gcc options controlling code generation, gcj has several options
specific to itself.
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. 
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.
-C This option is used to tell gcj to generate bytecode (.class files) rather than object
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.
When used with "-C", this causes all generated .class files to be put in the appropri-
ate subdirectory of directory. By default they will be put in subdirectories of the
current working directory.
By default, gcj generates code which checks the bounds of all array indexing opera-
tions. With this option, these checks are omitted, which can improve performance for
code that uses arrays extensively. Note that this can result in unpredictable behav-
ior 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 "ArrayIn-
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".
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.
When the optimization level is greather 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.
Some gcj code generations options affect the resulting ABI, and so can only be meaning-
fully given when "libgcj", the runtime package, is configured. "libgcj" puts the appro-
priate 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
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.
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.
On some systems, a library routine is called to perform integer division. This is
required to get exception handling correct when dividing by zero.
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.
1. 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.
gcc(1), gcjh(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7), and the Info entries for gcj
Copyright (C) 2001, 2002 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 1.1 or any later version published by the Free
Software Foundation; with the Invariant Sections being ``GNU General Public License'', 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).
gcc-3.2.2 2003-02-25 GCJ(1)