Re: How do I write directly to a memory address?

[Hyman Rosen <hyrosen@mail.com>]

On 2/7/2011 4:00 AM, Ludovic Brenta wrote:
> I recently found another bug in a C program that was caused
> by changing the value of a "passed-by-copy" parameter. Of
 > course the caller was unaffected but the algorithm inside
 > the function was broken.

In C or C++, a function parameter is an ordinary variable
which is initialized by the an expression in the function
call. If the parameter is not const, it may then be changed
during the operation of the function. C has worked this way
for something like forty years, so being surprised by this
now seems disingenuous.

There's no particular reason that a function parameter should
not be permitted to be changed, notwithstanding that you found
a case where doing so caused an error. One puts implementation,
for example, is

     int puts(char *s)
     {
         while (*s) if (putchar(*s++) == EOF) return EOF;
         return putchar('\n');
     }

and a C programmer would be puzzled when told that this function
would be better if he had to use another variable initialized to
the value of 's' before getting down to business.

I rather think that it's Ada that got it wrong here, by choosing
to combine the concept of passing by value or reference and the
concept of immutability. Evidence for that is the fact that in
several places the Ada standard has to say that a parameter must
be passed by reference while the declaration of the subprogram
does not give you that information (because the parameter is 'in').