Re: C structure padding

Dale Worley (drw@math.mit.edu) writes:
|For example: Suppose we have a machine with the length and alignment
|requirements:
| integer length 4 alignment 4
| char length 1 alignment 1
|
|If I have a structure:
| struct tag { int a; char b; };
...
|But-- If I just declare a variable with type struct tag, must the full
|8 bytes be allocated to it, or can the compiler give it 5 and pack
|some other variable(s) in the following 3 bytes?


I have been wondering about an almost identical question. Given


struct bigger { struct tag a; char b; };


can b be placed in the padding for a, so that sizeof(struct bigger)
is the same as sizeof(struct tag)?


There is probably a lot of code which assumes this won't happen,
but I can't find anything in the Standard which prohibits it.




Our moderator writes:
|[It's pretty clear that all structures have to be treated the same, since
|if p and q are pointers to structures, you can write *p = *q regardless of
|whether they're pointing to simple structures or into an array. -John]


Yes, but the code generated for a structure assignment does not
have to copy the padding.
--
Patrick Smith
pat@tesuji.qc.ca
uunet.ca!frumious!pat
--

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