# expm1 (3)

# NAME

expm1, expm1f, expm1l - exponential minus 1# SYNOPSIS

#include <math.h>

double expm1(doublex);float expm1f(floatx);long double expm1l(long doublex);

Link with

*-lm*.

Feature Test Macro Requirements for glibc (see

**feature_test_macros**(7)):

**expm1**():

_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

**expm1f**(),

**expm1l**():

_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 toexp(x) - 1The result 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).

Interface | Attribute | Value |

expm1 (), expm1f (), expm1l () | Thread safety | MT-Safe |

# CONFORMING TO

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