# Arch manual pages

 CACOS(3) Linux Programmer's Manual CACOS(3)

# 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

# CONFORMING TO

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);
exit(EXIT_FAILURE);
}
z = atof(argv) + atof(argv) * 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);
}
```