# man pages section 3: Extended Library Functions, Volume 2

Updated: Thursday, June 13, 2019

## log2f(3M)

### Name

log2, log2f, log2l - compute base 2 logarithm functions

### Synopsis

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

double log2(double x);```
`float log2f(float x);`
`long double log2l(long double x);`

### Description

These functions compute the base 2 logarithm of their argument x, log2(x).

### Return Values

Upon successful completion, these functions return the base 2 logarithm of x.

If x is ±0, a pole error occurs and log2(), log2f(), and log2l() return −HUGE_VAL, −HUGE_VALF, and −HUGE_VALL, respectively.

For finite values of x that are less than 0, or if x is −Inf a domain error occurs and a NaN is returned.

If x is NaN, a NaN is returned.

If x is 1, +0 is returned.

If x is +Inf, x is returned.

### Errors

These functions will fail if:

Domain Error

The finite value of x is less than 0, or x is −Inf.

If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception is raised.

Pole Error

The value of x is 0.

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

### Usage

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.

### Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
Availability
system/library/math
MT-Level
MT-Safe
Standard