why do you need to define that anyway? it's not like it's going to change...
----- Original Message ----- From: Jim Dougherty To: [email protected] Sent: Wednesday, February 11, 2009 3:15 PM Subject: Re: [c-prog] Re: Integer constant suffix [email protected] wrote: > > --- In [email protected] <mailto:c-prog%40yahoogroups.com> , > "jimdougherty" <j_doughe...@..> wrote: > > > > --- In [email protected] <mailto:c-prog%40yahoogroups.com> , > Brett McCoy <idragosani@> wrote: > > > > > > On Wed, Feb 11, 2009 at 11:31 AM, Jim Dougherty <j_dougherty@> wrote: > > > > > > > I was just browsing some C-code that I did not write and I saw a > line > > > > that said: > > > > > > > > #define MILISECONDS_PER_SECOND 1000L > > > > > > > > If I wrote the same line I would have written it without the 'L'. > > > > What is the point of the 'L'? > > > > > > It makes it a long integer. > > > > > > > I still don't get it. Can you give me an example where it would be > > wrong to use 1000 rather than 1000L ? > > It would be useful to do this in an environment where sizeof(long) > > sizeof(int) eg. sizeof(long)=4 (32 bits) and sizeof(int)=2 (16 bits). > Then: > > int n, x, y; > > y = (n * MILISECONDS_PER_SECOND) / x; > > Say n = 45678, x = 1500. Then 45678 * 1000 would overflow in the 16 > bit arithmetic used if MILISECONDS_PER_SECONDS was specified without > the L. With the L, 32 bit arithmetic would be used and the 'correct' > result would be obtained. > > Of course you don't need the L if you cast: > > y = ((long)n * MILISECONDS_PER_SECOND) / x; > > but I suppose including the L acts as a safety net. > > In an environment where sizeof(long) == sizeof(int), which is quite > common (but not the one I code for), I think you just gain a bit of > portability. > > John > Thanks, this help. I do embedded work and our longs are 32 bits and our ints are 16. The idea seems to be that someone was worried about numeric overflow. If that is the case I do not think I like the approach that was taken. #defining MILLISECONDS_PER_SECOND as a long will cause all calculations that use it to do 32 bit math which is substantially slower on our system than 16 bit math. It seems to me that a better approach would be to #define MILLISECONDS_PER_SECOND as 1000 (no L) and then to also use typecasting locally in those calculations that need to worry about overflow. Along the same lines, we certainly would not want to make all of our #define'd integer constants long. [Non-text portions of this message have been removed]
