Re: Commercial C To Ada 95 compiler

[dewar@gnat.com (Robert Dewar)]

> In fact this is not an error in C.  Programs can (and do) intentionally
> index past the end of arrays.  I have seen C code that allocates memory
> dynamically and uses array access to reference the memory.  It is nearly
> impossible to distinguish these cases.

This is an erroneous C program. Definitely it will be hard to translate
programs that are erroneous and have undefined semantics. Or from a more
formal point of view, ANY translation of such a program is correct! Many
C programmers think C has general address arithmetic. It does not. If you
want general address arithmetic, you must use some other language, e.g.
Ada, which does have this feature. 
 
> > Another incompatibility is trying to reference the control
> > variable in a "for" loop outside the scope of the "for" loop.
> > This is impossible in Ada and not uncommon in C. The C compiler
> > will allow this. The Ada compiler will not.
> 
> In this case a direct translation should translate the for loop to a while
> construct.  For a direct translation the generated code must behave exactly
> the same as the original code.

A better response is that there is no "for" loop in C. The thing that looks
like a for loop in C is (and is defined to be) simply a macro form for a while
loop. Most obviously it must be translated into a while loop.

In general the point is that a translator that does this kind of translation
must have all the intelligence and semantic knowledge of a C compiler (and
must be written by someone who knows the formal semantics and typical usage
of both languages comprehensively).