Calling existing C++ dll from native code causes an ACCESS_VIOLATION

Question

Hi everyone,
I'm having issues creating a Java app to call an existing c++ DLL. I've done extensive searching for a solution and found lots of goodies, but nothing to fix my problem yet.
I've gone through the JNI tutorial and created my java class, used javah, and created my native code according to the tutorial.
I'm new to dealing with DLLs so I'm not sure if there are some steps that I'm not taking.
The issue comes when I run the program. I get an EXCEPTION_ACCESS_VIOLATION.
the DLL I'm trying to call makes a call to a device on the USB port. I also have the source code and header files for the DLL but thought it would be easier to just have to deal with the already built DLL.
After putting some print statements in the native function, the violation comes when I try to call the function in the external DLL. The DLL itself loads correctly (as far as I know).
Here my native method that will load the external dll. I got the code to call the DLL method from:
http://goff.nu/techarticles/development/cpp/calldll.html
JNIEXPORT void JNICALL
Java_OCPMControl_OCPMSetPixelCount (JNIEnv *env, jobject obj, jint pixelCount)
//load the dll
HINSTANCE ocpmSerialDLL = LoadLibrary("OCPMSerialDLL");
/* get pointer to the function in the dll*/
FARPROC myDLLFunction = GetProcAddress(HMODULE(ocpmSerialDLL), "OCPMSetPixelCount");
/* Define the Function in the DLL for reuse. This is just prototyping
* the dll's function. A mock of it. Use "stdcall" for maximum compatibility.
typedef void (__stdcall * FUNC)(enum ePixelNum);
FUNC MyFunction;
MyFunction = FUNC(myDLLFunction);
printf("Calling function\n");
/* The actual call to the function contained in the dll */
MyFunction(PIXEL256);
/* Release the Dll */
FreeLibrary(ocpmSerialDLL);
When I run the program, I get the following error:
An unexpected exception has been detected in native code outside the VM.
Unexpected Signal : EXCEPTION_ACCESS_VIOLATION (0xc0000005) occurred at PC=0x182
91E6B
Function=[Unknown.]
Library=C:\sun\MyJava\ocpm\src\OCPMSerialDLL.dll
NOTE: We are unable to locate the function name symbol for the error
just occurred. Please refer to release documentation for possible
reason and solutions.
Current Java thread:
at OCPMControl.OCPMSetPixelCount(Native Method)
at OCPMControl.getData(OCPMControl.java:167)
at OCPMService.main(OCPMService.java:46)
Dynamic libraries:
0x00400000 - 0x00406000 c:\sun\j2sdk1.4.2_03\bin\java.exe
0x77F80000 - 0x77FFD000 C:\WINNT\system32\ntdll.dll
0x7C2D0000 - 0x7C332000 C:\WINNT\system32\ADVAPI32.dll
0x7C570000 - 0x7C628000 C:\WINNT\system32\KERNEL32.DLL
0x77D30000 - 0x77DA1000 C:\WINNT\system32\RPCRT4.DLL
0x78000000 - 0x78045000 C:\WINNT\system32\MSVCRT.dll
0x08000000 - 0x08138000 c:\sun\j2sdk1.4.2_03\jre\bin\client\jvm.dll
0x77E10000 - 0x77E75000 C:\WINNT\system32\USER32.dll
0x77F40000 - 0x77F7E000 C:\WINNT\system32\GDI32.DLL
0x77570000 - 0x775A0000 C:\WINNT\system32\WINMM.dll
0x10000000 - 0x10007000 c:\sun\j2sdk1.4.2_03\jre\bin\hpi.dll
0x007C0000 - 0x007CE000 c:\sun\j2sdk1.4.2_03\jre\bin\verify.dll
0x007D0000 - 0x007E9000 c:\sun\j2sdk1.4.2_03\jre\bin\java.dll
0x007F0000 - 0x007FD000 c:\sun\j2sdk1.4.2_03\jre\bin\zip.dll
0x18270000 - 0x1827E000 C:\sun\MyJava\ocpm\src\OCPMNative.dll
0x18290000 - 0x182AA000 C:\sun\MyJava\ocpm\src\OCPMSerialDLL.dll
0x68120000 - 0x681A1000 C:\sun\MyJava\ocpm\src\instrsup.dll
0x76B30000 - 0x76B6E000 C:\WINNT\system32\comdlg32.dll
0x70A70000 - 0x70AD5000 C:\WINNT\system32\SHLWAPI.DLL
0x71710000 - 0x71794000 C:\WINNT\system32\COMCTL32.DLL
0x782F0000 - 0x78538000 C:\WINNT\system32\SHELL32.DLL
0x77920000 - 0x77943000 C:\WINNT\system32\imagehlp.dll
0x72A00000 - 0x72A2D000 C:\WINNT\system32\DBGHELP.dll
0x690A0000 - 0x690AB000 C:\WINNT\system32\PSAPI.DLL
Heap at VM Abort:
Heap
def new generation total 576K, used 140K [0x10010000, 0x100b0000, 0x104f0000)
eden space 512K, 27% used [0x10010000, 0x10033368, 0x10090000)
from space 64K, 0% used [0x10090000, 0x10090000, 0x100a0000)
to space 64K, 0% used [0x100a0000, 0x100a0000, 0x100b0000)
tenured generation total 1408K, used 0K [0x104f0000, 0x10650000, 0x14010000)
the space 1408K, 0% used [0x104f0000, 0x104f0000, 0x104f0200, 0x10650000)
compacting perm gen total 4096K, used 1012K [0x14010000, 0x14410000, 0x1801000
0)
the space 4096K, 24% used [0x14010000, 0x1410d1f0, 0x1410d200, 0x14410000)
Local Time = Wed May 12 10:53:56 2004
Elapsed Time = 10
# The exception above was detected in native code outside the VM
# Java VM: Java HotSpot(TM) Client VM (1.4.2_03-b02 mixed mode)
# An error report file has been saved as hs_err_pid2120.log.
# Please refer to the file for further information.
Is there something else I have to do to tell the JVM that the external DLL call isn't an Access Violation?
Would it be easier to use the source files instead?
Thanks to anyone who can help me out here!
Scott Campbell

Answer 1

Thanks,
The problem has been solved. I wasn't linking the dll properly to access the external DLL and thus, my native DLL was looking for methods that it didn't know how to find. which was easily solved by adding a few options when creating my native dll.
Here is the link I found to solve the problem incase anyone else has issues like this. It is regarding implicit v.s. explicit linking to external DLLs in using visual C++.
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vccore/html/_core_link_an_executable_to_a_dll.asp
Scott.