# log1pl (3)

# NAME

log1p, log1pf, log1pl - logarithm of 1 plus argument# SYNOPSIS

#include <math.h>

double log1p(doublex);float log1pf(floatx);long double log1pl(long doublex);

Link with

*-lm*.

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 versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

|| _XOPEN_SOURCE >= 500

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

**log1pf**(),

**log1pl**():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

# DESCRIPTION

These functions return a value equivalent tolog (1 +The result is computed in a way that is accurate even if the value ofx)

*x*is near zero.

# RETURN VALUE

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

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

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

Interface | Attribute | Value |

log1p (), log1pf (), log1pl () | Thread safety | MT-Safe |

# CONFORMING TO

C99, POSIX.1-2001, POSIX.1-2008.# BUGS

Before version 2.22, the glibc implementation did not set*errno*to

**EDOM**when a domain error occurred. Before version 2.22, the glibc implementation did not set

*errno*to

**ERANGE**when a range error occurred.

# SEE ALSO

**exp**(3),

**expm1**(3),

**log**(3)