In SoftwareSerial, the ring buffer is defined as a signed char, which causes corruption

[NickBKeenan]

In SoftwareSerial.h the ring buffer is defined as:

ArduinoCore-renesas/libraries/SoftwareSerial/src/SoftwareSerial.h

Line 60 in bba294b

::RingBuffer<char> ringbuf;

This is a signed quantity. If a byte is received where the high bit is set, this causes unexpected and incorrect behavior.

This is SoftwareSerial::read():

ArduinoCore-renesas/libraries/SoftwareSerial/src/SoftwareSerial.cpp

Lines 328 to 335 in bba294b

	int SoftwareSerial::read()
	{
	int chr = -1;
	if (!rx_descr.ringbuf.empty()) {
	chr = rx_descr.ringbuf.get();
	}
	return chr;
	}

It returns an int, which is a signed quantity. Since get() is returning a char, which is also a signed quantity, if the highest bit in that byte is set all of the high-order bits in the int that is returned will be set. Code that checks for the read value to be less than zero will fail.

Defining it instead as:

::RingBuffer<uint8_t> ringbuf;

fixes the problem and gives expected behavior.