# logb (3p)

# 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(doublex); float logbf(floatx); long double logbl(long doublex);

# 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.1‐2008 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

*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;

*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 (

These functions may fail if:
*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.- 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.# SEE ALSO

*,*

*feclearexcept*()*,*

*fetestexcept*()*,*

*ilogb*()*The Base Definitions volume of POSIX.1‐2008,*

*scalbln*()*Section 4.19*,

*Treatment of Error Conditions for Mathematical Functions*,

*,*

**<float.h>**

**<math.h>**