It’s a given fact of life that something that’s deemed totally safe in one environment may be totally unsafe in another. Every German who has ever used an American sauna knows what I’m talking about.
Similar (but far less embarrassing!) traps lurk in situations where you reuse perfectly working C++ code in a C environment. Some time ago, I integrated a little home-grown C++ library into a plain C project. However, instead of the expected, proven functionality I got plenty of core dumps. After some assembly-level debugging, I came to the conclusion that I had found a compiler bug. Code along these lines
|
1 2 3 4 |
p += sizeof('A'); return *p; |
was compiled to this:
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1 2 3 4 5 6 |
mov rax, QWORD PTR p[rip] add rax, 4 ; 4? mov QWORD PTR p[rip], rax mov rax, QWORD PTR p[rip] |
Why the heck did the compiler insert an offset of 4 instead of 1?
The answer to this question, which is also the answer to our bug hunting adventure, can be found here.