Question

Background

I am trying to build a Java program that make use of an existing native library via JNI. There are two components of the native project that each build a shared library:

1. A core component that builds core.lib and core.dll
2. An application component that depends on the core component, and builds application.lib and application.dll

Using SWIG, I’ve created an application_wrap.c file, and corresponding Java files so that I can use JNI to access the native library.

What I would like to do is build a new shared library (lets call it application_jni.dll) containing this application_wrap.c file, so that I can load it in my Java program like this:

System.loadLibrary("application_jni");

Problem

Unfortunately, I can’t seem to figure out how to do this on Windows using the Visual Studio compiler. Here’s what I’ve tried:

First, compiling the application_wrap.c file generated by SWIG. This produces application_wrap.obj, and seems to work just fine.

cl /I "path/to/core/headers" /I "path/to/application/headers" -c application_wrap.c

Next, linking the application_wrap.obj against the existing shared libraries to create a new shared library:

link /dll /out:application_jni.dll application_wrap.obj core.lib application.lib

This results in a large number of errors that look like this:

application_wrap.obj : error LNK2019: unresolved external symbol __imp__function_in_core referenced in function_in_application_wrap

Any idea what could be causing these errors? I don’t have a lot of experience with compiling on Windows, so I wouldn’t be the least bit surprised to find that I’m missing some flags in the compile or link stages (or even misunderstanding how dll’s work).

A Few Extra Notes

Running

dumpbin /exports core.lib

shows that all of the functions mentioned by the error messages are in fact exported by the core shared library.

Edit

At Pavel’s suggestion I wrote a simple C program to see if it would link. I used the same compile and link commands as above, and I ran into the same error.

The C program test.c:

#include "application.h"

int main() {
  function_in_application();
}

The error:

test.obj : error LNK2019: unresolved external symbol __imp__function_in_application referenced in function _main

Answer 1

The libraries failed to link because core.dll and application.dll were compiled for 64 bit, and the default target for cl.exe is 32 bit. The linker failed because i was trying to link a 32 bit obj file to two 64 bit dll's. Compiling everything for 64 bit solved the problem.

A Few Things To Be Aware Of

• If you have the 64 bit compilers installed, you make cl.exe target 64 bit by running the the vcvarsall.bat script with an argument corresponding to your target platform (e.g. “amd64”). More in depth instructions can be found here.
• The default version of Visual Studio Express 2010 does not install the 64 bit compilers by default. You may need to run setup.exe again and check the appropriate custom install options to install them.
• The default version of Visual Studio Express 2012 does come with 64 bit compilers. I uninstalled the 2010 version, installed 2012, and had no problems after that.