Paul Eggert wrote:
> It is referring to the behavior of the VAX floating point unit, where the
> hardware representation that one thinks would represent -0.0 behaves
> sort of like a NaN. To avoid this problem, on a VAX copysign(0, -1)
> returns +0.0, not the usual -0.0. That is why the C and POSIX
> standards are worded the way they are.
Thanks for explaining. I have submitted the attached change request to the
Linux man pages maintainer.
> I suggested copysign instead of the signbit macro because copysign has
> been around a lot longer -- it was in the 1985 IEEE floating point
> standard -- and is optimized better. But if you want to play it safer
> (and slower :-) you can use signbit.
Ah, there is also signbit()! Thanks for mentioning it. It's optimized better
(here with gcc-4.3 on i386):
-----------------------------------------------------------------------
#include <math.h>
int using_signbit (long double x) { return signbit (x); }
int using_copysign (long double x) { return copysign (1.0L, x) < 0; }
-----------------------------------------------------------------------
is compiled into
using_signbit:
movl 12(%esp), %eax
andl $32768, %eax
ret
using_copysign:
subl $12, %esp
fldt 16(%esp)
fstpl (%esp)
movl 4(%esp), %edx
fld1
testl %edx, %edx
jns .L2
fstp %st(0)
fld1
fchs
.L2:
fldz
fucompp
fnstsw %ax
testb $69, %ah
sete %al
addl $12, %esp
movzbl %al, %eax
ret
--- man-pages-2.43/man3/copysign.3.bak 2005-10-19 09:07:02.000000000 +0200
+++ man-pages-2.43/man3/copysign.3 2007-04-01 01:02:21.000000000 +0200
@@ -26,7 +26,8 @@
.\" 386BSD man pages
.\" Modified 1993-07-24 by Rik Faith ([EMAIL PROTECTED])
.\" Modified 2002-08-10 by Walter Harms ([EMAIL PROTECTED])
-.TH COPYSIGN 3 2002-08-10 "GNU" "Linux Programmer's Manual"
+.\" Modified 2007-03-31 by Bruno Haible <[EMAIL PROTECTED]>
+.TH COPYSIGN 3 2007-03-31 "GNU" "Linux Programmer's Manual"
.SH NAME
copysign, copysignf, copysignl \- copy sign of a number
.SH SYNOPSIS
@@ -46,7 +47,8 @@
that of \fIx\fP, but whose sign matches that of \fIy\fP.
If \fIx\fP is a NaN, then a NaN with the sign of \fIy\fP is returned.
.SH NOTES
-The \fBcopysign\fP() functions may treat a negative zero as positive.
+On architectures where the floating-point formats are not IEEE 754 compliant,
+the \fBcopysign\fP() functions may treat a negative zero as positive.
.SH "CONFORMING TO"
C99, 4.3BSD.
This function is defined in IEC 559 (and the appendix with
