Re: Struct compares/copies

On Sat, 16 Jun 2007 16:09:18 GMT, bb@xxxxxxx wrote:

Q: I need to copy and compare C Structs.

Is this the safe and quick way to do it?:
sniip


Copying is easy: new = old;
As the comapre (new == old) made the compiler unhappy I didn't want to
trust the assign (old = new).

Trust has nothing to do with it. It is a question of what
capabilities the language provides. If you don't have a decent
reference manual, get one.


For safe and meaningful comparison, compare on a field-by-field basis,
comparing the most significant fields first. Typically one would write
a function to do this.
Well once I've found a change I do have to go through each field to
and action the changes. However the changes are rare, so I wanted to
get the 'compare' over and done with asap, hence the 'old == new' as
there can be quite a few fields in the structure (20+ at the mo).

It is not sweet -- the advice is wrong (except in the oddest of
situations). It is entirely possible for memcmp to return non-zero
when the two structures are equal in all important respects (i.e. have
identical values in all members).
Well I have an odd situation because it has worked for me. Can you (if
you have the time and inclination) tell me why memcmp would do this
(is it because of 'structure' padding/alignment and that two
'identical' structs not being identical)? I was concerned (hence the
question in the 1st place).

Padding is one situation where equal structures can have mismatched
bits. Pointers are another - it is possible for two pointers to the
same address to have different bit representations. A system that
supports -0 is another. 0 and -0 must compare equal but will not have
the same bit representation. There probably are other situations
also.


Remove del for email
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