From: zhuyj
> Sent: 19 October 2016 14:21
> __attribute__((packed)) is potentially unsafe on some systems. The
> symptom probably won't show up on an x86, which just makes the problem
> more insidious; testing on x86 systems won't reveal the problem. (On
> the x86, misaligned accesses are handled in hardware; if you
> dereference an int* pointer that points to an odd address, it will be
> a little slower than if it were properly aligned, but you'll get the
> correct result.)
>
> On some other systems, such as SPARC, attempting to access a
> misaligned int object causes a bus error, crashing the program.
__attribute__((packed)) causes the compiler to generate byte memory
access and shifts to access the unaligned data.
You don't get a fault, just rather more instructions that you had in mind.
You shouldn't really specify 'packed' unless there are real reasons
why the data structure will appear on misaligned addresses.
David
