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8. Common problems and their solutions

8.1 Java compilers and the chroot

Java is difficult to deal with in an automatic way. It is probably most preferable to use Oracle (previously Sun) Java, because that's the version contestants will be used to. The GNU Compiler for Java (GCJ) is easier to deal with but may lack some features.

With the default configuration, submitted programs are run within a minimal chroot environment. For this the programs have to be statically linked, because they do not have access to shared libraries.

For most languages compilers support this, but for Java, this is a bit problematic. The Oracle Java compiler `javac' is not a real compiler: a bytecode interpreter `java' is needed to run the binaries and thus this cannot simply run in a chroot environment.

There are some options to support Java as a language:

  1. One can build a bigger chroot environment which contains all necessary ingredients to let Java work within it. DOMjudge supports this with some manual setup. First of all, a chroot tree with Java support must be created. The script bin/dj_make_chroot creates one from Debian GNU/Linux sources; run that script without arguments for basic usage information. Next, edit the script lib/judge/chroot-startstop.sh and adapt it to work with your local system and uncomment the script in etc/judgehost-config.php.
  2. As an alternative the gcj compiler from GNU can be used instead of Oracle's version. This one generates true machine code and can link statically. However a few function calls cannot be linked statically (see `GCJ compiler warnings' in this FAQ). Secondly, the static library libgcj.a doesn't seem to be included in all GNU/Linux distributions: at least not in RedHat Enterprise Linux 4.
  3. One can disable the chroot environment in etc/judgehost-config.php by disabling USE_CHROOT. Disabling the chroot environment removes this layer of security against submissions that attempt to cheat, but it is a simple solution to getting Java to work, for demo or testing purposes. No guarantees about system security can be made when running a contest with chroot disabled.

8.2 The Java virtual machine (jvm) and memory limits

DOMjudge imposes memory limits on submitted solutions. These limits are imposed before the compiled submissions are started. On the other hand, the Java virtual machine is started via a compile-time generated script which is run as a wrapper around the program. This means that the memory limits imposed by DOMjudge are for the jvm and the running program within it. As the jvm uses approximately 300MB, this reduces the limit by this significant amount. See judge/compile_java_javac.sh for the implementation details.

If you see error messages of the form


Error occurred during initialization of VM
java.lang.OutOfMemoryError: unable to create new native thread

or
Error occurred during initialization of VM
Could not reserve enough space for object heap

Then the problem is probably that the jvm needs more memory than what is reserved by the Java compile script. You should try to increase the MEMRESERVED variable in judge/compile_java.sh and check that the configuration variable memory limit is set larger than MEMRESERVED. If that does not help, you should try to increase the configuration variable process limit (since the JVM uses a lot of processes for garbage collection).

8.3 Java class naming

Java requires a specific naming of the main class. When declaring the main class public, the filename must match the class name. Therefore one should not declare the main class public; from experience however, many teams do so. Secondly, the Java compiler generates a bytecode file depending on the class name. There are two ways to handle this.

The simplest Java compile script compile_java_javac.sh requires the main class to be named Main with method


public static void main(String args[])

The alternative (and default) is to use the script compile_java_javac_detect.sh, which automatically detects the main class and even corrects the source filename when it is declared public.

When using the GNU gcj compiler, the same holds and two similar scripts compile_java_gcj.sh and compile_java_gcj_detect.sh are available.

8.4 GCJ compiler warnings

When using the GNU GCJ compiler for compiling Java sources, it can give a whole lot of warning messages of the form

/usr/lib/gcc-lib/i386-linux/3.2.3/libgcj.a(gc_dlopen.o)(.text+0xbc):
In function `GC_dlopen': Using 'dlopen' in statically linked
applications requires at runtime the shared libraries from the glibc
version used for linking

These are generated because you are trying to compile statically linked sources, but some functions can not be static, e.g. the `dlopen' function above. These are warnings and can be safely ignored, because under normal programming contest conditions people are not allowed to use these functions anyway (and they are not accessible within the chroot-ed environment the program is run in).

To filter these warnings, take a look at judge/compile_java_gcjmod.sh and replace or symlink judge/compile_java.sh by/to this file.

8.5 C#/mono support

Using the mono compiler and runtime for C# gives rise to similar problems as with Java. Although the C# language has been added to DOMjudge, there's no support yet to run it within a chroot environment. So in that case, USE_CHROOT must be disabled.

8.6 Memory limit errors in the web interface

When uploading large testdata files, one can run into an error in the jury web interface of the form:

Fatal error: Allowed memory size of XX bytes exhausted (tried to
allocate YY bytes) in /home/domjudge/system/lib/lib.database.php
on line 154
This means that the PHP engine has run out of memory. The solution is to raise the memory limits for PHP. This can be done by either editing etc/apache.conf and raising the memory_limit, upload_max_filesize and post_max_size values to well above the size of your largest testcase. You can change these parameters under the jury directory or by directly editing the global Apache or php.ini configuration. Note also that max_file_uploads must be larger than the maximum number of testcases per problem to be able to upload and edit these in the web interface.

The optional PHP Suhosin module may also impose additional limits; check your error logging to see if these are triggered. You may also need to raise MySQL's max_allowed_packet parameter in /etc/mysql/my.cnf on both server and client.

8.7 Compiler errors: `runguard: root privileges not dropped'

Compiling failed with exitcode 255, compiler output:
/home/domjudge/system/bin/runguard: root privileges not dropped
When the above error occurs on submitting any source, this indicates that you are running the judgedaemon as root user. You should not run any part of DOMjudge as root; the parts that require it will gain root by themselves through sudo. Either run it as yourself or, probably better, create dedicated a user domjudge under which to install and run everything.

Also do not confuse this with the domjudge-run user: this is a special user to run submissions as and should also not be used to run normal DOMjudge processes; this user is only for internal use.


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