Function sigaddset 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn sigaddset(
    set: *mut sigset_t,
    signo: c_int,
) -> c_int

See https://pubs.opengroup.org/onlinepubs/9799919799/functions/sigaddset.html.

Adds the individual signal specified by signo to the signal set pointed to by set.

Upon success, returns 0. Upon failure, returns -1 and sets errno to indicate the error.

Safety

The sigset_t pointed to by set must be initialized by sigemptyset() or sigfillset() before calling this function or undefined behaviour will occur.