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[Linux manuál]

# logb, logbf, logbl: exponent nezávislý na radixech

Originální popis anglicky: logb, logbf, logbl - radix-independent exponent

Návod, kniha: POSIX Programmer's Manual

## STRUČNĚ

#include <math.h>

double logb(double x);

float logbf(float x);

long double logbl(long double x);

## POPIS / INSTRUKCE

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.

## NÁVRATOVÁ HODNOTA

Upon successful completion, these functions shall return the exponent of x.
If x is ±0, a pole error shall occur and logb(), logbf(), and logbl() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.
If x is NaN, a NaN shall be returned.
If x is ±Inf, +Inf shall be returned.

## CHYBY / ERRORY

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.

The following sections are informative.

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.

None.

None.

## SOUVISEJÍCÍ

feclearexcept() , fetestexcept() , ilogb() , scalb() , the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical Functions, <float.h>, <math.h> Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
 2003 IEEE/The Open Group
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