#include <stdio.h>

char *find_char(char const *source, char const *chars)
{
    int i, j;
    if ((*source != NULL) && (*chars != NULL))
    {
        for (j=0; *(source+j) != '\0'; j++)
        {
            for (i=0; *(chars+i) != '\0'; i++)
            {
                if (*(source+j) == *(chars+i))
                {
                    printf("i = %d\n", j);
                    return ((char *)(source+j));    //注：source类型为(char const *)，函数类型为(char *)，返回时需要进行强制类型转换
                }
            }
        }
        return NULL;
    }
    else
        return NULL;
}

int main()
{
    char source[] = "ABCDEB";
    char chars[] = "EF";

    char *p = find_char(source, chars);

    if (p != NULL)
    {
        printf("Find the substr at %d\n", p-source);
    }
    else
        printf("Substr no found!");

    getchar();
    return 0;
}