两层指针的参数

#cat redirect.h
#ifndef REDIRECT_PTR_H
#define REDIRECT_PTR_H

extern void get_a_day(const char **);

#endif

#cat redirect.c
#include "redirect_prt.h"

static const char *msg[] = {
    "Sunday","Monday","Tuesday","Thursday","Friday","Saturday"
};

void get_a_day(const char **pp){
    static int i = 0;
    *pp = msg[i%7];
    i++;
}

#cat main.c
#include <stdio.h>
#include "redirect.h"

int main(void){
    const char *firstday = NULL;
    const char *secondday = NULL;
    get_a_day(&firstday);
    get_a_day(&secondday);
    printf("%s\t%s\n",firstday,secondday);
    return 0;
}