[__david__]

> It's in fact very, very difficult to find a platform where that example won't work

No, it isn't. Many ARM processors will bus error on that code if (foo & 3) != 0. I believe PowerPC doesn't do unaligned word reads either...

It quite often has to do with the memory controller and not with the particular processor, though I believe x86 has to support unaligned reads. I've certainly worked first hand with ARMs that did not support it.

[sillysaurus]

That's interesting. What causes the bus error?

Would

  uint8_t foo[4];  *(uint32_t*)(&foo[0]) = 0;

also result in a bus error? Why?

[__david__]

That's the same thing, so yes, if foo is unaligned then it will cause a bus error. It causes it because the code is generate a store word assembly instruction (as opposed to store byte) and if the address is not aligned to 4 bytes then the memory controller hardware will raise a bus error.

Notice I keep saying "if the address is unaligned". The insidious part is that it probably will work for a while since it's likely that your "foo" array will happen to be aligned. But add one uint8_t variable to your structure or stack frame or wherever "foo" is defined and things could shift and suddenly it starts causing bus errors. It can be a very annoying type of heisenbug.

And bus errors are actually a good thing. I believe I've used hardware (an ARM or an SH2, can't remember) where the memory controller just ignored the last 2 bits during whole word reads and writes (which works fine as long as you only read aligned words). So if run your code on that hardware it doesn't give you an error, it just subtly "corrupts" your data. Yay!