/**************************************************************************
编写一函数：int substring(char *str,char *sub),判断字符串sub是否是字符串str
的子串，若是子串，则返回子串位置，否则返回-1.
***************************************************************************/

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int substring(char *str,char *sub)
{
    int lena,lenb,i = 0,j = 0;
    lena = strlen(str);
    lenb = strlen(sub);
    while(i < lena && j < lenb)
    {
        if(str[i] == sub[j])
        {
            j++;
            i++;
        }
        else
        {
            i = i + 1 - j;
            j = 0;
        }
        
    }
    if(j >= lenb)
        return i - lenb;
    return -1;
}

int main(void)
{
    char str[300];
    char sub[300];
    int m = 0;

    printf("please input str:");
    gets(str);
    printf("please input sub:");
    gets(sub);

    m = substring(str,sub);
    printf("m == %d\n",m);
    if(m == -1)
        printf("sub is not str son!\n");
    else
        printf("find the location is %d.\n",m);

    return 0;
}

while(i < lena && j < lenb)
    {
        if(str[i] == sub[j])
        {
            j++;
            i++;
        }
        else if(str[i]==sub[0])         
        {
           i++;
           j=1;
        }
        else {
            i++;
            j=0;
        }
        
    }

我这个程序的想法和你的想法是类似的，就是表述不一样。