The JNI GetEnv function is supposed to return JNI error codes which
are all negative numbers. Thus,

   if ((jvm->GetEnv((void **)&interface, version)) < 0) {
      ... error processing ...

is the standard usage pattern. For JVMDI,

   if ((jvm->GetEnv((void **)&jvmdi, JVMDI_VERSION_1)) < 0) {
      ... error processing ...

the error test will fail and an uninitialized interface pointer
will be returned if debugging is not enabled,

    } else if (version == JVMDI_VERSION_1) {
      if (jvmdi::enabled()) {
        *penv = (void *)jvmdi::GetInterface_1(vm);
        return JNI_OK;
      } else {
        return JVMDI_ERROR_ACCESS_DENIED;
      }

since the code returns a JVMDI error code which is a positive
number and is not a JNI error code as specified. Note also that
current versions (JVMDI_VERSION_1, JVMDI_VERSION_1_1, ...) cannot
be specified (see how JNI and JVMPI handle this).