Lpc2000

Thiadmer Riemersma (ITB CompuPhase) wrote:

>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?
>
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>
No, it is the exact reason you declare a structure to be packed.  As 
David says in the other posting, look at the code being generated via 
listing or JTAG assembly view in your debugger.

Regards,

TomW



>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
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>Yahoo! Groups Links
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-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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