expm1(3)

NAME#

expm1, expm1f, expm1l - exponential minus 1

LIBRARY#

Math library (libm, -lm)

SYNOPSIS#

#include <math.h>

double expm1(double \nx\n);\n
\nfloat expm1f(float \nx\n);\n
\nlong double expm1l(long double \nx\n);

expm1():

\n
    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
\n
        || _XOPEN_SOURCE >= 500
\n
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
\n
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

expm1f(), expm1l():

\n
    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
\n
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
\n
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION#

These functions return a value equivalent to

\n
    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).

STANDARDS#

C11, POSIX.1-2008.

HISTORY#

C99, POSIX.1-2001. BSD.

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#

exp(3), log(3), log1p(3)