ieee(3m) — RISC
Name
copysign, drem, finite, logb, scalb − copysign, remainder, exponent manipulations
Syntax
#include <math.h>
double copysign(x,y)
double x,y;
double drem(x,y)
double x,y;
int finite(x)
double x;
double logb(x)
double x;
double scalb(x,n)
double x;
int n;
Description
These functions are required, or recommended by the IEEE standard 754 for floating−point arithmetic.
The copysign function returns x with its sign changed to y’s.
The drem(x,y) function returns the remainder r := x − n∗y where n is the integer nearest the exact value of x/y. Additionally if |n−x/y|=1/2, then n is even. Consequently the remainder is computed exactly and |r| ≤ |y|/2. Note that drem(x,0) is the exception (see DIAGNOTICS).
Finite(x)= 1 just when −∞ < x < +∞,
= 0 otherwise(when |x| = ∞ or x is NaN)
The logb(x)returns a signed integer converted to double−precision floating−point and so chosen that 1 ≤ |x|/2**n < 2 unless x = 0 or |x| = ∞ or x lies between 0 and the Underflow Threshold.
Scalb(x,n) = x∗(2**n) computed, for integer n, without first computing 2**N.
Diagnostics
IEEE 754 defines drem(x,0) and drem(∞,y) to be invalid operations that produce a NaN.
IEEE 754 defines logb(±∞) = +∞ and logb(0) = −∞, and requires the latter to signal Division−by−Zero.
Restrictions
IEEE 754 currently specifies that logb(denormalized no.) = logb(tiniest normalized no. > 0) but the consensus has changed to the specification in the new proposed IEEE standard p854, namely that logb(x) satisfy
1 ≤ scalb(|x|,−logb(x)) < Radix ... = 2 for IEEE 754
for every x except 0, ∞ and NaN. Almost every program that assumes 754’s specification will work correctly if logb follows 854’s specification instead.
IEEE 754 requires copysign(x,NaN) = ±x but says nothing else about the sign of a NaN.