Re: Redirecting IO from C++ native library

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <>
Sat, 16 Sep 2006 23:52:29 -0400
Josef Svitak wrote:

Chris Uppal wrote:

I doubt if that -- or anything else -- will work.

Right. I think I've come around to that view...

If you have access to the same C/C++ compiler
as the DLL you want to capture from, then there
may be some possibilities.

Look at this example. I am using Windows and
Mingw C compiler.

First we emulate the current DLL you have.

public class Nat1 {
     public native void hello();
     static {


#include <stdio.h>

#include <jni.h>

#include "Nat1.h"

JNIEXPORT void JNICALL Java_Nat1_hello(JNIEnv *cntx, jobject me)
     printf("Hello world\n");

A true classic.

Now to capture the output from this DLL we create a new DLL.

public class Nat2 {
     public native void init();
     public native String done();
     static {


#include <stdio.h>

#include <fcntl.h>
#include <io.h>

#include <jni.h>

#include "Nat1.h"

#define MAXBUF 20480

static int oldstdout;
static int pipehandles[2];
static char buf[MAXBUF];

JNIEXPORT void JNICALL Java_Nat2_init(JNIEnv *cntx, jobject me)
     oldstdout = _dup(_fileno(stdout));

JNIEXPORT jstring JNICALL Java_Nat2_done(JNIEnv *cntx, jobject me)
     int n;
     n = _read(pipehandles[0],buf,sizeof(buf));
     buf[n] = '\0';
     return (*cntx)->NewStringUTF(cntx,buf);

Now we just need a test program.

public class TestProgram {
     public static void main(String[] args) throws Exception {
         Nat2 n2 = new Nat2();
         Nat1 n1 = new Nat1();
         System.out.println("#" + n2.done() + "#");

Building and running:

javac -classpath .
javah -classpath . -jni Nat1
gcc -c -I\sunjava\jdk1.5.0\include -I\sunjava\jdk1.5.0\include\win32
Nat1.c -o Nat1.obj
gcc -s -shared -Wl,--export-all,--kill-at Nat1.obj -o Nat1.dll
javac -classpath .
javah -classpath . -jni Nat2
gcc -c -I\sunjava\jdk1.5.0\include -I\sunjava\jdk1.5.0\include\win32
Nat2.c -o Nat2.obj
gcc -s -shared -Wl,--export-all,--kill-at Nat2.obj -o Nat2.dll
javac -classpath .
java -classpath . TestProgram

And the output:

#Hello world
Hello world
Hello world

The method should work with C++ IO as well (cout instead of stdout).

This technique requires:
   - DLL's being dynamicly linked against the C/C++ RTL
   - DLL's being compiled with the same compiler

So it may not be possible for you.

But on the other hand it may be possible !


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