Separate flush/drain a bit better (#58)

Author: AndreRenaud

README.md

@@ -204,7 +204,17 @@ Set the state of a hardware pin of the UART. See *simple_uart_get_pin*
 ```c
 int simple_uart_flush(struct simple_uart *uart);
 ```
-Ensure all outstanding data has been written to the wire and is not being buffered by the OS.
+Discard all pending input and output data buffered by the OS without waiting for it to be transmitted.
+
+
+### Drain
+```c
+int simple_uart_drain(struct simple_uart *uart);
+```
+Block until all pending output data has been transmitted. Unlike *simple_uart_flush*, this does not discard data — it waits for the transmit buffer to empty.
+
+#### Return:
+*0* on success. *< 0* on failure.
 
 
 ### Break

simple_uart.c

@@ -986,11 +986,8 @@ int simple_uart_flush(struct simple_uart *uart)
     if (!uart)
         return -EINVAL;
 
-#if defined(__linux__)
-    if (ioctl(uart->fd, TCFLSH, TCIOFLUSH) < 0)
-        return -errno;
-#elif defined(__APPLE__)
-    if (tcdrain(uart->fd) < 0)
+#ifndef _WIN32
+    if (tcflush(uart->fd, TCIOFLUSH) < 0)
         return -errno;
 #else
     DWORD purge_all = PURGE_RXABORT | PURGE_RXCLEAR | PURGE_TXABORT | PURGE_TXCLEAR;
@@ -999,3 +996,18 @@ int simple_uart_flush(struct simple_uart *uart)
 #endif
     return 0;
 }
+
+int simple_uart_drain(struct simple_uart *uart)
+{
+    if (!uart)
+        return -EINVAL;
+
+#ifndef _WIN32
+    if (tcdrain(uart->fd) < 0)
+        return -errno;
+#else
+    if (!FlushFileBuffers(uart->port))
+        return win32_errno();
+#endif
+    return 0;
+}

simple_uart.h

@@ -77,10 +77,15 @@ int simple_uart_get_pin(struct simple_uart *uart, int pin);
 int simple_uart_set_pin(struct simple_uart *uart, int pin, bool high);
 
 /**
- * Make sure all write data has been flushed out of the UART
+ * Make sure all read/write data has been flushed (discarded) out of the UART
  */
 int simple_uart_flush(struct simple_uart *uart);
 
+/**
+ * Block until all pending output has been transmitted
+ */
+int simple_uart_drain(struct simple_uart *uart);
+
 /**
  * Send the break symbol
  */