# logb

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           logb, logbf, logbl — radix-independent exponent
    


## SYNOPSIS



           #include <math.h>
    
           double logb(double _x_);
           float logbf(float _x_);
           long double logbl(long double _x_);
    


## DESCRIPTION



           The functionality described on this reference page is aligned with
           the ISO C standard. Any conflict between the requirements
           described here and the ISO C standard is unintentional. This
           volume of POSIX.1‐2017 defers to the ISO C standard.
    
           These functions shall compute the exponent of _x_ , which is the
           integral part of log _r_ |_x_ |, as a signed floating-point value, for
           non-zero _x_ , where _r_ is the radix of the machine's floating-point
           arithmetic, which is the value of FLT_RADIX defined in the
           _< float.h>_ header.
    
           If _x_ is subnormal it is treated as though it were normalized; thus
           for finite positive _x_ :
    
               1 <= _x_ * FLT_RADIX-logb(x) < FLT_RADIX
    
           An application wishing to check for error situations should set
           _errno_ to zero and call _feclearexcept_(FE_ALL_EXCEPT) before calling
           these functions. On return, if _errno_ is non-zero or
           _fetestexcept_(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW |
           FE_UNDERFLOW) is non-zero, an error has occurred.
    


## RETURN VALUE



           Upon successful completion, these functions shall return the
           exponent of _x_.
    
           If _x_ is ±0, _logb_(), _logbf_(), and _logbl_() shall return -HUGE_VAL,
           -HUGE_VALF, and -HUGE_VALL, respectively.
    
           On systems that support the IEC 60559 Floating-Point option, a
           pole error shall occur;
           otherwise, a pole error may occur.
    
           If _x_ is NaN, a NaN shall be returned.
    
           If _x_ is ±Inf, +Inf shall be returned.
    


## ERRORS



           These functions shall fail if:
    
           Pole Error  The value of _x_ is ±0.
    
                       If the integer expression (_math_errhandling_ &
                       MATH_ERRNO) is non-zero, then _errno_ shall be set to
                       **[ERANGE]**.  If the integer expression (_math_errhandling_
                       & MATH_ERREXCEPT) is non-zero, then the divide-by-zero
                       floating-point exception shall be raised.
    
           These functions may fail if:
    
           Pole Error  The value of _x_ is 0.
    
                       If the integer expression (_math_errhandling_ &
                       MATH_ERRNO) is non-zero, then _errno_ shall be set to
                       **[ERANGE]**.  If the integer expression (_math_errhandling_
                       & MATH_ERREXCEPT) is non-zero, then the divide-by-zero
                       floating-point exception shall be raised.
    
           _The following sections are informative._
    


## EXAMPLES



           None.
    


## APPLICATION USAGE



           On error, the expressions (_math_errhandling_ & MATH_ERRNO) and
           (_math_errhandling_ & MATH_ERREXCEPT) are independent of each other,
           but at least one of them must be non-zero.
    


## RATIONALE



           None.
    


## FUTURE DIRECTIONS



           None.
    


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