LOGB(3M) | Mathematical Library Functions | LOGB(3M) |

logb, logbf, logbl - radix-independent exponent

c99 [flag... ]file...-lm[library... ] #include <math.h>doublelogb(doublex);

floatlogbf(floatx);

long doublelogbl(long doublex);

cc [flag... ]file...-lm[library... ] #include <math.h>doublelogb(doublex);

floatlogbf(floatx);

long doublelogbl(long doublex);

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:

- o
- For SUSv3-conforming applications compiled with the
**c99**compiler driver (see standards(7)), the exponent of*x*as if*x*were normalized is returned.

- o
- 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(7) for descriptions of the following attributes:

ATTRIBUTE TYPE | ATTRIBUTE VALUE |

Interface Stability | Standard |

MT-Level | MT-Safe |

math.h(3HEAD), feclearexcept(3M), fetestexcept(3M), ilogb(3M), matherr(3M), scalb(3M), attributes(7), standards(7)

July 12, 2006 | SunOS 5.11 |