Add sp_drain() function.

libserialport.h.in

@@ -526,6 +526,14 @@ enum sp_return sp_write(struct sp_port *port, const void *buf, size_t count);
  */
 enum sp_return sp_flush(struct sp_port *port, enum sp_buffer buffers);
 
+/**
+ * Wait for buffered data to be transmitted.
+ *
+ * @return SP_OK on success, SP_ERR_FAIL on failure, or SP_ERR_ARG
+ *         if an invalid port is passed.
+ */
+enum sp_return sp_drain(struct sp_port *port);
+
 /**
  * @}
  * @defgroup Errors Obtaining error information

serialport.c

@@ -535,6 +535,23 @@ enum sp_return sp_flush(struct sp_port *port, enum sp_buffer buffers)
 	return SP_OK;
 }
 
+enum sp_return sp_drain(struct sp_port *port)
+{
+	CHECK_PORT();
+
+#ifdef _WIN32
+	/* Returns non-zero upon success, 0 upon failure. */
+	if (FlushFileBuffers(port->hdl) == 0)
+		return SP_ERR_FAIL;
+#else
+	/* Returns 0 upon success, -1 upon failure. */
+	if (tcdrain(port->fd) < 0)
+		return SP_ERR_FAIL;
+#endif
+
+	return SP_OK;
+}
+
 enum sp_return sp_write(struct sp_port *port, const void *buf, size_t count)
 {
 	CHECK_PORT();