Lpc2000

Hello everyone,

I just saw that I have a couple of structure fields that are not well
aligned. That is, a 4-byte pointer starts at an odd memory address
(instead of at a multiple of 4). The funny thing is, the code runs fine.

What's up? is GNU GCC too smart? Is this a side-effect of packing?

Some background: I have had a couple of nasty bugs in the firmware
that turned out to be caused by mis-alignment. When another nasty bug
came up earlier this week, I thought "alignment" again, and set out
(finally) to write that utility that helps me find misaligned
variables, structure fields, etc. This time, the nasty bug turned out
to be a stack overrun, but I made the utility regardless and ran it on
the code.

I found that in a few instances, structures could start at an odd
address, even though the first element is a pointer. This is what all
mis-aligned structures have in common:
- their first element is a pointer to char
- all members have __attribute((packed))
- the structure itself has __attribute((packed)) too

Other observations:
- both structures in RAM and in ROM can be mis-aligned
- both global and "static local" variables can be mis-aligned

My guess is that the first member being a pointer to char is just
coincidence. So probably this has to do with structure packing. This
implies that GCC must have an alternative method to access the
mis-aligned structure fields, and in fact, it means that GCC must use
this alternative (and probably slower) access method for all accesses
to packed structures.

Can anyone confirm/refute this?

Kind regards,
Thiadmer