# NAME

cacos, cacosf, cacosl - complex arc cosine

# LIBRARY

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

# 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))``

# ATTRIBUTES

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

 Interface Attribute Value cacos(), cacosf(), cacosl() Thread safety MT-Safe

# STANDARDS

C11, POSIX.1-2008.

# HISTORY

glibc 2.1. C99, POSIX.1-2001.

# EXAMPLES

``````/* 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);
}``````