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

# NAME¶

expm1, expm1f, expm1l - exponential minus 1# SYNOPSIS¶

#include <math.h>

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

*-lm*.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

**expm1**():

**expm1f**(),

**expm1l**():

# DESCRIPTION¶

These functions return a value equivalent toexp(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 |

# 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)# COLOPHON¶

This page is part of release 5.03 of the Linux*man-pages*project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

2017-09-15 |