---
title: log2
description: "计算以2为底的对数值。"
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sourcePath: /source/zh/kunpengboostkithistory/240RC2/accel/kunpengaccel_kml_16_0201.html
indexId: 78f3dc5d888864f68c896269ffa139cc4ecc8022d68dafea4d2e682681fa3dc476
---
# log2

计算以2为底的对数值。

#### 接口定义

C interface：

float log2f(float x);

double log2(double x);

long double log2l(long double x);

Fortran interface：

RES = LOG2F(X);

RES = LOG2(X);


#### 参数

| 参数名 | 类型 | 描述 | 输入/输出 |
| --- | --- | --- | --- |
| x | 在log2f中，x是单精度浮点类型。 在log2中，x是双精度浮点类型。 在log2l中，x是长双精度浮点类型。 | 表示输入数据的浮点值。 | 输入 |


#### 返回值

- 返回以2为底x的对数函数值y，y ∈ (-INF, +INF)。
- 输入x为±0，返回-∞。
- 输入x为1，返回0。
- 输入x为负数，返回nan。
- 输入x为-∞，返回nan。
- 输入x为+∞，返回+∞。
- 输入x为nan，返回nan。


#### 依赖

C: "km.h"


#### 示例

C interface：

```
// typical usage
double x1 = 2.0, x2 = 0.5, x3 = 125, x4 = 5;
// special handing
double a = 0.0, b = INFINITY, c = -INFINITY, d = NAN, e = 1.0;
// print result
printf("base-5 logarithm of 125 = %.15f\n", log2(x3)/log2(x4));
printf("log2(1.0) = %.15f\n", log2(e));
printf("log2(2.0) = %.15f\n", log2(x1));
printf("log2(0.5) = %.15f\n", log2(x2));
printf("log2(0.0) = %.15f\n", log2(a));
printf("log2(INFINITY) = %.15f\n", log2(b));
printf("log2(-INFINITY) = %.15f\n", log2(c));
printf("log2(NAN) = %.15f\n", log2(d));
/*
*  base-5 logarithm of 125 = 3.000000000000000
*  log2(1.0) = 0.000000000000000
*  log2(2.0) = 1.000000000000000
*  log2(0.5) = -1.000000000000000
*  log2(0.0) = -inf
*  log2(INFINITY) = inf
*  log2(-INFINITY) = nan
*  log2(NAN) = nan
*
* */
```

Fortran interface：

```
REAL(8) :: X = 0.5
PRINT*,  LOG2(X)
!
! OUTPUT
!     -1.000000000000000
!
```