"Did you mean 'mature'?"

Google is sometimes amusing. Google proposes 'mature' for 'mathurl' that I discussed in my last post. Also, the sponsored link is funny in this context at least if you understand Finnish.



One more thing about the search. Peculiarly the mathURL site itself isn't appearing as the top result but my post about it.

Lessons on JNI and dynamic libraries in Linux

I was experimenting for the first time with two things: The Java Native Library (JNI) and home-made dynamically linked libraries in Linux. It took some awkward moments until it all worked due to amateurish handling of the subject, and some bad attention. Still, it was rather amusing, and shall be documented.

By the Sun JNI example, I began by creating a Java class (C_Gcd.java) and writing the method declaration for the native method called gcd. Finally I added a static code block to the class that loads the library with System.loadLibrary, which I passed the library name (gcd) as an argument.

Next I generated the C headers (C_Gcd.h) from my Java source file using javah. After that, I created a .cpp file (I decided to use C++) in which the implementation of the native method would be. Next I wrote the actual gcd implementation.

Okay, that was easy. Next we compile the C++ code and make a shared library out of it. Here is what went wrong: I forgot that all dynamic library names are prefixed with 'lib'. I created a file called gcd.so.

Now, thinking everything was going fine, I wrote another Java class that would be using the native gcd method of the C_Gcd class. Now, I noticed that I want the gcd method to be static to avoid instatiating a C_Gcd object. So I happily went and added the static keyword to the gcd method declaration in the Java source file. Here was the second mistake: I forgot that changing the Java method declaration may, and probably will, lead to changes in the output of javah.

Now, believing it should run fine, I compiled everything again and run the Java application but got:

Exception in thread "main" java.lang.UnsatisfiedLinkError: no gcd in java.library.path

Google reminded me of LD_LIBRARY_PATH, but setting it to the current path did not help. I kept having the same error.

Next I went to investigate the system with ldconfig. At some point I realised that not only is it a convention that all libraries are prefixed with lib, they must be! Unfortunately I went and changed the argument of System.loadLibrary to libgcd, which is of course wrong. So here was the third mistake: we refer to a library libfoo with mere foo, but I expressed, using the same example, liblibfoo. I still got the very same error.

After a moment I corrected the lattest glitch, and now, the error message changed:

Exception in thread "main" java.lang.UnsatisfiedLinkError: Gcd.gcd(II)I

It was still an UnsatisfiedLinkError, but it dropped the talk about java.library.path.

Next, feeling desparete, I decided to take a look on the Java bytecode using javap. It looked completely healthy. I decided to take a break.

After the break, I opened the C++ sources again and began reading with no prejudice -- and there it was -- I finally saw that the C function declaration and its definition had been in conflict all the time. Corrected it, recompiled, and, uh, it finally worked!

And here we summarize on how to get JNI working painlessly under Linux:

• Make sure to have LD_LIBRARY_PATH point to the library path. (*)
  
• Make sure to name the library libFoo.so.
• Make sure to refer to the library with mere Foo.
• Be careful when changing the native method signatures.
  

(*) or launch the JVM with: java -Djava.library.path=. YourApplication in case your library is in the current directory.

For completeness, here's the source (with class names cleaned a bit):

== Gcd.java ==

public class Gcd {
    public static native int gcd(int a, int b) throws IllegalArgumentException;
    static {
        System.loadLibrary("Gcd");
    }   
}

== Gcd.h ==

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Gcd */
    
#ifndef _Included_Gcd
#define _Included_Gcd
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     Gcd
 * Method:    gcd
 * Signature: (II)I
 */
JNIEXPORT jint JNICALL Java_Gcd_gcd
  (JNIEnv *, jclass, jint, jint);

#ifdef __cplusplus
}
#endif
#endif

== Gcd.cpp ==

#include "Gcd.h"

JNIEXPORT jint JNICALL
Java_Gcd_gcd(JNIEnv *env, jclass cl, jint a, jint b)
{   
    if (a < 0 || b < 0) {
        jclass ex = env->FindClass( 
                "java/lang/IllegalArgumentException");
        env->ThrowNew(ex, "Both arguments must be positive.");
        env->DeleteLocalRef(ex);
        return -1;
    }
    while (a != b) {
        if (a > b) {
            a -= b;
        } else {
            b -= a;
        }
    }
    return a;
}

== Test.java ==

public class Test {
    public static void main(String[] args) {
        System.out.println("gcd(12,24) = " + Gcd.gcd(12, 24));
    }
}

== Makefile ==

CC=g++
INCLUDE=-I/usr/lib/jvm/java-1.5.0-sun-1.5.0.16/include/linux/ -I/usr/lib/jvm/java-1.5.0-sun-1.5.0.16/include

libGcd.so: Gcd.o
        $(CC) -shared -o libGcd.so Gcd.o

Gcd.o: Gcd.cpp 
        $(CC) $(INCLUDE) -O3 -c Gcd.cpp

clean:
        rm -f libGcd.so Gcd.o

Nokia Ovi barely usable

What is going on with the world's largest mobile phone company? Their strategy is to become big on the net but, man, for me their Ovi is a huge disappointment.

OK, it works fine from my N82 but with Firefox 3 (Linux) some features don't work at all! Yesterday I was trying to define my mobile phone model for Ovi (which should be automatically determined since I had already created an account using my phone's web browser) and the UI just didn't work. I couldn't press "next". The button wasn't there. And another thing, the UI was _really_ slow - not far from unusable.

So I tried the newest Opera and this time the button appeared and I managed to set my mobile model for the system. Still, the system was painfully slow.

What was positive I managed to upload an image to the system from my phone which was my initial goal. And the sync feature worked.

Today I logged in to Ovi again from my desktop - maybe the slowness was just a temporary problem? Nah, it's still slow.

Considering Ovi, Nokia's web technology is insufficient. They should take example from Google on how to make web apps that are platform independent (in my experience, having experimented Google apps from several software and hardware platforms with no problems) and user friendly in easiness of use and responsiveness.

Job Interview Failure

I have been fortunate enough in my career to avoid IT job interviews up to this point. It has been that I have been asked not the other way around. It is quite amusing that I have worked as an interviewer but never been interviewed myself. So yesterday I went to my first IT job interview to see what it is like to be on the other end of the table.

Now I'm going to write what are generally the three most important human features in hiring context.

The most important part of yourself to show in an interview is that you are exceptionally passionate about your subject. There are a lot of people who know how to design and write programs competently, but those with passion are the ones with the greatest chance to stand up and achieve something remarkable. Real passion is something very rare.

The second most important part of yourself is that you are specialized. The greatest companies do not seek people that are generally competent in every aspect, but the people who are the best in some aspect. Usually the passionate are also the most specialized.

The third most important part of yourself is to show that you are not a hoax, that you know what you are talking about. This is the easiest part to do. Everybody with the slightest sophistication can show this. With the passionate this is self-evident.

What comes to my first interview yesterday, I think I emerged as an enthusiast and not as a complete clown. But I made one terrible mistake. I drew myself as a generalist - not a specialist. I did not convince the interviewer, nor did I deserve it. Albeit knowing all this, I somehow managed to fail.

---

So I accepted the research trainee position in TKK.