Linux manual pages Section 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);
 
[Note] Note
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
[Note] Note

Link with −lm.

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

CONFORMING TO

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 version 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 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

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