Int3 is a special single-byte (CC, if I recall correctly) form of the INT instruction (which is CD imm8, I think) to raise an interrupt. Interrupt #3 is the debugging interrupt, so by overwriting any instruction with CC, you place a breakpoint there.
Sure, strtok is a terrible misfeature, a relic of ancient times, but it’s plainly the heritage of C, not C++ (just like e.g. strcpy). The C++ problems are things like braced initialization list having different meaning depending on the set of available constructors, or the significantly non-zero cost of various abstractions, caused by strange backward-compatible limitations of the standard/ABI definitions, or the distinctness of
vector<bool>
etc.