On Thu, Jul 25, 2013 at 3:11 PM, Zdenek Styblik
<zdenek.styb...@gmail.com> wrote:
>> Both. The gcc compiler cannot pick out used, but uninitialized fields
>> from a structure (other compilers might), but IMHO it's better to
>> explicitly initialize the structure near where it's going to be used
>> so that people can see that you're explicitly initializing it and know
>> that you didn't just forget about it.
>>
> [...code example...]
>
> I sort of wish I knew why, but ok and I'll try to remember. Still, the
> diff between ``int i = 0;'' V. ``int i; i = 0;'' is just ... I guess
> beyond my understanding, resp. knowledge :|
Let's consider an example:
int foo(int arg)
{
int i;
if (arg < 1)
i = 1;
else if (arg >= 2)
i = 2;
printf("i=%d\n", i);
}
Now in this case if 'arg' is 1, then the value printed will be
undefined. However the compiler will spit a warning "i may be used
uninitialized" (or something to that effect), so you know that you
have an unused case there. If you just blindly initialize 'int i=0;'
at the top of the function then you will not get that warning. Maybe
you really wanted i to be 3 if arg was 1, but you just forgot to
handle the case where arg == 1. A better style would be:
int foo(int arg)
{
int i;
if (arg < 1)
i = 1;
else if (arg >= 2)
i = 2;
else
i = 0;
printf("i=%d\n", i);
}
because then you know that you have all of the cases covered and the
compiler will not spit a warning about uninitialized variables being
used.
Now of course there are exceptions to this "rule". Something like:
int foo (int arg)
{
int ret = 0;
if (arg)
{
ret = func1();
}
return (ret);
}
Is fine and you see that a lot. In this case you have a known default
value that you are setting ret to. You're not just initializing ret
to something just to initialize it or (worse yet) to get rid of
warnings about ret being used without being initialized. However to
my mind it's better to not initialize ret in the declaration and just
initialize it close to where it's first used.:
int foo (int arg)
{
int ret;
/* default to success! */
ret = 0;
if (arg)
{
ret = func1();
}
return (ret);
}
This just tells me, as a reader, that the author _knows_ that ret may
not be set by the body of the function and wants ret set to some
default value. But this is a minor quibble for small functions.
>> Also, remember that:
>>
>> uint8_t data[256] = {0};
>>
>> does not zero all the elements of 'data', only the first element.
>
> Damn! You're absolutely right! I always forget about this. I did write
> it down this time and I hope I'll remember it from now on.
> Also, 'data' is being initialized like two lines below. Ehm.
yeah that another thing that these patches had.. Something like:
int ret = 0;
ret = scanf(fd.....);
The ret = 0 in the declaration is totally unnecessary (and will just
be optimized out) since we're immediately going to set ret to the
return value from scanf.
> The problem is nobody does it(init).
> And further you go from your
> declarations the easier is to forget about it. Then only one bit is
> initialized and the rest of structure is left at "random". But this is
> beyond discussion whether to initialize in the declaration or later in
> the code.
> Simply - ok.
I agree, there are not hard and fast rules for everything, you have to
use a bit of sense, I'm just saying that it's better not to do the
initialization in the declaration unless there's a good reason to
since it can suppress these valuable compile-time warnings.
thanks
dan
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