[wpiutil] Add GetSystemTime() (#3840)

This portably gets the time in microseconds since the Unix epoch.

wpiutil/src/main/java/edu/wpi/first/util/WPIUtilJNI.java

@@ -62,6 +62,8 @@ public final class WPIUtilJNI {
 
   public static native long now();
 
+  public static native long getSystemTime();
+
   public static native void addPortForwarder(int port, String remoteHost, int remotePort);
 
   public static native void removePortForwarder(int port);

wpiutil/src/main/native/cpp/jni/WPIUtilJNI.cpp

@@ -116,6 +116,18 @@ Java_edu_wpi_first_util_WPIUtilJNI_now
   }
 }
 
+/*
+ * Class:     edu_wpi_first_util_WPIUtilJNI
+ * Method:    getSystemTime
+ * Signature: ()J
+ */
+JNIEXPORT jlong JNICALL
+Java_edu_wpi_first_util_WPIUtilJNI_getSystemTime
+  (JNIEnv*, jclass)
+{
+  return wpi::GetSystemTime();
+}
+
 /*
  * Class:     edu_wpi_first_util_WPIUtilJNI
  * Method:    addPortForwarder

wpiutil/src/main/native/cpp/timestamp.cpp

@@ -16,7 +16,7 @@
 #endif
 
 // offset in microseconds
-static uint64_t zerotime() noexcept {
+static uint64_t time_since_epoch() noexcept {
 #ifdef _WIN32
   FILETIME ft;
   uint64_t tmpres = 0;
@@ -35,7 +35,7 @@ static uint64_t zerotime() noexcept {
 #else
   // 1-us intervals
   return std::chrono::duration_cast<std::chrono::microseconds>(
-             std::chrono::high_resolution_clock::now().time_since_epoch())
+             std::chrono::system_clock::now().time_since_epoch())
       .count();
 #endif
 }
@@ -66,7 +66,7 @@ static uint64_t update_frequency() {
 }
 #endif
 
-static const uint64_t zerotime_val = zerotime();
+static const uint64_t zerotime_val = time_since_epoch();
 static const uint64_t offset_val = timestamp();
 #ifdef _WIN32
 static const uint64_t frequency_val = update_frequency();
@@ -96,6 +96,10 @@ uint64_t wpi::Now() {
   return (now_impl.load())();
 }
 
+uint64_t wpi::GetSystemTime() {
+  return time_since_epoch();
+}
+
 extern "C" {
 
 uint64_t WPI_NowDefault(void) {
@@ -110,4 +114,8 @@ uint64_t WPI_Now(void) {
   return wpi::Now();
 }
 
+uint64_t WPI_GetSystemTime(void) {
+  return wpi::GetSystemTime();
+}
+
 }  // extern "C"

wpiutil/src/main/native/include/wpi/timestamp.h

@@ -33,6 +33,14 @@ void WPI_SetNowImpl(uint64_t (*func)(void));
  */
 uint64_t WPI_Now(void);
 
+/**
+ * Return the current system time in microseconds since the Unix epoch
+ * (January 1st, 1970 00:00 UTC).
+ *
+ * @return Time in microseconds.
+ */
+uint64_t WPI_GetSystemTime(void);
+
 #ifdef __cplusplus
 }  // extern "C"
 #endif
@@ -62,6 +70,14 @@ void SetNowImpl(uint64_t (*func)());
  */
 uint64_t Now(void);
 
+/**
+ * Return the current system time in microseconds since the Unix epoch
+ * (January 1st, 1970 00:00 UTC).
+ *
+ * @return Time in microseconds.
+ */
+uint64_t GetSystemTime();
+
 }  // namespace wpi
 #endif