Incorrect Result from cosine function with inf input

[Jackson Vanover]

Using version 2.6 of GSL from ftp://ftp.gnu.org/gnu/gsl/gsl-2.6.tar.gz

OS is Ubuntu 18.04.2 LTS

Hardware is a Dell workstation with a 3.60 GHz Intel i7-4790 and 32 GB
of RAM

Compiler is gcc 7.4.0 with the -w option

Description: when passing positive infinity to gsl_sf_cos_pi, the
result is 1 which doesn't make sense as the cosine function oscillates
between -1 and 1

#include <gsl/gsl_sf.h>
#include <stdio.h>
#include <gsl/gsl_math.h>

int main(void){
    double out;
out = gsl_sf_cos_pi(GSL_POSINF);

printf("%f\n", out);
    return 0;
}

Best,
Jackson Vanover