On Mar 16 2014 10:03 PM, Dave Hylands wrote:
> Hi John,
>
>
> On Sun, Mar 16, 2014 at 6:57 PM, John Kasunich
> <[email protected]>
> wrote:
>>
>>
>>
>> On Sun, Mar 16, 2014, at 09:41 PM, Dave Hylands wrote:
>> > Hi,
>> >
>> > I'm jumping in late here, but why not just do:
>> >
>> > *(hal_data_u *)data_addr = pin->dummysig;
>> >
>> > which is a structure copy, and the compiler will optimize it to
>> use the
>> > best way of copying based on the size?
>> >
>>
>> I didn't know you could copy structures like that in C.
>>
>> Does it work for any size structure?
>>
>> When was that added to the language? I'm 99.4% certain
>> that in the K&R days you had to copy structs yourself.
>> Is it maybe a C99 thing?
>
> Many K&R variants didn't support structure copying. It was an
> extension to
> K&R C:
> http://cm.bell-labs.com/cm/cs/who/dmr/cchanges.pdf
>
> It was introduced as part of the language specification when ANSI C
> was
> introduced, so its been around for quite a while.
>
> It works with any size structure. IIRC gcc uses memcpy (or a function
> like
> memcpy) for large structures and optimizes it to do word copies for
> small
> structures.
>
> I've used it for many many years (I learned C using K&R in
> university, but
> fortunately never had to actually use it for working).
> Function prototypes made C manageable :)
Yes, but you need to be careful and make sure that it will do what you
want. For fairly flat structures:
struct bla {
int some, thing;
float more;
}
it will do fine, but for:
struct woof {
struct bla *bla1, *bla2;
char *names[];
}
you do not know apreori if you should do a deepcopy or shallow (ie do
you copy the pointers, or do you follow down the pointers and copy all
the subrecords).
Other than that, it is a very useful feature.
EBo --
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