#include <math.h>

double log1p(double x);

float log1pf(float x);

long double log1pl(long double x);

Link with -lm.

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



or cc -std=c99

log1pf(), log1pl():


or cc -std=c99


log1p(x) returns a value equivalent to

    log (1 + x)

It 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

An invalid floating-point exception (FE_INVALID) is raised.

Pole error: x is -1

A divide-by-zero floating-point exception (FE_DIVBYZERO) is raised.

These functions do not set errno.


Multithreading (see pthreads(7))

The log1p(), log1pf(), and log1pl() functions are thread-safe.


C99, POSIX.1-2001.

RELATED TO log1pl…


This page is part of release 3.74 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at