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 generalpurpose math library.
 libmsve
 (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:
 If you want only selected processes to run with the SVE version, you can include both libraries in your OS image
and use the l m or l msve option to
qcc
to link explicitly against the appropriate one.
 If you want all processes to use the SVE version, include libmsve.so in your OS image
and set up a symbolic link from libm.so to libmsve.so.
Use the l m option to
qcc
to link against the library.
Note:
Compile your program with the fnobuiltin option to prevent the compiler from using a
builtin version of the function.
Description:
The ilogb(), ilogbf(), and ilogbl() functions
compute the integral part of:
log_{r} 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:
 Call feclearexcept(FE_ALL_EXCEPT) before calling
ilogb(), ilogbf(), or ilogbl().
 On return, if fetestexcept(FE_ALL_EXCEPT)
is nonzero, then an error has occurred.
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 floatingpoint number.
Classification:
C11,
POSIX 1003.1
Safety: 

Cancellation point 
No 
Interrupt handler 
Yes 
Signal handler 
Yes 
Thread 
Yes 