'\" t .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk) .\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk .\" .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" References consulted: .\" Linux libc source code .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991) .\" 386BSD man pages .\" Modified 1993-07-24 by Rik Faith (faith@cs.unc.edu) .\" Modified 2002-07-25 by Walter Harms .\" (walter.harms@informatik.uni-oldenburg.de) .\" .TH acos 3 2024-05-02 "Linux man-pages 6.9.1" .SH NAME acos, acosf, acosl \- arc cosine function .SH LIBRARY Math library .RI ( libm ", " \-lm ) .SH SYNOPSIS .nf .B #include .P .BI "double acos(double " x ); .BI "float acosf(float " x ); .BI "long double acosl(long double " x ); .fi .P .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .P .BR acosf (), .BR acosl (): .nf _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19: */ _DEFAULT_SOURCE || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE .fi .SH DESCRIPTION These functions calculate the arc cosine of .IR x ; that is the value whose cosine is .IR x . .SH RETURN VALUE On success, these functions return the arc cosine of .I x in radians; the return value is in the range [0,\ pi]. .P If .I x is a NaN, a NaN is returned. .P If .I x is +1, +0 is returned. .P If .I x is positive infinity or negative infinity, a domain error occurs, and a NaN is returned. .P If .I x is outside the range [\-1,\ 1], a domain error occurs, and a NaN is returned. .SH ERRORS See .BR math_error (7) for information on how to determine whether an error has occurred when calling these functions. .P The following errors can occur: .TP Domain error: \fIx\fP is outside the range [\-1,\ 1] .I errno is set to .BR EDOM . An invalid floating-point exception .RB ( FE_INVALID ) is raised. .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lbx lb lb l l l. Interface Attribute Value T{ .na .nh .BR acos (), .BR acosf (), .BR acosl () T} Thread safety MT-Safe .TE .SH STANDARDS C11, POSIX.1-2008. .SH HISTORY C99, POSIX.1-2001. .P The variant returning .I double also conforms to C89, SVr4, 4.3BSD. .SH SEE ALSO .BR asin (3), .BR atan (3), .BR atan2 (3), .BR cacos (3), .BR cos (3), .BR sin (3), .BR tan (3)