As a followup to this question: Can I safely connect two GPIO pins to each other (ie. for testing purposes)? And can I safely connect an input pin to either +VDD (3.3 V) or to Ground? Or do I need a resistor each time? I would expect that the maximum input current for the pins is equal to the maximum output current, so that this shouldn't damage anything.
Note: I know that connecting the 5V output to anything could fry the Pi. This question only is about Pins running with 3.3V.
It is safe to connect a GPIO set as an INPUT to any voltage between 0 to 3V3 volts.
It is safe to connect a GPIO set as an INPUT to a GPIO set as an OUTPUT.
There is a risk in connecting together two GPIO both set as OUTPUTs. One may be set to 0V and the other to 3V3 which is in effect a short circuit.
To mitigate the risk of setting two GPIO as OUTPUTs to a different level it is prudent to always have something like a 300 ohm resistor in series whenever you connect one GPIO to another GPIO.
Having a resistor in series between GPIO will never do any harm.
