c语言，任意位置插入字符或者字符串

char *insert(char *s1,char *s2,int n)
{
    int len1 = 0, len2 = 0,i,j = 0,k = 0;
    char str3[100];
    if(s1 == NULL)
    {
        return NULL;
    }
    if(s2 == NULL)
    {
        return s1;
    }
    len1 = strlen(s1);
    if(n>strlen(s1))
    {
        return "\0";
    }
    len2 = strlen(s2);
    for(i = 0;i<n;i++)
        j++;
    for(i = 0;i<len1;i++)
    {
        str3[k++] = s1[i];
    }
    for ( i = 0; i < len2; i++)
    s1[j++] = s2[i];
    for( i = n; i < len1; i++)
    s1[j++] = str3[i];
    s1[j] = '\0';
    return s1;
}