% lec 23: undefined behavior

# administrivia

Thursday: hw4 due

Friday: wrapup / Q&A

next Monday: Q&A (560)

next Wednesday: final exam, open book/notes/laptop

# most time-consuming bugs

memory bugs:
dangling pointer,
double free,
"`&var`" vs "`var`",
leak ("800 valgrind warnings"),
null pointers ("3 days"),
variable lifetime ("1 week")

logical bugs:
wrong functions ("`getnameinfo`"),
wrong variables ("`head`" vs "`header`"),
wrong seek offset/count

others:
missing semicolons,
missing parentheses,
`size_t` vs `uint32_t`,
performance bugs,
"no idea what happened"

# ex19 quotes

> The[re] are so many bugs in CSE333 that drove me crazy.

> I've tried my best to repress most of the pain this class has
> caused me...

# undefined behavior

- can lead to unstable code:
intended code altered by compilers due to undefined behavior
    - useful code unexpectedly gone
    - _not_ a compiler bug: legal optimizations
    - _not_ a spec bug: spec allows anything to happen

- joint work with Nickolai Zeldovich, Frans Kaashoek, Armando Solar-Lezama