On Sat, 25 Nov 2000, Andries Brouwer wrote:
> On Sun, Nov 26, 2000 at 09:11:18AM +1100, Herbert Xu wrote:
>
> > No information is lost.
>
> Do I explain things so badly? Let me try again.
> The difference between
>
> static int a;
>
> and
>
> static int a = 0;
>
> is the " = 0". The compiler may well generate the same code,
It does not. That's the whole point: the (functionally redundant) =0 wastes
another sizeof(int) bytes in the kernel image.
> but I am not talking about the compiler. I am talking about
> the programmer. This " = 0" means (to me, the programmer)
> that the correctness of my program depends on this initialization.
If you want to document your code like this, put it in a comment. That's what
they are there for. Or, if coding a function which explicitly relies on a
variable being 0, have that function set the variable to zero.
> Its absense means (to me) that it does not matter what initial
> value the variable has.
Which is silly. The variable is explicitly defined to be zero anyway, whether
you put this in your code or not.
> This is a useful distinction. It means that if the program
>
> static int a;
>
> int main() {
> /* do something */
> }
>
> is used as part of a larger program, I can just rename main
> and get
>
> static int a;
>
> int do_something() {
> ...
> }
>
> But if the program
>
> static int a = 0;
>
> int main() {
> /* do something */
> }
>
> is used as part of a larger program, it has to become
>
> static int a;
>
> int do_something() {
> a = 0;
> ...
> }
Just put:
static int a; /* must be set to zero in foobar() */
> You see that I, in my own code, follow a certain convention
> where presence or absence of assignments means something
> about the code.
Unfortunately, this handy documentation shortcut of yours bloats the kernel
unnecessarily.
> If now you change "static int a = 0;"
> into "static int a;" and justify that by saying that it
> generates the same code,
It does NOT generate the same code - that's the point. It generates smaller but
functionally equivalent code. The first version zeroes a TWICE, in effect; this
is completely unnecessary, and just bloats the kernel.
> then I am unhappy, because now
> if I turn main() into do_something() I either get a buggy
> program, or otherwise I have to read the source of main()
> again to see which variables need initialisation.
Oh no! You mean you might actually have to look at the code you're changing?!
This is hardly a valid reason for bloating the kernel! If you put the "this
variable must be zero when foo() is called" in a comment, rather than as a C
statement, it is equally clear to you - but avoids bloating the kernel.
> In a program source there is information for the compiler
> and information for the future me. Removing the " = 0"
> is like removing comments. For the compiler the information
> remains the same. For the programmer something is lost.
So put that comment in a real comment, rather than a redundant statement!
James.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Thu Nov 30 2000 - 21:00:14 EST