## Name

cacos, cacosf, cacosl — complex arc cosine

## Synopsis

```#include <complex.h>
```
 ```double complex cacos(``` double complex z`)`;

 ```float complex cacosf(``` float complex z`)`;

 ```long double complex cacosl(``` long double complex z`)`;

Note Link with `−lm`.

## DESCRIPTION

The `cacos`() function calculates 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))
```

## VERSIONS

These functions first appeared in glibc in version 2.1.

C99.

## EXAMPLE

```/* Link with "−lm" */

#include <complex.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.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);
}
```

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