# Manual Reference Pages  - EXPM1 (3)

### NAME

expm1, expm1f, expm1l - exponential minus 1

Synopsis
Description
Return Value
Errors
Bugs
Colophon

### 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 || _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.

### 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.