ap_set_flag_slot
This sets a field within a structure to either 1 or 0.
Example
AP_INIT_FLAG( "ModExampleFlag", ap_set_flag_slot, (void *)APR_OFFSETOF(mod_example_config_rec, someFlag), RSRC_CONF, "Description of this directive" )
Display
This directive would set the field named someFlag within the struct mod_example_config_rec to either 0 or 1 based on the value specified in the Apache configuration files.