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
. The value of x
should be in the range [−1,1].
An application wishing to check for error situations should set errno
zero and call feclearexcept
(FE_ALL_EXCEPT) before calling these
functions. On return, if errno
is non-zero or
(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
, in the range [0,π] radians.
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.
is NaN, a NaN shall be returned.
is +1, +0 shall be returned.
is ±Inf, a domain error shall occur, and a NaN shall be
These functions shall fail if:
- Domain Error
- The x argument is finite and is not in the range
[−1,1], or is ±Inf.
The following sections are informative.
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 be raised.
On error, the expressions ( math_errhandling
& MATH_ERRNO) and (
& MATH_ERREXCEPT) are independent of each other,
but at least one of them must be non-zero.
The Base Definitions volume of POSIX.1‐2008, Section 4.19
Treatment of Error Conditions for Mathematical Functions
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 discrepancy 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 .
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