JNI with parameters on Linux

[paul wheaton]

Well, I got JNI to work fine on Linux as long as I don't do anything interesting.
I'm now trying to pass in a string and get a string back. My java program takes a string from the command line and sends it to the JNI. The JNI reverses the string and returns it. The java program prints the results.
If I run the program with no parameters, I get the default message. If I pass a parameter, I get some sort of crash. Here is what I have:

----------------------- Reverse.java

class Reverse 
{
    public native String reverseString( String s );
    static 
    {
        System.loadLibrary("reverse");
    }
    public static void main( String[] args )
    {
        if ( args.length == 1 )
        {
            Reverse r = new Reverse();
            System.out.println( r.reverseString( args[0] ) );
        }
        else
        {
            System.out.println("how bout a string pal");
        }
    }
}

---------------------- ReverseImp.c
#include <jni.h>
#include "Reverse.h"
#include <stdio.h>
#define BUFFY_LEN 1000
    
                                
JNIEXPORT jstring JNICALL Java_Reverse_reverseString( JNIEnv* env , jobject obj , jstring js )     
{
    char buffy[ BUFFY_LEN ];
    int i ;
    const char* s = (*env)->GetStringUTFChars( env , js , 0 );
    buffy[ BUFFY_LEN - 1 ] = '\0' ;
    i = BUFFY_LEN - 2 ; // where to put the first character
    while( *s != '\0' )
    {
        buffy[ i ] = *s ;
        s++ ;
        i-- ;
    }
    (*env)->ReleaseStringUTFChars( env, js , s );
    s = &buffy[ i + 1 ];
    return (*env)->NewStringUTF( env , buffy );
}

------------------------- what I get when I run it
[root@localhost jni]# java Reverse TheTick
SIGSEGV   11*  segmentation violation
        stackpointer=0xbffff284
Full thread dump Classic VM (Linux_JDK_1.2_pre-release-v2, native threads):
    "Finalizer" (TID:0x404c2320, sys_thread_t:0x80cc7d0, state:CW, native ID:0xc
04) prio=8
        at java.lang.Object.wait(Native Method)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:112)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:127)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:174)
    "Reference Handler" (TID:0x404c23b0, sys_thread_t:0x80c8018, state:CW, nativ
e ID:0x803) prio=10
        at java.lang.Object.wait(Native Method)
        at java.lang.Object.wait(Object.java:424)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:114)
    "SIGQUIT handler" (TID:0x404c23e0, sys_thread_t:0x80c0ab0, state:R, native I
D:0x402) prio=5
    "main" (TID:0x404c21e0, sys_thread_t:0x804c1b8, state:R, native ID:0x400) pr
io=5
        at Reverse.reverseString(Native Method)
        at Reverse.main(Compiled Code)
Monitor Cache Dump:
    java.lang.ref.Reference$Lock@404C23C0/404F7B20: <unowned>
        Waiting to be notified:
            "Reference Handler" (0x80c8018)
    java.lang.ref.ReferenceQueue$Lock@404C2338/404F8028: <unowned>
        Waiting to be notified:
            "Finalizer" (0x80cc7d0)
Registered Monitor Dump:
    PCMap lock: <unowned>
    utf8 hash table: <unowned>
    JNI pinning lock: <unowned>
    JNI global reference lock: <unowned>
    BinClass lock: <unowned>
    Class linking lock: <unowned>
    System class loader lock: <unowned>
    Code rewrite lock: <unowned>
    Heap lock: <unowned>
    Monitor cache lock: owner "main" (0x804c1b8) 1 entry
    Thread queue lock: owner "main" (0x804c1b8) 1 entry
    Monitor registry: owner "main" (0x804c1b8) 1 entry
   
-------------------------------------------  

Can anyone tell me what I'm doing wrong?

[paul wheaton]

To answer my own question ...
I change the value of s before passing it back to ReleaseStringUTFChars(), so it has no idea what to release.
It turns out that there was a second flaw in the c program: I was returning buffy instead of s.