How do I get an error message when failing to load a JVM via JNI?

I would like to retrieve an error message that explains why the jvm failed to load. From the examples provided here:

http://java.sun.com/docs/books/jni/html/invoke.html

I extracted this example:

 /* Create the Java VM */
 res = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);

 if (res < 0) {
     // retrieve verbose error here?
     fprintf(stderr, "Can't create Java VM\n");
     exit(1);
 }

In my specific case I'm providing invalid arguments in the vm_args and would expect to see what I get on the command line: "Unrecognized option: -foo=bar"

On further testing it looks like the jvm is putting the message I want to stdout or stderr. I believe I would need to capture stdout and stderr to get the error I'm looking for (unless of course there is a simpler way). I'm coding in C++ so if someone can show a way to capture the error into a stringstream that would be ideal.

Thanks, Randy

From stackoverflow

• When I wrote this code I would also get the error via stdout/stderr.

  The best way to redirect stdout/stderr in your process is by using freopen. Here is a StackOverflow question specifically about that subject.

  However, once that call is passed, you will then have a JNIEnv, and all further error checking can and should be done by calling JNIEnv::ExceptionOccurred(), which will may return a Throwable object that you can then interrogate with your JNI code.

• After getting a hold of stdout and stderr, which you'll need anyway, add -Xcheck:jni to your jvm command line to get extra jni-related warnings from the jvm.

• I was able to get what I needed by using the "vfprintf" option described here:

  http://java.sun.com/products/jdk/faq/jnifaq-old.html

  although I used the jdk1.2 options. This code snippet summarizes my solution:

  static string jniErrors;
  
  static jint JNICALL my_vfprintf(FILE *fp, const char *format, va_list args)
  {
      char buf[1024];
      vsnprintf(buf, sizeof(buf), format, args);
      jniErrors += buf;
      return 0;
  }
  
  ...
  
  JavaVMOption options[1];
  options[0].optionString = "vfprintf";
  options[0].extraInfo = my_vfprintf;
  
  JavaVMInitArgs vm_args;
  memset(&vm_args, 0, sizeof(vm_args));
  vm_args.nOptions = 1;
  vm_args.options = options;
  vm_args.version = JNI_VERSION_1_4;
  vm_args.ignoreUnrecognized = JNI_FALSE;
  
  JNIEnv env;
  JavaVM jvm;
  
  jint res = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);
  
  if (res != JNI_OK)
      setError(jniErrors);
  
  jniErrors.clear();
  

  Also interesting was that I could not for the life of me capture stdout or stderr correctly with any of the freopen or dup2 tricks. I could load my own dll and redirect correctly but not the jvm. Anyway, it's better this way because I wanted the errors in memory rather than in a file.