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From: rbutterworth@watmath.UUCP
Newsgroups: comp.lang.c
Subject: ANSI really does want you to == float values.
Message-ID: <6909@watmath.UUCP>
Date: Mon, 13-Apr-87 13:30:38 EST
Article-I.D.: watmath.6909
Posted: Mon Apr 13 13:30:38 1987
Date-Received: Tue, 14-Apr-87 00:44:12 EST
Distribution: comp
Organization: U of Waterloo, Ontario
Lines: 40

Since most people seem to be yelling into the wind and not
noticing what the others are saying, this is probably a bad
time to submit this, but anyway.

The ANSI standard defines functions such as strtod() that return
a double value, HUGE_VAL, upon failure.  That seems to imply that
the following pieces of code all should do the comparison the same
way.

    if (HUGE_VAL == strtod(args)) ...

    val = strtod(args);
    if (val == HUGE_VAL) ...

    bad = HUGE_VAL;
    val = strtod(args);
    if (val == bad) ...

Now does that mean that HUGE_VAL (and -HUGE_VAL) are defined so
that the comparisons will work correctly regardless of registers,
guard bits, optimization, etc.?  If so, do such numbers really
exist on all machines?
Probably not.  e.g. if "val" and "bad" above are registers with
extra guard bits that aren't automatically zeroed when the values
are loaded, they usually won't compare equal.

On the other hand if there isn't supposed to be anything special
about these numbers, does that mean that the compiler should
generate correct code for all doubles?  If so, is this even
possible on all machines?
Again, probably not.  At least not unless you are willing to put
up with some very inefficient code.  (e.g. whenever any two floating
values are compared they are first normalized and stored in temporary
memory somewhere.)

Or is the compiler itself supposed to know that HUGE_VAL is
special and generate "correct" comparison code for that case?

Or does any of this really matter?
Who bothers to check error status returns anyway?