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# Technical Reference: Base Operating System and Extensions, Volume 1

## log2, log2f, or log2l Subroutine

### Purpose

Computes base 2 logarithm.

### Syntax

#include <math.h>
double log2 (x)
double x;
float log2f (x)
float x;
long double log2l (x)
long double x;

### Description

The log2, log2f, and log2l subroutines compute the base 2 logarithm of the x parameter, log_{2} (x).

An application wishing to check for error situations should set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these subroutines. Upon return, if errno is nonzero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW)
is nonzero, an error has occurred.

### Parameters

x |
Specifies the value to be computed. |

### Return Values

Upon successful completion, the log2, log2f, and log2l subroutines return the base 2
logarithm of x.

If x is ±0, a pole error occurs and the log2, log2f, and log2l subroutines 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.

### Related Information

feclearexcept Subroutine and fetestexcept Subroutine.

math.h in AIX 5L Version 5.2 Files Reference.

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