ilogb(), ilogbf(), ilogbl()

Updated: October 26, 2022

Compute the integral part of a logarithm

Synopsis:

#include <math.h>

int ilogb( double x );

int ilogbf( float x );

int ilogbl( long double x );

Arguments:

x
The number for which you want to compute the integral part of the logarithm.

Library:

libm
The general-purpose math library.
libm-sve
(QNX Neutrino 7.1 or later) A library that optimizes the code for ARMv8.2 chips that have Scalable Vector Extension hardware.

Your system requirements will determine how you should work with these libraries:

Note: Compile your program with the -fno-builtin option to prevent the compiler from using a built-in version of the function.

Description:

The ilogb(), ilogbf(), and ilogbl() functions compute the integral part of:

logr |x|

as a signed integral value, for nonzero finite x, where r is the radix of the machine's floating point arithmetic.

To check for error situations, use feclearexcept() and fetestexcept(). For example:

Returns:

The exponent part of x, in integer format.

If x is: These functions return: Errors:
0.0 FP_ILOGB0
NaN FP_ILOGBNAN
±Inf INT_MAX

These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.

Classification:

C11, POSIX 1003.1

Safety:  
Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes