C strongly typed?

[Nick Maclaren]

Henry Spencer writes:

> ....  C's type system is not extensible unless
> you count "struct", but the language is strongly typed -- mixing random
> types is not allowed.

I am afraid that I must disagree with this.  While mixing RANDOM types is
not allowed, even ANSI C permits a bewildering variety of type punning.
For example:

    double x, y;
    memcpy(&x,&y,sizeof(double));

The ANSI standard (and K&R) explicitly require that any data type may be
treated as an array of characters (under certain circumstances, such as
the above).  A huge proportion of the library relies upon this to work at
all (e.g. much of string.h, some of stdlib.h, some of stdio.h).

    union {void *a; char *b;} fred;
    fred.a = ...;
    ... = fred.b;

This example is a curiosity in ANSI C:  while it is illegal, and the compiler
is entitled to throw it out, it is also required to work!  The reason is that
'char *' and 'void *' are different types but are required to have the same
representation and alignment.

There are a large number of more obscure cases, many of which are relied
upon by traditional C programs.  Good, portable ones avoid such constructions
if at all possible, but sometimes their use is essential.

Nick Maclaren
University of Cambridge Computer Laboratory
nmm at cl.cam.ac.uk