UnsatisfiedLinkErrors with a jar file, JNI and C++

I’ve been busy porting legacy C code to C++. Yeah, real fun. I also have a java gui which calls the C code using the Java Native Interface (JNI). This layer also had to be updated in order to use the C++ code. Everything worked great on windows. I then had to make sure everything built and ran correctly on linux and solaris. I am just not a linux or solaris person. Never have been, never will be. However, I did discover something useful.
I built my jar file and shared objects relatively easily. Next I tried running the jar file. The jar file linked to my shared object ok, but it couldn’t find the function being called. I kept getting an UnsatisfiedLinkError. I guessed the reason was due to C++ name mangling.
Javah is used to create the jni header files. When the header files are created, javah automatically adds:

#ifdef __cplusplus
extern “C” {

to prevent C++ from mangling function names. I believe the command I used to look at the function names in the shared object was nm. Weird thing is, the names kind of looked mangled. I did some research on the internet, but found nothing useful. Everything worked fine on Windows, the header files use #ifdef cplusplus to prevent name mangling, so how could the function names be mangled? I don’t know. I was desperate, so took the #ifdef cplusplus and put it in the C++ source file too. Right at the top, after the includes and before the first function. I ended the #ifdef cplusplus at the very end of the file. I recompiled and tried running the jar file again. What do you know? No more UnsatisfiedLinkErrors. The jar file found the functions in the shared object just fine. Thank goodness.

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