expm1(3) Library Functions Manual expm1(3)

# NAME

expm1, expm1f, expm1l - exponential minus 1

# LIBRARY

Math library (libm, -lm)

# 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 <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE```

expm1f(), expm1l():

```
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc <= 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 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:

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

# STANDARDS

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

# BUGS

Before glibc 2.17, on certain architectures (e.g., x86, but not x86_64) expm1() raised a bogus underflow floating-point exception for some large negative x values (where the function result approaches -1).

Before approximately glibc 2.11, expm1() raised a bogus invalid floating-point exception in addition to the expected overflow exception, and returned a NaN instead of positive infinity, for some large positive x values.

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