# logb(3m) [bsd man page]

IEEE(3M) IEEE(3M)NAME

copysign, drem, finite, logb, scalb - copysign, remainder, exponent manipulationsSYNOPSIS

#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 for, or recommended by the IEEE standard 754 for floating-point arithmetic. Copysign(x,y) returns x with its sign changed to y's. Drem(x,y) returns the remainder r := x - n*y where n is the integer nearest the exact value of x/y; moreover if |n-x/y|=1/2 then n is even. Consequently the remainder is computed exactly and |r| <= |y|/2. But drem(x,0) is exceptional; see below under DIAGNOSTICS. Finite(x) = 1 just when< x < +infinity, = 0 otherwise (when |x| = infinity or x is NaN or x is the VAX's reserved operand.) Logb(x) returns x's exponent n, a signed integer converted to double-precision floating-point and so chosen that 1 <= |x|/2**n < 2 unless x = 0 or (only on machines that conform to IEEE 754) |x| = infinity or x lies between 0 and the Underflow Threshold; see below under "BUGS". Scalb(x,n) = x*(2**n) computed, for integer n, without first computing 2**n.-infinityDIAGNOSTICS

IEEE 754 defines drem(x,0) and drem(infinity,y) to be invalid operations that produce a NaN. On a VAX, drem(x,0) returns the reserved op- erand. No infinity exists on a VAX. IEEE 754 defines logb(+-infinity) = +infinity and logb(0) =, requires the latter to signal Division-by-Zero. But on a VAX, logb(0) = 1.0 - 2.0**31 =-infinity,147,483,647.0. And if the correct value of scalb(x,n) would overflow on a VAX, it returns the reserved oper- and and sets errno to ERANGE.-2SEE ALSO

floor(3M), math(3M), infnan(3M)AUTHOR

Kwok-Choi NgBUGS

Should drem(x,0) and logb(0) on a VAX signal invalidity by setting errno = EDOM? Should logb(0) return -1.7e38? IEEE 754 currently specifies that logb(denormalized no.) = logb(tiniest normalized no. > 0) but the consensus has changed to the specifica- tion 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, infinity 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. A NaN (Not a Number) is similar in spirit to the VAX's reserved operand, but very different in important details. Since the sign bit of a reserved operand makes it look negative, copysign(x,reserved operand) = -x; should this return the reserved operand instead?4.3 Berkeley DistributionMay 12, 1986 IEEE(3M)

## Check Out this Related Man Page

ieee(3m) ieee(3m)Namecopysign, drem, finite, logb, scalb - copysign, remainder, exponent manipulationsSyntax#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;DescriptionThese functions are required, or recommended by the IEEE standard 754 for floating-point arithmetic. The function returns x with its sign changed to y's. The 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 is the exception (see Diagnostics). Finite(x) = 1 just when< x < +infinity, = 0 otherwise (when |x| = infinity or x is NaN) The a signed integer converted to double-precision floating-point and so chosen that 1 <= |x|/2**n < 2 unless x = 0 or |x| = infinity or x lies between 0 and the Underflow Threshold. Scalb(x,n) = x*(2**n) computed, for integer n, without first computing 2**N.-infinityDiagnosticsIEEE 754 defines drem(x,0) and drem(infinity,y) to be invalid operations that produce a NaN. IEEE 754 defines logb(+-infinity) = +infinity and logb(0) =, requires the latter to signal Division-by-Zero.-infinityRestrictionsIEEE 754 currently specifies that logb(denormalized no.) = logb(tiniest normalized no. > 0) but the consensus has changed to the specifica- tion 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, infinity 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.See Alsofloor(3M), fp_class(3), math(3M) RISC ieee(3m)