# log1p(3) — Linux manual page

```log1p(3)                Library Functions Manual                log1p(3)
```

## NAME         top

```       log1p, log1pf, log1pl -  logarithm of 1 plus argument
```

## LIBRARY         top

```       Math library (libm, -lm)
```

## SYNOPSIS         top

```       #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

log1pf(), log1pl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

## DESCRIPTION         top

```       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.
```

## RETURN VALUE         top

```       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.
```

## ERRORS         top

```       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.
```

## ATTRIBUTES         top

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

┌──────────────────────────────────────┬───────────────┬─────────┐
│Interface                             │ Attribute     │ Value   │
├──────────────────────────────────────┼───────────────┼─────────┤
│log1p(), log1pf(), log1pl()           │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
```

## STANDARDS         top

```       C11, POSIX.1-2008.
```

## HISTORY         top

```       C99, POSIX.1-2001.
```

## BUGS         top

```       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.
```

```       exp(3), expm1(3), log(3)