Determing alignment of (char *) pointer

[rushfort at esunix.UUCP]

In article <487 at cartan.Berkeley.EDU> desj at brahms (David desJardins) writes

>   Why not use
>
>	if (p != (char *) (long *) p) ... ?
>
>   This should give you the relevant information (can a long be stored at
>the location pointed to by p?) without any machine dependencies.  I suppose
>on some (broken) compilers it might not work...

Not quite.  This test merely checks whether casting a character pointer into a
long pointer and then back into a character pointer preserves all of the bits.
A compiler is not forced to check alignment on casts.  According to K&R
Appendix A, Section 14.4 (Explicit pointer conversions):

	Certain conversions involving pointers are permitted but have
	implementation-dependent aspects.
[...]
	A pointer to one type may be converted to a pointer to another
	type.  The resulting pointer may cause addressing exceptions
	upon use if the subject pointer does not refer to an object
	suitably aligned in storage.

Note that K&R specifically allows an implementation to leave a pointer
alone when casting between pointer types.  I tried this out on a VAX
running Ultrix 1.2 and a SUN 2 running some form of 4.2 BSD and both
chose to leave a mis-aligned pointer alone.  The bottom line is that
there is no PORTABLE way to determine whether a character pointer is
suitably aligned such that it can point to an object of a different
type.
-- 
                Kevin C. Rushforth
                Evans & Sutherland Computer Corporation

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