# expm1

```EXPM1(3)                   Linux Programmer's Manual                  EXPM1(3)

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():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _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

DESCRIPTION
These functions return a value equivalent to

exp(x) - 1

The 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  sub-
traction 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  at-
tributes(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 float-
ing-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.