```LOGB(3P)                  POSIX Programmer's Manual                 LOGB(3P)
```

## PROLOG         top

```       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         top

```       logb, logbf, logbl — radix-independent exponent
```

## SYNOPSIS         top

```       #include <math.h>

double logb(double x);
float logbf(float x);
long double logbl(long double x);
```

## DESCRIPTION         top

```       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 logr |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         top

```       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         top

```       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         top

```       None.
```

## APPLICATION USAGE         top

```       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         top

```       None.
```

## FUTURE DIRECTIONS         top

```       None.
```

## SEE ALSO         top

```       feclearexcept(3p), fetestexcept(3p), ilogb(3p), scalbln(3p)

The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment
of Error Conditions for Mathematical Functions, float.h(0p),
math.h(0p)
```

## COPYRIGHT         top

```       Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .

Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                            LOGB(3P)
```

Pages that refer to this page: math.h(0p)ilogb(3p)