From the original report:

A double value (e.g., DBL_MAX) can have up to 309 decimal digits before the decimal point. When formatted with a precision of 100 (%.100f), the resulting string can exceed 409 characters (309 integer digits + 1 decimal point + 100 fractional digits + 1 null terminator). This exceeds the 350-byte stack buffer, leading to a stack overflow. Using long double exacerbates this, potentially writing thousands of bytes out of bounds