Function setting_get 

pub unsafe extern "C" fn setting_get(
    context: *mut c_context,
    key: *const c_char,
    callback: get_string_callback,
    user_data: *mut c_void,
) -> err

@defgroup settings Nix configuration settings @{ / /** @brief Retrieves a setting from the nix global configuration.

This function requires nix_libutil_init() to be called at least once prior to its use.

@param[out] context optional, Stores error information @param[in] key The key of the setting to retrieve. @param[in] callback Called with the setting value. @param[in] user_data optional, arbitrary data, passed to the callback when it’s called. @see nix_get_string_callback @return NIX_ERR_KEY if the setting is unknown, or NIX_OK if the setting was retrieved successfully.