cacos, cacosf, cacosl — complex arc cosine


#include <complex.h>

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

Link with -lm.


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))


These functions first appeared in glibc in version 2.1.


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.


/* Link with "-lm" */

#include <complex.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>

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]);

    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));


See Also

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


This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at

Referenced By

acos(3), ccos(3), complex(7).

The man pages cacosf(3) and cacosl(3) are aliases of cacos(3).

2019-03-06 Linux Programmer's Manual