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Linux 2.6 - man page for acoshl (linux section 3)

ACOSH(3)						     Linux Programmer's Manual							  ACOSH(3)

acosh, acoshf, acoshl - inverse hyperbolic cosine function
#include <math.h> double acosh(double x); float acoshf(float x); long double acoshl(long double x); Link with -lm. Feature Test Macro Requirements for glibc (see feature_test_macros(7)): acosh(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L; or cc -std=c99 acoshf(), acoshl(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L; or cc -std=c99
The acosh() function calculates the inverse hyperbolic cosine of x; that is the value whose hyperbolic cosine is x.
On success, these functions return the inverse hyperbolic cosine of x. If x is a NaN, a NaN is returned. If x is +1, +0 is returned. If x is positive infinity, positive infinity is returned. If x is less than 1, a domain error occurs, and the functions return a NaN.
See math_error(7) for information on how to determine whether an error has occurred when calling these functions. The following errors can occur: Domain error: x is less than 1 errno is set to EDOM. An invalid floating-point exception (FE_INVALID) is raised.
C99, POSIX.1-2001. The variant returning double also conforms to SVr4, 4.3BSD, C89.
asinh(3), atanh(3), cacosh(3), cosh(3), sinh(3), tanh(3)
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at 2010-09-20 ACOSH(3)