# cacos

```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 = ca-
cos(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 at-
tributes(7).

+-----------------------------+---------------+---------+
|Interface                    | Attribute     | Value   |
+-----------------------------+---------------+---------+
|cacos(), cacosf(), cacosl()  | Thread safety | MT-Safe |
+-----------------------------+---------------+---------+
CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.

EXAMPLE

#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);
}