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LOG(3) Linux Programmer's Manual LOG(3)

NAME

`     log, logf, logl - natural logarithmic function`

SYNOPSIS

`     #include <math.h>`
```     double log(double x);
float logf(float x);
long double logl(long double x);```
`     Link with -lm.`
` Feature Test Macro Requirements for glibc (see feature_test_macros(7)):`
```     logf(), logl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE```

DESCRIPTION

`     These functions return the natural logarithm of x.`

RETURN VALUE

`     On success, these functions return the natural logarithm of x.`
`     If x is a NaN, a NaN is returned.`
`     If x is 1, the result is +0.`
`     If x is positive infinity, positive infinity 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 (including negative infinity),  then  a  domain  error
occurs, and a NaN (not a number) is returned.```

ERRORS

```     See  math_error(7) for information on how to determine whether an error
has occurred when calling these functions.```
`     The following errors can occur:`
```     Domain error: x is negative
errno is set  to  EDOM.   An  invalid  floating-point  exception
(FE_INVALID) is raised.```
```     Pole error: x is zero
errno  is set to ERANGE.  A divide-by-zero floating-point excep-
tion (FE_DIVBYZERO) is raised.```

ATTRIBUTES

```     For  an  explanation  of  the  terms  used   in   this   section,   see
attributes(7).```
```     +----------------------+---------------+---------+
|Interface             | Attribute     | Value   |
+----------------------+---------------+---------+
|log(), logf(), logl() | Thread safety | MT-Safe |
+----------------------+---------------+---------+```

CONFORMING TO

`     C99, POSIX.1-2001, POSIX.1-2008.`
`     The variant returning double also conforms to SVr4, 4.3BSD, C89.`

BUGS

```     In glibc 2.5 and earlier, taking the log() of a NaN  produces  a  bogus
invalid floating-point (FE_INVALID) exception.```

`     cbrt(3), clog(3), log10(3), log1p(3), log2(3), sqrt(3)`
```     This  page  is  part of release 4.16 of the Linux man-pages project.  A
`                                2017-09-15                            LOG(3)`