Native C code calling from Java Swing GUI

Question

I am trying to read and write to the parallel port, I implemented the writing and reading in C, now I want to import that code into a java GUI application. I managed to include the C .so file into the java project and when calling the functions directly in the Java solutions main() method they work just fine.

I tried to call the native functions when a button is pressed, but it won't work, the application crashes. I am running the application as root, root priviledge is needed to change and read the parallel ports values.

How I am trying to call the native function:

 private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
       try
       {
           int portNR=Integer.parseInt(jTextField1.getText());
           int value=Integer.parseInt(jTextField2.getText());
        ParalellComanderApp.setPort(portNR,value );
       }
       catch (Exception e)
       {
           System.err.println(e.getMessage());
       }
    }

The Native function in C:

JNIEXPORT void JNICALL Java_paralellcomander_ParalellComanderApp_setPort
  (JNIEnv *env, jobject obj, jint port, jint value)
{
     outb(value,MAIN_PORT+port); 
     printf("Setting port %d to value %d\n",port,value);
}

Crash message:

    A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007f00adaf9833, pid=6516, tid=139640785835776
#
# JRE version: 6.0_23-b23
# Java VM: OpenJDK 64-Bit Server VM (20.0-b11 mixed mode linux-amd64 compressed oops)
# Derivative: IcedTea6 1.11pre
# Distribution: Ubuntu oneiric (development branch), package 6b23~pre10-0ubuntu5
# Problematic frame:
# C  [libAccessParalel.so+0x833]  inb+0x17
#
# An error report file with more information is saved as:
# /home/bari/NetBeansProjects/ParalellComander/dist/hs_err_pid6516.log
#
# If you would like to submit a bug report, please include
# instructions how to reproduce the bug and visit:
#   https://bugs.launchpad.net/ubuntu/+source/openjdk-6/
# The crash happened outside the Java Virtual Machine in native code.
# See problematic frame for where to report the bug.

Sorry for the long post. Can anybody help me?

Answer 1

Rather than using JNI and running your GUI as root, I would make the C part into a stand alone program (that runs with the bare minimum privileges - i.e. it binds to the parallel port and then revokes all other privileges) which talks to the Java app over a network socket. That way you have more control over what passes between the part that faces the end user and the part that has elevated privileges, and are less vulnerable to attack. It would also be easier to debug and test, because you can test the network communication to the C program using telnet or netcat without even involving the GUI part.