Re: Struct compares/copies


<bb@xxxxxxx> ha scritto nel messaggio
news:s618731ejdtvf2rm0p8ipj6ll0dhsdfitr@xxxxxxxxxx
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).

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).

Even if there is no padding, the structures

struct words {
char word1[8] = { 'H', 'e', 'l', 'l', 'o', '\0', 42, 73 }
char word2[8] = { 'w', 'o', 'r', 'l,' 'd', '\0', 1, 95 }
} a;

and

struct words {
char word1[8] = { 'H', 'e', 'l', 'l', 'o', '\0', 12, 23 }
char word2[8] = { 'w', 'o', 'r', 'l,' 'd', '\0', 47, 79 }
} b;

don't memcmp equal, even if any reasonable person shouldn't
distinguish them in most cases.


.

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