Add functionality to flush Rx/Tx serial buffers

src/main/c/Posix/SerialPort_Posix.c

@@ -2,7 +2,7 @@
  * SerialPort_Posix.c
  *
  *       Created on:  Feb 25, 2012
- *  Last Updated on:  Dec 01, 2021
+ *  Last Updated on:  Dec 07, 2021
  *           Author:  Will Hedgecock
  *
  * Copyright (C) 2012-2021 Fazecast, Inc.
@@ -33,6 +33,7 @@
 #include <sys/ioctl.h>
 #include <sys/time.h>
 #include <termios.h>
+#include <time.h>
 #include <unistd.h>
 #if defined(__linux__)
 #include <linux/serial.h>
@@ -52,6 +53,7 @@ jfieldID eventListenerRunningField;
 jfieldID disableConfigField;
 jfieldID isDtrEnabledField;
 jfieldID isRtsEnabledField;
+jfieldID autoFlushIOBuffersField;
 jfieldID baudRateField;
 jfieldID dataBitsField;
 jfieldID stopBitsField;
@@ -135,6 +137,7 @@ JNIEXPORT void JNICALL Java_com_fazecast_jSerialComm_SerialPort_initializeLibrar
 	disableConfigField = (*env)->GetFieldID(env, serialCommClass, "disableConfig", "Z");
 	isDtrEnabledField = (*env)->GetFieldID(env, serialCommClass, "isDtrEnabled", "Z");
 	isRtsEnabledField = (*env)->GetFieldID(env, serialCommClass, "isRtsEnabled", "Z");
+	autoFlushIOBuffersField = (*env)->GetFieldID(env, serialCommClass, "autoFlushIOBuffers", "Z");
 	baudRateField = (*env)->GetFieldID(env, serialCommClass, "baudRate", "I");
 	dataBitsField = (*env)->GetFieldID(env, serialCommClass, "dataBits", "I");
 	stopBitsField = (*env)->GetFieldID(env, serialCommClass, "stopBits", "I");
@@ -185,6 +188,7 @@ JNIEXPORT jlong JNICALL Java_com_fazecast_jSerialComm_SerialPort_openPortNative(
 	const char *portName = (*env)->GetStringUTFChars(env, portNameJString, NULL);
 	unsigned char disableExclusiveLock = (*env)->GetBooleanField(env, obj, disableExclusiveLockField);
 	unsigned char disableAutoConfig = (*env)->GetBooleanField(env, obj, disableConfigField);
+	unsigned char autoFlushIOBuffers = (*env)->GetBooleanField(env, obj, autoFlushIOBuffersField);
 
 	// Ensure that the serial port still exists and is not already open
 	serialPort *port = fetchPort(&serialPorts, portName);
@@ -220,6 +224,13 @@ JNIEXPORT jlong JNICALL Java_com_fazecast_jSerialComm_SerialPort_openPortNative(
 				errno = 0;
 			port->handle = -1;
 		}
+		else if (autoFlushIOBuffers)
+		{
+			// Sleep to workaround kernel bug about flushing immediately after opening
+			const struct timespec sleep_time = { 0, 10000000 };
+			nanosleep(&sleep_time, NULL);
+			Java_com_fazecast_jSerialComm_SerialPort_flushRxTxBuffers(env, obj, (jlong)(intptr_t)port);
+		}
 	}
 	else
 		port->errorNumber = errno;
@@ -427,6 +438,18 @@ JNIEXPORT jboolean JNICALL Java_com_fazecast_jSerialComm_SerialPort_configTimeou
 	return JNI_TRUE;
 }
 
+JNIEXPORT jboolean JNICALL Java_com_fazecast_jSerialComm_SerialPort_flushRxTxBuffers(JNIEnv *env, jobject obj, jlong serialPortPointer)
+{
+	serialPort *port = (serialPort*)(intptr_t)serialPortPointer;
+	if (tcflush(port->handle, TCIOFLUSH))
+	{
+		port->errorLineNumber = __LINE__ - 2;
+		port->errorNumber = errno;
+		return JNI_FALSE;
+	}
+	return JNI_TRUE;
+}
+
 JNIEXPORT jint JNICALL Java_com_fazecast_jSerialComm_SerialPort_waitForEvent(JNIEnv *env, jobject obj, jlong serialPortPointer)
 {
 	// Initialize the local variables

src/main/c/Posix/com_fazecast_jSerialComm_SerialPort.h

@@ -129,6 +129,14 @@ JNIEXPORT jboolean JNICALL Java_com_fazecast_jSerialComm_SerialPort_configPort
 JNIEXPORT jboolean JNICALL Java_com_fazecast_jSerialComm_SerialPort_configTimeouts
   (JNIEnv *, jobject, jlong, jint, jint, jint, jint);
 
+/*
+ * Class:     com_fazecast_jSerialComm_SerialPort
+ * Method:    flushRxTxBuffers
+ * Signature: (J)Z
+ */
+JNIEXPORT jboolean JNICALL Java_com_fazecast_jSerialComm_SerialPort_flushRxTxBuffers
+  (JNIEnv *, jobject, jlong);
+
 /*
  * Class:     com_fazecast_jSerialComm_SerialPort
  * Method:    waitForEvent

src/main/c/Windows/SerialPort_Windows.c

@@ -2,7 +2,7 @@
  * SerialPort_Windows.c
  *
  *       Created on:  Feb 25, 2012
- *  Last Updated on:  Dec 06, 2021
+ *  Last Updated on:  Dec 07, 2021
  *           Author:  Will Hedgecock
  *
  * Copyright (C) 2012-2021 Fazecast, Inc.
@@ -49,6 +49,7 @@ jfieldID eventListenerRunningField;
 jfieldID disableConfigField;
 jfieldID isDtrEnabledField;
 jfieldID isRtsEnabledField;
+jfieldID autoFlushIOBuffersField;
 jfieldID baudRateField;
 jfieldID dataBitsField;
 jfieldID stopBitsField;
@@ -366,6 +367,7 @@ JNIEXPORT void JNICALL Java_com_fazecast_jSerialComm_SerialPort_initializeLibrar
 	disableConfigField = (*env)->GetFieldID(env, serialCommClass, "disableConfig", "Z");
 	isDtrEnabledField = (*env)->GetFieldID(env, serialCommClass, "isDtrEnabled", "Z");
 	isRtsEnabledField = (*env)->GetFieldID(env, serialCommClass, "isRtsEnabled", "Z");
+	autoFlushIOBuffersField = (*env)->GetFieldID(env, serialCommClass, "autoFlushIOBuffers", "Z");
 	baudRateField = (*env)->GetFieldID(env, serialCommClass, "baudRate", "I");
 	dataBitsField = (*env)->GetFieldID(env, serialCommClass, "dataBits", "I");
 	stopBitsField = (*env)->GetFieldID(env, serialCommClass, "stopBits", "I");
@@ -396,6 +398,7 @@ JNIEXPORT jlong JNICALL Java_com_fazecast_jSerialComm_SerialPort_openPortNative(
 	jstring portNameJString = (jstring)(*env)->GetObjectField(env, obj, comPortField);
 	const wchar_t *portName = (wchar_t*)(*env)->GetStringChars(env, portNameJString, NULL);
 	unsigned char disableAutoConfig = (*env)->GetBooleanField(env, obj, disableConfigField);
+	unsigned char autoFlushIOBuffers = (*env)->GetBooleanField(env, obj, autoFlushIOBuffersField);
 
 	// Ensure that the serial port still exists and is not already open
 	serialPort *port = fetchPort(&serialPorts, portName);
@@ -423,6 +426,8 @@ JNIEXPORT jlong JNICALL Java_com_fazecast_jSerialComm_SerialPort_openPortNative(
 			CloseHandle(port->handle);
 			port->handle = INVALID_HANDLE_VALUE;
 		}
+		else if (autoFlushIOBuffers)
+			Java_com_fazecast_jSerialComm_SerialPort_flushRxTxBuffers(env, obj, (jlong)(intptr_t)port);
 	}
 	else
 	{
@@ -573,6 +578,18 @@ JNIEXPORT jboolean JNICALL Java_com_fazecast_jSerialComm_SerialPort_configTimeou
 	return JNI_TRUE;
 }
 
+JNIEXPORT jboolean JNICALL Java_com_fazecast_jSerialComm_SerialPort_flushRxTxBuffers(JNIEnv *env, jobject obj, jlong serialPortPointer)
+{
+	serialPort *port = (serialPort*)(intptr_t)serialPortPointer;
+	if (PurgeComm(port->handle, PURGE_RXABORT | PURGE_RXCLEAR | PURGE_TXABORT | PURGE_TXCLEAR) == 0)
+	{
+		port->errorLineNumber = __LINE__ - 2;
+		port->errorNumber = GetLastError();
+		return JNI_FALSE;
+	}
+	return JNI_TRUE;
+}
+
 JNIEXPORT jint JNICALL Java_com_fazecast_jSerialComm_SerialPort_waitForEvent(JNIEnv *env, jobject obj, jlong serialPortPointer)
 {
 	// Create an asynchronous event structure

src/main/java/com/fazecast/jSerialComm/SerialPort.java

@@ -421,7 +421,7 @@ public final class SerialPort
 	private volatile String comPort, friendlyName, portDescription;
 	private volatile boolean eventListenerRunning = false, disableConfig = false, disableExclusiveLock = false;
 	private volatile boolean rs485Mode = false, rs485ActiveHigh = true, rs485RxDuringTx = false, rs485EnableTermination = false;
-	private volatile boolean isRtsEnabled = true, isDtrEnabled = true;
+	private volatile boolean isRtsEnabled = true, isDtrEnabled = true, autoFlushIOBuffers = false;
 	private SerialPortInputStream inputStream = null;
 	private SerialPortOutputStream outputStream = null;
 
@@ -592,6 +592,7 @@ public final class SerialPort
 	private final native long closePortNative(long portHandle);			// Closes serial port
 	private final native boolean configPort(long portHandle);			// Changes/sets serial port parameters as defined by this class
 	private final native boolean configTimeouts(long portHandle, int timeoutMode, int readTimeout, int writeTimeout, int eventsToMonitor);	// Changes/sets serial port timeouts as defined by this class
+	private final native boolean flushRxTxBuffers(long portHandle);     // Flushes underlying RX/TX device buffers
 	private final native int waitForEvent(long portHandle);				// Waits for serial event to occur as specified in eventFlags
 	private final native int bytesAvailable(long portHandle);			// Returns number of bytes available for reading
 	private final native int bytesAwaitingWrite(long portHandle);		// Returns number of bytes still waiting to be written
@@ -966,6 +967,27 @@ public final class SerialPort
 		return outputStream;
 	}
 
+	/**
+	 * Flushes the serial port's Rx/Tx device buffers.
+	 * <p>
+	 * If this function is called before the port is open, then the buffers will be flushed immediately after opening the port.
+	 * If called on an already-open port, flushing of the buffers will happen immediately.
+	 * <p>
+	 * Note that flushing serial buffers will always work on real serial ports; however, many USB-to-serial converters
+	 * do not accurately implement this functionality, so the flushing may not always work as expected, especially immediately
+	 * after opening a new port.
+	 * 
+	 * @return Whether the IO buffers were (or will be) successfully flushed.
+	 */
+	public final synchronized boolean flushIOBuffers()
+	{
+		autoFlushIOBuffers = true;
+
+		if (portHandle > 0)
+			return flushRxTxBuffers(portHandle);
+		return true;
+	}
+
 	/**
 	 * Sets all serial port parameters at one time.
 	 * <p>