---
title: log
description: "计算实数以自然对数e为底的对数值。"
url: https://www.hikunpeng.com/document/detail/zh/kunpengboostkithistory/2200/accel/kunpengaccel_kml_16_0119.html
sourcePath: /source/zh/kunpengboostkithistory/2200/accel/kunpengaccel_kml_16_0119.html
indexId: 888368441fd023361122c2583398b13f57ac84e6a0cc580fce00d8cd66f5a5a874
---
# log

计算实数以自然对数e为底的对数值。

#### 接口定义

C interface：

float logf(float x);

double log(double x);

double log_18(double x); // 仅在高精度版本提供

Fortran interface：

RES = LOGF(X);

RES = LOG(X);


#### 参数

| 参数名 | 类型 | 描述 | 输入/输出 |
| --- | --- | --- | --- |
| x | 在logf中，x是单精度浮点类型。 在log中，x是双精度浮点类型。 在log\_18中，x是双精度浮点类型。 | 表示输入数据的浮点值。 | 输入 |


#### 返回值

- 返回以e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+∞。
- 输入x为nan，返回nan。


#### 依赖

C: "km.h"

Fortran: "km.f03"


#### 示例

C interface：

```
double pi = acos(-1);
// typical usage
double x1 = pi/2, x2 = 2.0, 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", log(x3)/log(x4));
printf("log(1.0) = %.15f\n", log(e));
printf("log(pi/2) = %.15f\n", log(x1));
printf("log(2.0) = %.15f\n", log(x2));
printf("log(0.0) = %.15f\n", log(a));
printf("log(INFINITY) = %.15f\n", log(b));
printf("log(-INFINITY) = %.15f\n", log(c));
printf("log(NAN) = %.15f\n", log(d));
/*
*  base-5 logarithm of 125 = 3.000000000000000
*  log(1.0) = 0.000000000000000
*  log(pi/2) = 0.451582705289455
*  log(2.0) = 0.693147180559945
*  log(+0.0) = -inf
*  log(INFINITY) = inf
*  log(-INFINITY) = nan
*  log(NAN) = nan
*
* */
```

Fortran interface：

```
REAL(8) :: X = 2
PRINT*,  LOG(X)
!
! OUTPUT
!     0.693147180559945
!
```