ACOS(3POSIX) POSIX Programmer's Manual ACOS(3POSIX)
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
this interface may differ (consult the corresponding Linux manual page for details of
Linux behavior), or the interface may not be implemented on Linux.
acos, acosf, acosl — arc cosine functions
double acos(double x);
float acosf(float x);
long double acosl(long double x);
The functionality described on this reference page is aligned with the ISO C standard. Any
conflict between the requirements described here and the ISO C standard is unintentional.
This volume of POSIX.1‐2008 defers to the ISO C standard.
These functions shall compute the principal value of the arc cosine of their argument x.
The value of x should be in the range [−1,1].
An application wishing to check for error situations should set errno to zero and call
feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-
zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero,
an error has occurred.
Upon successful completion, these functions shall return the arc cosine of x, in the range
For finite values of x not in the range [−1,1], a domain error shall occur, and either a
NaN (if supported), or an implementation-defined value shall be returned.
If x is NaN, a NaN shall be returned.
If x is +1, +0 shall be returned.
If x is ±Inf, a domain error shall occur, and a NaN shall be returned.
These functions shall fail if:
The x argument is finite and is not in the range [−1,1], or is ±Inf.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then
errno shall be set to [EDOM]. If the integer expression (math_errhandling &
MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception shall
The following sections are informative.
On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling &
MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero.
cos(), feclearexcept(), fetestexcept(), isnan()
The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment of Error Conditions
for Mathematical Functions, <math.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrep‐
ancy between this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original Standard can be
obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 ACOS(3POSIX)