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c99 [flag... ]file... -lm[library... ] #include <math.h>

**double logb**(**double ***x*);

**float logbf**(**float ***x*);

**long double logbl**(**long double ***x*);

cc [flag... ]file... -lm[library... ] #include <math.h>

**double logb**(**double ***x*);

**float logbf**(**float ***x*);

**long double logbl**(**long double ***x*);

These functions 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.

Upon successful completion, these functions return the exponent of *x*.

If *x* is subnormal:

•
For SUSv3-conforming applications compiled with the **c99** compiler driver (see **standards**(5)), the exponent of *x* as if *x* were normalized is returned.

•
Otherwise, if compiled with the **cc** compiler driver, -1022, -126, and -16382 are returned for **logb()**, **logbf()**, and **logbl()**, respectively.

If *x* is ±0, a pole error occurs and **logb()**, **logbf()**, and **logbl()** return -**HUGE_VAL**, -**HUGE_VALF**, and -**HUGE_VALL**, respectively.

If *x* is NaN, a NaN is returned.

If *x* is ±Inf, +Inf is returned.

These functions will fail if:

**Pole Error**
The value of *x* is ±0.

If the integer expression (**math_errhandling** & **MATH_ERREXCEPT**) is non-zero, the divide-by-zero floating-point exception is raised.

The **logb()** function sets **errno** to **EDOM** if the value of *x* is 0.

An application wanting to check for exceptions should call **feclearexcept**(**FE_ALL_EXCEPT**) before calling these functions. On return, if **fetestexcept**(**FE_INVALID** | **FE_DIVBYZERO** | **FE_OVERFLOW**
| **FE_UNDERFLOW**) is non-zero, an exception has been raised. An application should either examine the return value or check the floating point exception flags to detect exceptions.

An application can also set **errno** to 0 before calling **logb()**. On return, if **errno** is non-zero, an error has occurred. The **logbf()** and **logbl()** functions do not set **errno**.

See **attributes**(5) for descriptions of the following attributes:

ATTRIBUTE TYPE | ATTRIBUTE VALUE |

Interface Stability | Standard |

MT-Level | MT-Safe |

**feclearexcept**(3M), **fetestexcept**(3M), **ilogb**(3M), **math.h**(3HEAD), **matherr**(3M), **scalb**(3M), **attributes**(5), **standards**(5)

- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUES
- ERRORS
- USAGE
- ATTRIBUTES
- SEE ALSO

This document was created by man2html, using the manual pages.

Time: 02:39:32 GMT, October 02, 2010