Generating .dll files for JNI under Windows/Cygwin/GCC

July 28, 2004 at 2:51 am · Filed under Java, Windows

In Windows you can use the Cygwin GCC compiler/linker to produce the .dll files needed for JNI (Java Native Interface) applications. Take for example the JNI HelloWorld example from Sun.

But there seem to be several obstacles. First I got compiler errors (concerning undefined jlong type) and after resolving this I wondered how to produce a Win32 native .dll file.

The solution is as follows (my environment is J2SE 1.4.2-b4, Cygwin GCC 3.3.1):

Changes to JDK headers needed for GCC

(Take from here) GCC doesn’t have a __int64 built-in, and this patch basically uses “long long” instead.

1. Edit the file /include/win32/jni_md.h, Where   is the installation root (eg., c:/jdk1.4.2).

2. Replace the segment:

   typedef long jint;
typedef __int64 jlong;
typedef signed char jbyte;

   with:

   typedef long jint;
#ifdef __GNUC__typedef long long jlong;
#else
typedef __int64 jlong;
#endif
typedef signed char jbyte;

Modify the GCC spec file

This modification is only important if the next step (compiling and linking) produces an linker error “dllcrt2.o not found”.

(Take from here) In the Cygwin shell edit the file “/lib/gcc-lib/i686-pc-cygwin/3.3.1/specs” (replace 3.3.1 with your GCC version). Find the text “dllcrt2” and replace it with the absolute (Cygwin) path to dllcrt2.o, e.g. “/lib/mingw/dllcrt2”.

Compile and Link the .dll file

(Take from here) Call the compiler:

gcc -L /lib/mingw -Wl,--kill-at -mno-cygwin -shared -I /include/ -I /include/win32/ HelloWorldImpl.c -o hello.dll

Replace with the Cygwin path to your J2SE, e.g. “/cygdrive/c/Java/jdk1.4.2”.

That’s it. Now I get “Hello World!”. Fine.

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