b64.c | Base64 encode/decode | Messaging library

I have written a base 64 encoding / decoding library for the Arduino IDE (yes, I am aware such libraries already exist. This is for my own education as much as anything practical). My target microcontroller is an Espressif ESP32.

In my base64 decoding function, the index variable i is affected by the presence of a Serial.println() debug statement. It is the strangest thing I have ever seen and I cannot figure out why it should make a difference if there is debug printing or not.

Using the test program base64.ino and the b64.cpp functions below, here are two samples of my serial output. In the first example, I'm using a Serial.println(i); in the function b64dec() just after the for loop. In the second example, the Serial.println(i); is commented out. That is the only difference and I get two drastically different results.

Am I missing something here? Is it a compiler optimization gone wonky? My understanding of C variable scoping is that there are only global, function, and parameter levels. The i in the for loop should be the same as the int i = 0; a few lines above it. I don't believe it's buffer eoverflow either, since the debugging output of 13 for the decoded length is accurate for a 12 character message and its NULL terminator.

This is what I expect to get any time I run it: