Re: encapsulating directory operations

Richard Heathfield <rjh@cpax.org.uk> writes:

On 30/05/2025 10:20, David Brown wrote:
>

On 29/05/2025 14:38, Richard Heathfield wrote:
>

This really is a very simple point, but perhaps a simple analogy
will help to clarify it.  You don't throw out your 3/4" just
because you've bought a 19mm.  There is room for both in the
toolbox, and why write 3/4" on your new spanner?  It /isn't/ a
3/4" spanner even though it's very like it, so why pretend
otherwise?

>
Your analogy does not cover C99 vs C90.

>
It does if we can construct a program that is legal C90 but not
legal C99, which is easy enough, or (slightly harder but still not
that hard) a program that is legal in both dialects but which
gives different output under C99 than it does for C90.
>
$ cat c9099.c; \
    gcc -W -Wall -ansi -pedantic -o c90 c9099.c; \
    gcc -o c99 c9099.c; \
   ./c90; \
   ./c99
#include <stdio.h>
>
int main(void)
{
  int a = 42;
>
  int b = a //* comment */ 6;
  ;
  printf("Soln = %d\n", b);
>
  return 0;
}
Soln = 7
Soln = 42

It's a straightfoward exercise to write a program that discovers
whether a given .c file has this potential ambiguity.  Of course,
essentially no actual source code will, but the point is the
problem is easy to detect, and also correct.

This observation suggests a related question.  Is there a source
of potential ambiguity between C90 and C99 that is not so easy to
detect and correct?  I can think of one, but it falls into the
realm of depending on implementation-specific behavior, so could
just as easily be seen as a portability issue rather than a
language version issue.

Obviously it's a contrived example, but then examples pointing out
the consequences of language differences invariably are.

Some are.  Some are not.  In the case of C90 and C99, probably
all such cases are contrived, but certainly for other language
pairs there are cases that might arise unintentionally, even for
similar languages (notably C and C++).