303 // The function below is actually exposed by sun.misc.VM and not

   304 // java.lang.System, but we choose to keep it here so that it stays next

   305 // to JVM_CurrentTimeMillis and JVM_NanoTime

   306 

   307 const jlong MAX_DIFF_SECS = 0x0100000000;   //  2^32

   308 const jlong MIN_DIFF_SECS = -MAX_DIFF_SECS; // -2^32

   309 

   310 JVM_LEAF(jlong, JVM_GetNanoTimeAdjustment(JNIEnv *env, jclass ignored, jlong offset_secs))

   311   JVMWrapper("JVM_GetNanoTimeAdjustment");

   312   jlong seconds;

   313   jlong nanos;

   314 

   315   os::javaTimeSystemUTC(seconds, nanos);

   316 

   317   // We're going to verify that the result can fit in a long.

   318   // For that we need the difference in seconds between 'seconds'

   319   // and 'offset_secs' to be such that:

   320   //     |seconds - offset_secs| < (2^63/10^9)

   321   // We're going to approximate 10^9 ~< 2^30 (1000^3 ~< 1024^3)

   322   // which makes |seconds - offset_secs| < 2^33

   323   // and we will prefer +/- 2^32 as the maximum acceptable diff

   324   // as 2^32 has a more natural feel than 2^33...

   325   //

   326   // So if |seconds - offset_secs| >= 2^32 - we return a special

   327   // sentinel value (-1) which the caller should take as an

   328   // exception value indicating that the offset given to us is

   329   // too far from range of the current time - leading to too big

   330   // a nano adjustment. The caller is expected to recover by

   331   // computing a more accurate offset and calling this method

   332   // again. (For the record 2^32 secs is ~136 years, so that

   333   // should rarely happen)

   334   //

   335   jlong diff = seconds - offset_secs;

   336   if (diff >= MAX_DIFF_SECS || diff <= MIN_DIFF_SECS) {

   337      return -1; // sentinel value: the offset is too far off the target

   338   }

   339 

   340   // return the adjustment. If you compute a time by adding

   341   // this number of nanoseconds along with the number of seconds

   342   // in the offset you should get the current UTC time.

   343   return (diff * (jlong)1000000000) + nanos;

   344 JVM_END