typedef enum { ... } SilcConfigType;

    This identifies the parameter type that an option has. This parameter
    is very important because the callback's *val pointer points to a
    memory location containing the previously specified data type.
    For example, if you specified an option with an integer parameter
    callback's *val will be a pointer to an integer.

    typedef enum {
      SILC_CONFIG_ARG_TOGGLE,       /* TOGGLE on,off; yes,no; true, false; */
      SILC_CONFIG_ARG_INT,          /* callback wants an integer */
      SILC_CONFIG_ARG_STR,          /* callback expects \0-terminated str */
      SILC_CONFIG_ARG_STRE,         /* same as above, but can also be empty */
      SILC_CONFIG_ARG_BLOCK,        /* this is a sub-block */
      SILC_CONFIG_ARG_SIZE,         /* like int, but accepts suffixes kMG */
      SILC_CONFIG_ARG_NONE,         /* does not expect any args */
    } SilcConfigType;