#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)):



or cc -std=c99

expm1f(), expm1l():


or cc -std=c99


expm1(x) returns a value equivalent to

    exp(x) - 1

It 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.


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.


Multithreading (see pthreads(7))

The expm1(), expm1f(), and expm1l() functions are thread-safe.


C99, POSIX.1-2001.


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.

RELATED TO expm1f…


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