Bart <bc@[EMAIL PROTECTED]
> writes:
> On May 7, 10:40 pm, Keith Thompson <ks...@[EMAIL PROTECTED]
> wrote:
>> Bart <b...@[EMAIL PROTECTED]
> writes:
>> > On May 6, 11:45 am, "Kevin" <yushuma...@[EMAIL PROTECTED]
> wrote:
>> >> Source:
>> >> #include <stdio.h>
>> >> #include <ctype.h>
>> >> void main()
>> >> {
>> >> char a[]="this is the beautiful world!";
>> >> char b[20];
>> >> strcpy(b,a);
>> >> printf("char array b size is(%d),The content of b
is:%s\n",sizeof(b),b);
>> >> if(strcmp(a,b)==0)
>> >> printf("a equal to b!\n");
>>
>> >> }
>>
>> >> Now , question as follows:
>>
>> >> 1. Why "b" size unequal "a" size , but "b" can output "a" content?
>> >> 2. Why "if(strcmp(a,b)==0)" is true?
>>
>> > a and b are different types:
>>
>> Yes.
>>
>> > a is a pointer to a string
>>
>> No, a is an array of type char[29] (the length of the literal used to
>> initialize it plus 1 for the trailing '\0').
>
> Yes, of course, I read it as char *a (I think char a[] is pointer in
> some other context).
``char a[]'' as a function parameter declaration declares ``a'' as a
pointer (unfortunately, IMHO).
>> > b /is/ a string.
>>
>> No, b is an array of type char[20]. An array can *contain* a string.
>
> So some informality.. Here the intention is clearly a string.
The intention is that b is an array that contains a string (or rather,
it would contain a string after the strcpy() call *if* b were big
enough). Before the strcpy() call, the contents of b are
indeterminate; it may or may not contain a string.
>> A "string" in C is a a data format, not a data type.
>>
>> > But a can obviously point to a string identical to what's in b.
>>
>> The object named ``a'' can't point to anything, since it's an array,
>> not a pointer. However, the expression ``a'', in most but not all
>> contexts, has the value of a pointer to (the address of) the first
>> element of the object named ``a''.
>>
>> > The size of a will be 4 or whatever, the size of b will be 20
>> > (although it should be more in this case otherwise your "this is..."
>> > string will overflow it).
>>
>> No, sizeof a == 20.
>
> Don't know about that, it seems a bit more. But different from sizeof
> b anyway, which was the original confusion.
Sorry, my mistake. sizeof a == 29; sizeof b == 20.
>> > strcmp() compares two pointers to strings; but b is automatically
>> > converted to such a pointer, thanks to the way C handles arrays.
>
> Well if a and b are both arrays, then the reason why strcmp matches
> them but sizeof is different is a little different!
>
> (strcmp() only matches characters up to the nul terminator, ignoring
> any trailing characters in the array that still contribute to the
> sizeof property.)
Right. But again, the program as originally posted invokes undefined
behavior. One likely outcome is that, after the call to strcpy(), b
will contain the 20 characters "this is the beautifu" *without* a
trailing '\0'. (There might happen to be a trailing '\0' after the
end of b -- or the strcpy might have clobbered something vital before
anything is printed.)
--
Keith Thompson (The_Other_Keith) <kst-u@[EMAIL PROTECTED]
>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
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