Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!amdcad!ames!ucbcad!ucbvax!hplabs!pyramid!epimass!jbuck
From: jbuck@epimass.EPI.COM (Joe Buck)
Newsgroups: comp.unix.wizards
Subject: Re: casting to float without converting to float?
Message-ID: <1815@epimass.EPI.COM>
Date: 8 Jan 88 17:52:42 GMT
References: <11171@brl-adm.ARPA>
Reply-To: jbuck@epimass.EPI.COM (Joe Buck)
Organization: Entropic Processing, Inc., Cupertino, CA
Lines: 32

In article <11171@brl-adm.ARPA> weiser.pa@Xerox.COM writes:
>I have the following problem:  I have a value which I happen to know
>has the proper bits to represent a floating point number (either 32
>or 64).  However, it is of type int.  I would now like to tell the C
>compiler to treat this thing as a float.  How can I do it without
>taking the address of the thing (which may not have an address
>because it is in a register or is an expression) or using a
>subroutine call (for which I do not wish to pay the cost)?  

Casts are conversions.  Because of the way they work with pointers
on byte-addressible machines, many beginners seem to think that
	(type)x
means to treat the bit pattern in x as if it were of type "type",
but it most emphatically doesn't.  But, we have unions to deal with
this problem.  If you have a machine in which both ints and floats
have the same length, the following will work.

union {
    int i_format;
    float f_format;
} fubar;

#define CONVERT(x) (fubar.i_format = (x), fubar.f_format)

This is absolutely horrible and nonportable (it's not predictable if
the lengths of ints and floats isn't the same), but it will work for
your specific problem.
-- 
- Joe Buck  {uunet,ucbvax,sun,decwrl,<smart-site>}!epimass.epi.com!jbuck
	    Old internet mailers: jbuck%epimass.epi.com@uunet.uu.net

Argue for your limitations and you get to keep them.   -- Richard Bach