log1p, log1pf, log1pl - logarithm of 1 plus argument

Math library (`libm`

, `-lm`

)

```
#include <math.h>
double log1p(double x);
float log1pf(float x);
long double log1pl(long double x);
```

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

```
log1p():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

```
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

These functions return a value equivalent to

` log (1 + x)`

The result is computed in a way that is accurate even if the value of
`x`

is near zero.

On success, these functions return the natural logarithm of *(1 +
x)*.

If `x`

is a NaN, a NaN is returned.

If `x`

is positive infinity, positive infinity is
returned.

If `x`

is -1, a pole error occurs, and the functions return
-**HUGE_VAL**, -**HUGE_VALF**, or
-**HUGE_VALL**, respectively.

If `x`

is less than -1 (including negative infinity), a domain
error occurs, and a NaN (not a number) is returned.

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 less than -1 -
`errno`

is set to**EDOM**(but see BUGS). An invalid floating-point exception (**FE_INVALID**) is raised. - Pole error:
`x`

is -1 -
`errno`

is set to**ERANGE**(but see BUGS). A divide-by-zero floating-point exception (**FE_DIVBYZERO**) is raised.

For an explanation of the terms used in this section, see attributes(7).

Interface | Attribute | Value |

Thread safety | MT-Safe |

C11, POSIX.1-2008.

C99, POSIX.1-2001.

Before glibc 2.22, the glibc implementation did not set
`errno`

to **EDOM** when a domain error
occurred.

Before glibc 2.22, the glibc implementation did not set
`errno`

to **ERANGE** when a range error
occurred.