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 sub-
traction 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 at-
tributes(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 float-
ing-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)
Linux man-pages 6.03 2023-02-05 expm1(3)
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