Program won't crash when it should

(GCC C)
I ran a pointer over each member of an integer array, storing one character of input in each cell.

The program seems to consistently crash when I try to store X+3 or more characters, where X is the size of the array, but if I do not exceed X+2 it will not crash.

Why does it crashes exactly at the point it does, but not for X+1 or X+2?

>>40
>>69

There is absolutely nothing undefined in the original program. Absolutely. Also note that the C standard has no notion of devices.

Undefined behavior is something that only exists at the language level. One could never argue, for example, that a fwrite() which happens to write to a dying hard disk
is "underfined behavior", or that an auto int x; which happened to be allocated on a failing memory circuitry during execution is "undefined behavior".

Don't mix things up.

The >>58 program, however, is indeed undefined.