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# EXPM1

Section: Linux Programmer's Manual (3)
Updated: 2008-08-05

## 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);

```

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

expm1(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE; or cc -std=c99
expm1f(), expm1l(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; 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
An overflow floating-point exception (FE_OVERFLOW) is raised.

These functions do not set errno.

## 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.

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

## COLOPHON

This page is part of release 3.21 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/.

## Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
BUGS
COLOPHON

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