# EXPM1

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
ATTRIBUTES
CONFORMING TO
BUGS
SEE ALSO
COLOPHON

## NAME

expm1, expm1f, expm1l − exponential minus 1

## SYNOPSIS

#include <math.h>

double expm1(double x);
float expm1f(float
x);
long double expm1l(long double
x);

Link with −lm.

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

expm1():

_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99

expm1f(), expm1l():

_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99

## DESCRIPTION

expm1(x) returns a value equivalent to

exp(x) − 1

It is computed in a way that is accurate even if the value of x is near zero—a case where exp(x) − 1 would be inaccurate due to subtraction of two numbers that are nearly equal.

## RETURN VALUE

On success, these functions return exp(x) − 1.

If x is a NaN, a NaN is returned.

If x is +0 (−0), +0 (−0) is returned.

If x is positive infinity, positive infinity is returned.

If x is negative infinity, −1 is returned.

If the result overflows, a range error occurs, and the functions return -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.

## 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:
Range error, overflow

errno is set to ERANGE (but see BUGS). An overflow floating-point exception (FE_OVERFLOW) is raised.

## ATTRIBUTES

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

C99, POSIX.1-2001.

## BUGS

For some large negative x values (where the function result approaches −1), expm1() raises a bogus underflow floating-point exception.

For some large positive x values, expm1() raises a bogus invalid floating-point exception in addition to the expected overflow exception, and returns a NaN instead of positive infinity.

Before version 2.11, the glibc implementation did not set errno to ERANGE when a range error occurred.

## SEE ALSO

exp(3), log(3), log1p(3)

## COLOPHON

This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man−pages/.