# cacos(3) — Linux manual page

```cacos(3)                Library Functions Manual                cacos(3)
```

## NAME         top

```       cacos, cacosf, cacosl - complex arc cosine
```

## LIBRARY         top

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

## SYNOPSIS         top

```       #include <complex.h>

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

## DESCRIPTION         top

```       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         top

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

## STANDARDS         top

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

## HISTORY         top

```       glibc 2.1.  C99, POSIX.1-2001.
```

## EXAMPLES         top

```       /* 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);
}
```

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