🚀 go-pugleaf

In article <1992Nov13.184421.11065@taumet.com>, steve@taumet.com (Steve Clamage) writes:
: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) writes:
: >I would like to know what the standard says about the following program:
: [ writes the contents of a pointer with fwrite, reads it back with fread ]
: [ that is, whether the value read == the value written ]
: 
: ANSI C Standard 4.9.2:
: 
: "Data read in from a binary stream shall compare equal to the data that
: were earlier written out to the stream, under the same implementation."
: 
: In the example, the file was opened in binary mode, so this applies.

That is exactly the kind of answer I hoped for when I posted to comp.std.c.
Thank you.  There remains, alas, one fine point of detail.  What does it
_mean_ to say "COMPARE EQUAL to the data ..."?

Consider this example:

	static struct foo {char a, b;} x, y = {2,3};
	fwrite(&x, sizeof x, 1, f);
	rewind(f);
	fread(&y, sizeof y, 1, f);

In this case, it is not possible to ask the question x == y because
== is not defined on structs.  So "compare equal" cannot refer to ==
but must refer to something like memcmp().  But I always did accept that

	char dummy;
	char *p = &dummy;
	char block[sizeof p];

	memcpy(block, &p, sizeof p);
	fwrite(&p, sizeof p, 1, f);
	rewind(f);
	fread(&p, sizeof p, 1, f);

must yield a state in which

	memcmp(block, &p, sizeof p) ==> 0

The question is whether the fact that the current value of p has the
same _bytes_ that it used to have means that it is necessarily the
same _pointer_.  The thing is, the wide licence allowed to casts has
taught me to be very cautious about assuming much when pointers and
bits are involved.  I know that casts are conversions, not merely type
relabellings, and that fwrite() and fread() do not involve casts.  I
mention casts only to explain why I am perhaps excessively wary.