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ILOGB(3) Library Functions Manual ILOGB(3)

ilogb, ilogbf, ilogbl
extract exponent

#include <math.h>
int
ilogb(double x);
int
ilogbf(float x);
int
ilogbl(long double x);

ilogb() returns x's exponent n, in integer format. ilogb(±infinity) returns INT_MAX and ilogb(0) returns INT_MIN. The ilogbf() function is a single precision version of ilogb(). The ilogbl() function is an extended precision version of ilogb().

ffs(3), frexp(3)

IEEE Std 754-1985

The ilogb, ilogbf and ilogbl functions appeared in 4.3BSD, NetBSD 1.1 and OpenBSD 4.5, respectively.
April 26, 2016 OpenBSD-current