code: func CloseOnExec(fd int, set bool) error {
  	flag := uintptr(0)
  	if set {
  		flag = syscall.FD_CLOEXEC
  	}
  	_, _, err := syscall.RawSyscall(syscall.SYS_FCNTL, uintptr(fd), syscall.F_SETFD, flag)
  	if err != 0 {
  		return syscall.Errno(err)
  	}
  	return nil
  }
docs: // CloseOnExec sets or clears FD_CLOEXEC flag on a file descriptor