Architecture independent level 1 (DDI/DKI).

The free_rtn structure is referenced by the datab structure. When freeb(9F) is called to free the message, the driver's message-freeing routine (referenced through the free_rtn structure) is called, with arguments, to free the data buffer.

void     (*free_func)()      /* user's freeing routine */
char     *free_arg           /* arguments to free_func() */

The free_rtn structure is defined as type frtn_t.

esballoc(9F), freeb(9F), datab(9S)

STREAMS Programming Guide