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**NAME**

expm1, expm1f, expm1l - exponential minus 1

**SYNOPSIS**

#include<math.h>doubleexpm1(doublex);floatexpm1f(floatx);longdoubleexpm1l(longdoublex);Link with-lm. Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):expm1(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L; orcc-std=c99expm1f(),expm1l(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L; orcc-std=c99

**DESCRIPTION**

expm1(x)returns a value equivalent to exp(x) - 1 It is computed in a way that is accurate even if the value ofxis near zero—a case whereexp(x)-1would be inaccurate due to subtraction of two numbers that are nearly equal.

**RETURN** **VALUE**

On success, these functions returnexp(x)-1. Ifxis a NaN, a NaN is returned. Ifxis +0 (-0), +0 (-0) is returned. Ifxis positive infinity, positive infinity is returned. Ifxis 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**

Seemath_error(7) for information on how to determine whether an error has occurred when calling these functions. The following errors can occur: Range error, overflowerrnois set toERANGE(but see BUGS). An overflow floating-point exception (FE_OVERFLOW) is raised.

**CONFORMING** **TO**

C99, POSIX.1-2001.

**BUGS**

For some large negativexvalues (where the function result approaches -1),expm1() raises a bogus underflow floating-point exception. For some large positivexvalues,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 seterrnotoERANGEwhen a range error occurred.

**SEE** **ALSO**

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

**COLOPHON**

This page is part of release 3.35 of the Linuxman-pagesproject. A description of the project, and information about reporting bugs, can be found at http://man7.org/linux/man- pages/. 2010-09-12 EXPM1(3)