expm1 man page

expm1, expm1f, expm1l — exponential minus 1


#include <math.h>

double expm1(double x);

float expm1f(float x);

long double expm1l(long double x);

Link with -lm.

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


|| _XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

expm1f(), expm1l():

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE


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.


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.


For an explanation of the terms used in this section, see attributes(7).

expm1(), expm1f(), expm1l()Thread safetyMT-Safe

Conforming to

C99, POSIX.1-2001, POSIX.1-2008.


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)

Referenced By

exp(3), log1p(3).

Explore man page connections for expm1(3).

expm1f(3) and expm1l(3) are aliases of expm1(3).