Section (3) expm1
expm1, expm1f, expm1l — exponential minus 1
||long double x
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
near zero—a case where exp(x) − 1 would be
inaccurate due to subtraction of two numbers that are nearly
On success, these functions return exp(x) − 1.
x is a NaN, a
NaN is returned.
x is +0
(−0), +0 (−0) is returned.
x is positive
infinity, positive infinity is returned.
x is negative
infinity, −1 is returned.
If the result overflows, a range error occurs, and the
functions return -
HUGE_VALF, or -
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
errnois 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).
For some large negative
x values (where the function
result approaches −1),
expm1() raises a bogus underflow
For some large positive
expm1() raises a bogus invalid
floating-point exception in addition to the expected overflow
exception, and returns a NaN instead of positive
Before version 2.11, the glibc implementation did not set
errno to ERANGE when a range error occurred.
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Modified 2002-07-27 Walter Harms