# acos(3) — Linux manual page

```acos(3)                 Library Functions Manual                 acos(3)
```

## NAME         top

```       acos, acosf, acosl - arc cosine function
```

## LIBRARY         top

```       Math library (libm, -lm)
```

## SYNOPSIS         top

```       #include <math.h>

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

Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):

acosf(), acosl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

## DESCRIPTION         top

```       These functions calculate the arc cosine of x; that is the value
whose cosine is x.
```

## RETURN VALUE         top

```       On success, these functions return the arc cosine of x in
radians; the return value is in the range [0, pi].

If x is a NaN, a NaN is returned.

If x is +1, +0 is returned.

If x is positive infinity or negative infinity, a domain error
occurs, and a NaN is returned.

If x is outside the range [-1, 1], a domain error occurs, and a
NaN is returned.
```

## ERRORS         top

```       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 outside the range [-1, 1]
errno is set to EDOM.  An invalid floating-point exception
(FE_INVALID) is raised.
```

## ATTRIBUTES         top

```       For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface                           │ Attribute     │ Value   │
├─────────────────────────────────────┼───────────────┼─────────┤
│ acos(), acosf(), acosl()            │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
```

## STANDARDS         top

```       C11, POSIX.1-2008.
```

## HISTORY         top

```       C99, POSIX.1-2001.

The variant returning double also conforms to C89, SVr4, 4.3BSD.
```

```       asin(3), atan(3), atan2(3), cacos(3), cos(3), sin(3), tan(3)