Re: [avr-chat] Strange struct problem

[Russell Shaw]

Daniel O'Connor wrote:
> Hi,
> I am getting this odd problem with structs. If the size of the struct 
> is larger than 4 bytes I can't access it.. It's most peculiar.
>
> I have a test case (attached).
>
> Build it with..
> avr-gcc -O2 -fno-strict-aliasing -pipe  -g -Wall -Wunreachable-code 
> -DF_CPU=16000000 -mmcu=atmega32 -Wa,-adhlmsn=testcase.lst -c testcase.c -o 
> testcase.o
> avr-gcc -O2 -fno-strict-aliasing -pipe  -g -Wall -Wunreachable-code -DF_CPU=16000000 -mmcu=atmega32  -Wl,-Map=testcase.map,--cref  -g testcase.o -o testcase.elf 
> avr-objcopy -j .text -O ihex testcase.elf testcase.hex
> avr-objdump -S testcase.elf > testcase.dmp
>
> If BREAKME is defined all the struct accesses return 255 instead of what
> they should be (I think :)
>
> I am wondering if there is some oddity with GCC optimising for <= 4 byte
> structs which unbreaks my code, but I can't see why it is broken.
>
> Any help gratefully received!

Beware of structure padding leaving holes.

http://www.eskimo.com/~scs/C-faq/q2.12.html
http://www.eskimo.com/~scs/C-faq/q2.13.html