prom_cngetc
{ prom_cnprobe, prom_cninit, prom_cngetc, prom_cnputc },
int prom_cngetc(dev_t);
prom_cngetc,
cn_tab->cn_getc == prom_cngetc);
extern int prom_cngetc(dev_t);
if (cn_tab->cn_getc == prom_cngetc) {