Re: [PATCH hail] const-correctness tweaks

On 10/20/2010 04:00 AM, Jim Meyering wrote:
> Jeff Garzik wrote:
>
>> On 10/06/2010 08:07 AM, Jim Meyering wrote:
>>>
>>> Make write_cb callback's buffer parameter const, like all write-lik=
e
>>> functions.
>>> Give a few "char *" parameters the "const" attribute.
>>>
>>> Signed-off-by: Jim Meyering<[email protected]>
>>> ---
>>>
>>> It looks like most of hail's interfaces are const-correct,
>>> but one stood out because it provokes a warning when I tried to
>>> pass a const-correct write_cb function to hstor_get from iwhd:
>>>
>>>       proxy.c:382: warning: passing argument 4 of 'hstor_get' from =
\
>>>         incompatible pointer type
>>>       /usr/include/hstor.h:173: note: expected \
>>>         'size_t (*)(void *, size_t, size_t,  void *)' but argument =
is of type \
>>>         'size_t (*)(const void *, size_t,  size_t,  void *)'
>>>
>>> In case you feel comfortable fixing this, here's a patch:
>>
>>
>> Sorry for not getting back to this; I had hoped to solve some
>> additional problems that cropped up, but didn't have time.  So, to
>> forestall further delay,
>>
>> libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I.. -I../include -pthrea=
d
>> -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include
>> -I/usr/include/libxml2 -O2 -Wall -Wshadow -g -MT hutil.lo -MD -MP -M=
=46
>> .deps/hutil.Tpo -c hutil.c -o hutil.o
>> hutil.c: In function =E2=80=98hreq_hdr_push=E2=80=99:
>> hutil.c:145: warning: assignment discards qualifiers from pointer
>> target type
>> hutil.c:146: warning: assignment discards qualifiers from pointer
>> target type
>>
>> warnings appear after this patch.  When solving these warnings with
>> const' markers, it quickly becomes a bit of a rat's nest.
>>
>> At a minimum, the write_cb callback signature must match libcurl's,
>> which does not use 'const'.  I can see this makes sense from libcurl
>> implementation's perspective, even if it does not really match the
>> constness one expects from a foo-get function.
>
> Hi Jeff.
>
> Sorry I didn't notice that the first time.
> I built with ./autogen.sh&&  ./configure&&  make.
> It looks like you recommend -Wall -Wshadow.
>
> The two warnings above are the only ones I see with the patch,
> and they're easy to fix.  When storing const pointer params into
> a struct like that, I've found that it's best to cast away the "const=
",
> which really does reflect the semantics: by using "const" on the
> parameter, I view it as promising not to deref through the pointer
> *in that function*.  Since it's usually not reasonable to make
> the struct member "const" (as you saw, it propagates too far
> and often ends up being contradictory), the lesser evil of the cast
> is preferable here.
>
> If you're still game, the following incremental patch seems to be
> enough for me:  Let me know and I'll resubmit the full one.

Well, my primary concern now originates from curl_easy_setopt(3)=20
documentation:

        CURLOPT_WRITEFUNCTION
               Function pointer that  should  match  the  following
	      prototype: size_t  function(  void  *ptr,  size_t  size,
	      size_t nmemb, void *stream);

hstor's callback is passed directly to libcurl, so we seem to be bound=20
by outside constraints, no?

	Jeff