The decc$feature_get_name function returns a pointer to a null-
    terminated string containing the name of the feature for the
    entry specified by index. The index value can be 0 to the highest
    allocated feature. If there is no feature corresponding to the
    index value, then the function returns a NULL pointer.

    On error, NULL is returned and errno is set to indicate the
    error.

    See also decc$feature_get, decc$feature_get_index, decc$feature_
    get_value, decc$feature_set, decc$feature_set_value,
    decc$feature_show, and decc$feature_show_all.