GUI crashes when calling native code..

Question

Hi,
I have interfaced to an existing C program using the Java Native Interface and I am controlling the C program using a GUI. The problem is that everytime I click on the button to call the native method, the GUI crashes... The bizarre thing is that the C code seems to actually execute properly and no exceptions are thrown..
Anyone come across this problem?
/john

Answer 1

Hi,
Thanks for the replies...
The GUI completely disappears. No error file is generated. No exceptions are thrown. Here it is in more detail..
The C code is invoked using the java native interface. The C code converts a MIDI file to a text file. It is normally run in DOS and accepts two parameters. You would run it in DOS normally as follows:
mf2t Example1.mid Example1.txt.
In the GUI I select the MIDI file and specify the output file (Example1.txt). I then pass these two parameters to the C code. The C code originally used argc and argv to accept the parameters. I replaced these with "fileArray" and "parameter"... "mf2t" replaces "main" in the native code...
On the java side the code looks like this:
static public native boolean mf2t(String s6, String s7);
public String infile; // Input MIDI file
public String textOut; // Output text file
private void MIDIButtonActionPerformed(java.awt.event.ActionEvent evt) {
Object target=evt.getSource();
if(target == MIDIButton) {
if(mf2t(infile,textOut)){
InfoLabel.setText("MIDI to text conversion complete");
The code is built on the C side into a DLL using MS Visual Studio.
On the C side the code looks something like this:
static char *fileArray[5];
static int argument=3;
static char midiArray[25];
static char txtArray[25];
void mf2t(int argument,char * fileArray[]);
// Entry point to native C code is here
JNIEXPORT jboolean JNICALL
Java_MainWindow_mf2t (JNIEnv *env, jobject j1, jstring midi, jstring txt)
const jbyte *midiFile;
const jbyte *txtFile;
midiFile=(*env)->GetStringUTFChars(env,midi, NULL);
txtFile=(*env)->GetStringUTFChars(env,txt, NULL);
// The lines above just convert the java string into a C equivalent..
strcpy(midiArray,midiFile);
strcpy(txtArray,txtFile);
fileArray[0]="mf2t.exe"; // Here I get fileArray to point to the converted strings
fileArray[1]=midiArray;
fileArray[2]=txtArray;
mf2t(argument,fileArray); // Then I pass this to a native method
(*env)->ReleaseStringUTFChars(env,midi, midiFile);
(*env)->ReleaseStringUTFChars(env,txt, txtFile);
return 1;
void mf2t(int argument,char * fileArray[]){
// native code in here
I think it may have something to do with threads.. I'm not sure though.. It's impossible to know whats going on in the C code when the GUI crashes. If anything looks strange it's because I left out some code here for the sake of brevity... Still if you see anything that stands out let me know..
Thanks,
John