---
name: "cacos"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/cacos"
---

# cacos(3)

complex arc cosine

## NAME

cacos, cacosf, cacosl - complex arc cosine

## LIBRARY

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

## SYNOPSIS

```
#include <complex.h>
```

**double complex cacos(double complex** *z***);** **float complex cacosf(float complex** *z***);** **long double complex cacosl(long double complex** *z***);**

## DESCRIPTION

These functions calculate the complex arc cosine of *z*. If *y = cacos(z)*, then *z = ccos(y)*. The real part of *y* is chosen in the interval \[0,pi\].

One has:

```
    cacos(z) = -i * clog(z + i * csqrt(1 - z * z))
```

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **cacos**(), **cacosf**(), **cacosl**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

glibc 2.1. C99, POSIX.1-2001.

## EXAMPLES

```
/* Link with "-lm" */
#include <complex.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
    double complex z, c, f;
    double complex i = I;
    if (argc != 3) {
        fprintf(stderr, "Usage: %s <real> <imag>\n", argv[0]);
        exit(EXIT_FAILURE);
    }
    z = atof(argv[1]) + atof(argv[2]) * I;
    c = cacos(z);
    printf("cacos() = %6.3f %6.3f*i\n", creal(c), cimag(c));
    f = -i * clog(z + i * csqrt(1 - z * z));
    printf("formula = %6.3f %6.3f*i\n", creal(f), cimag(f));
    exit(EXIT_SUCCESS);
}
```

## SEE ALSO

[**ccos**(3)](/ccos/3.md), [**clog**(3)](/clog/3.md), [**complex**(7)](/complex/7.md)