Concat operator + with random() value leads to crash

[sebromero]

I found that when I concatenate a string directly (without wrapping it in a String object) with a number obtained from random() I get strange results.
When random() is used with a parameter, the resulting string contains garbage obtained from memory where it probably shouldn't read from.
When using random() without parameter the program crashes. On Portenta I get the red S.O.S flash, on the MKR1000 for example it just stops responding. I started to investigate but haven't found the culprit yet. It crashes somewhere in concat() when reading the string length using strlen().

void setup() {  
  Serial.begin(9600);
  while(!Serial);  

  //String newMessage = "Hello World " + random(1024); // Prints garbage:�����������3�������m���c���⸮�������Envie M7
  String newMessage = "Hello World " + random(); // Crashes
  //String newMessage = "Hello World " + String(random()); // Works
  //String newMessage = "Hello World " + String(random(1024)); // Works
  
  Serial.println(newMessage);
}
  
void loop() {}

[sebromero]

Maybe something for you @per1234 ? :-)

[matthijskooijman]

I believe what happens here is that "Hello world" resolves to a const char* to the first character of the string, and the + random() just adds an offset to this pointer. If that offset is more than the string length, you will end up pointing to random memory, which might look like a very long string if there is no NUL byte in memory anywhere near. It might even end up pointing to an invalid address, which can lead to a hardfault on ARM processors, which I suspect is what you're seeing.

You could probably try:

String newMessage = "Hello World " + 2;

and this would print llo World, or:

String newMessage = "Hello World " + 0xffffff;

(or some other similarly big number which goes beyond the end of RAM) and this will also crash.

I'm inclined to close this as not a bug, since this is expected behavior (C-style strings do not support concatenation with +) and even though it's somewhat confusing, I don't think really think there is any way we could handle this particular code in a better way (maybe the compiler could generate a warning in the constant case, but probably not when adding e.g. random()).

[sebromero]

@matthijskooijman What you're saying makes total sense! Thanks for sharing that. There is indeed an assignment operator String & operator = (const char *cstr); for char pointers. Hmm, yes, you're right, it's expected behaviour in that sense. Quite a bit of a trap for beginners. Well, it even got me ;-) I can't think of a way to avoid this problem currently. Let's close this and hopefully someone who comes across this issue finds the explanation here.