[Courses] [C Doubt]

[Conor Daly]

On Wed, Sep 18, 2002 at 09:58:16PM -0700 or thereabouts, Akkana wrote:
> 
> This works fine, but you're allocating more memory than you need ...
> you really only need (e - s + 1) bytes (+1 because you'll probably
> want to write a null at the end, since most functions that take strings
> expect them to be null-terminated).  This also has the nice bonus
> of making the function a little faster since you don't need to call
> strlen() (which has to loop over str to find out how long it is).
> 
> So I'd suggest modifying your function very slightly:
> 
>   str1=(char *)malloc(e-s+1);
                       ^^^^^^^

Maybe be explicit about the order of evaluation here.  In this case, 

y = (e - s + 1)

will evaluate as

x = e - s

followed by

y = x + 1

so all is well but for other operations you could get caught.  eg.

y = (e - s * 2)

evaluates as

x = s * 2
y = e - x

instead

y = ((e - s) * 2)

evaluates as 

x = e - s
y = x * 2

I _personally_ would use

y = ((e - s) + 1)

in the malloc.  It doesn't make any difference here but being in the habit
of explicitly specifying the order of evaluation avoids bugs in more complex
formulae

Conor
-- 
Conor Daly 
Met Eireann, Glasnevin Hill, Dublin 9, Ireland
Ph +353 1 8064276 Fax +353 1 8064247
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