```LOGB(3)                   Linux Programmer's Manual                  LOGB(3)
```

## NAME         top

```       logb, logbf, logbl - get exponent of a floating-point value
```

## SYNOPSIS         top

```       #include <math.h>

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

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

logb():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
logbf(), logbl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

## DESCRIPTION         top

```       These functions extract the exponent from the internal floating-point
representation of x and return it as a floating-point value.  The
used for the system's floating-point representation.  If FLT_RADIX is
2, logb(x) is equal to floor(log2(x)), except that it is probably
faster.

If x is subnormal, logb() returns the exponent x would have if it
were normalized.
```

## RETURN VALUE         top

```       On success, these functions return the exponent of x.

If x is a NaN, a NaN is returned.

If x is zero, then a pole error occurs, and the functions return
-HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.

If x is negative infinity or positive infinity, then positive
infinity is returned.
```

## ERRORS         top

```       See math_error(7) for information on how to determine whether an
error has occurred when calling these functions.

The following errors can occur:

Pole error: x is 0
A divide-by-zero floating-point exception (FE_DIVBYZERO) is
raised.

These functions do not set errno.
```

## ATTRIBUTES         top

```       For an explanation of the terms used in this section, see
attributes(7).

┌─────────────────────────┬───────────────┬─────────┐
│Interface                │ Attribute     │ Value   │
├─────────────────────────┼───────────────┼─────────┤
│logb(), logbf(), logbl() │ Thread safety │ MT-Safe │
└─────────────────────────┴───────────────┴─────────┘
```

## CONFORMING TO         top

```       C99, POSIX.1-2001, POSIX.1-2008.
```

```       ilogb(3), log(3)
```       This page is part of release 4.10 of the Linux man-pages project.  A