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

**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:

- 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 |

# 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.

# SEE ALSO

2023-02-05 | Linux man-pages 6.03 |