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#### 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);

```

#### 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))

```

#### VERSIONS

```       These functions first appeared in glibc in version 2.1.

```

#### ATTRIBUTES

```       For an explanation of the terms used in this section, see attributes(7).

┌─────────────────────────────┬───────────────┬─────────┐
│Interface                    │ Attribute     │ Value   │
├─────────────────────────────┼───────────────┼─────────┤
│cacos(), cacosf(), cacosl()  │ Thread safety │ MT-Safe │
└─────────────────────────────┴───────────────┴─────────┘
```

#### CONFORMINGTO

```       C99, POSIX.1-2001, POSIX.1-2008.

```

#### 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);
}

```

#### SEEALSO

```       ccos(3), clog(3), complex(7)

```

#### COLOPHON

```       This page is part of release 5.02 of the Linux man-pages project.  A  description  of  the