#include <stdio.h>
char *mystrchr(char *, char );
int main(void)
{
    char str[81];
    char ch;
    char *p;
    do
    {
        puts("input range string:");
        gets(str);
        puts("input match char:");
        ch = getchar();
        getchar();
        p = mystrchr(str, ch);
        if ( p )
        {
            puts("Find!");
            puts(p);
        }
        else puts("Can't find!");
        puts("input any char except q to go on.");
        gets(str);
    }
    while(*str != 'q');
    puts("Quit.");
    return 0;
}
char *mystrchr(char *p, char ch)
{
    char *p_save = p;
    if(*p == '\0') return NULL;
    while(1)
    {
        if(*p == ch)
        {
            return p_save;
        }
        else
        {
            if(*++p == '\0')
            return NULL;
            p_save = p;
        }
    }
}
mystrchr函数中return就退出函数了，那函数中最后一句是什么意思

if(*++p == '\0')
            return NULL;
if条件满足就退出，不满足执行 p_save = p;

if(*++p == '\0')
            return NULL;
            p_save = p;

这样子看就清楚

if(*++p == '\0')
    {
        return NULL;
    }
        p_save = p;