# acos(3p) — Linux manual page

```ACOS(3P)                POSIX Programmer's Manual               ACOS(3P)
```

## PROLOG         top

```       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.
```

## NAME         top

```       acos, acosf, acosl — arc cosine functions
```

## SYNOPSIS         top

```       #include <math.h>

double acos(double x);
float acosf(float x);
long double acosl(long double x);
```

## DESCRIPTION         top

```       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.1‐2017 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.
```

## RETURN VALUE         top

```       Upon successful completion, these functions shall return the arc
cosine of x, 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.

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.
```

## ERRORS         top

```       These functions shall fail if:

Domain Error
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 be raised.

The following sections are informative.
```

## EXAMPLES         top

```       None.
```

## APPLICATION USAGE         top

```       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.
```

## RATIONALE         top

```       None.
```

## FUTURE DIRECTIONS         top

```       None.
```

```       cos(3p), feclearexcept(3p), fetestexcept(3p), isnan(3p)

The Base Definitions volume of POSIX.1‐2017, Section 4.20,
Treatment of Error Conditions for Mathematical Functions,
math.h(0p)
```

```       Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group.  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.opengroup.org/unix/online.html .