rtu layer: fix parsing of settings string

Throw exceptions on error instead of silently changing values.
Check settings string for given stop bit value.

Up until now, the stop bits value was hard coded to be 2 if the parity
mode was set to none or 1 if the parity mode was set to even or odd.

So a setting of "115200E1" would result in a stop bit value of 1.
But "115200E2" would result also in a stop bit value of 1.
And "115200N1" would result in a stop bit value of 2.

This is not a restriction of libmodbus. The library supports all these
modes just fine. In fact they are using 115200N1 in the documentation
for modbus_new_rtu().

Rather this is a bug in the string parsing that libmodbuspp introduces
in its constructor.
libmodbus has no string parsing in modbus_new_rtu() and takes in the
arguments as parameters directly.

Fixes: b0f3dc4

Author: tamara-schmitz

include/modbuspp/rtulayer.h

@@ -169,23 +169,22 @@ namespace Modbus {
 
       /**
        * @brief Extracts the baudrate from a settings string.
-       * @return the baudrate found. if no value is found, returns the default
-       * value, ie 19200.
+       * @return the baudrate found. If no valid value is found, an exception is thrown.
        */
       static int baud (const std::string & settings);
 
       /**
        * @brief Extracts the parity from a settings string.
-       * @return the parity found. if no value is found, returns the default
-       * value, ie E for Even parity.
+       * @return the parity found. If no valid value is found, an exception is thrown.
        */
       static char parity (const std::string & settings);
 
       /**
        * @brief Return the stop bits from a settings string.
        *
-       * @return the number returned is determined based on the parity found.
-       * If the parity is None, this function returns 2, otherwise returns 1.
+       * @return the number of stop bits.
+       * It is parsed from the last character of the settings string.
+       * If the last character is neither '1' or '2' an exception is thrown.
        */
       static int stop (const std::string & settings);
 

src/rtulayer.cpp

@@ -234,40 +234,48 @@ namespace Modbus {
   // ---------------------------------------------------------------------------
   // static
   int RtuLayer::baud (const std::string & settings) {
-    int b;
     try {
-      b = std::stoi (settings);
+      return std::stoi (settings);
     }
     catch (...) {
-      b = 19200;
+      throw std::invalid_argument ("RtuLayer settings\"" + settings + "\" has an invalid baud rate setting.");
     }
-    return b;
   }
 
   // ---------------------------------------------------------------------------
   // static
   char RtuLayer::parity (const std::string & settings) {
-    char p = 'N';
     size_t s = settings.length();
 
     if (s >= 2) {
       char c = settings[s - 2];
-      if ( (c == 'E') || (c == 'O')) {
-        return c;
+      switch (c) {
+        case 'N':
+        case 'E':
+        case 'O':
+          return c;
       }
     }
-    return p;
+
+    throw std::invalid_argument ("RtuLayer settings\"" + settings + "\" has an invalid parity setting.");
   }
 
   // ---------------------------------------------------------------------------
   // static
   int RtuLayer::stop (const std::string & settings) {
+    size_t s = settings.length();
 
-    if (parity (settings) == 'N') {
-
-      return 2;
+    if (s >= 3) {
+      char c = settings[s - 1];
+      switch (c) {
+        case '1':
+          return 1;
+        case '2':
+          return 2;
+      }
     }
-    return 1;
+
+    throw std::invalid_argument ("RtuLayer settings\"" + settings + "\" has an invalid stop bit setting.");
   }
 
   // ---------------------------------------------------------------------------