C语言求两个字符串的最长公共子串

#include "stdio.h"
#include "string.h"
#include "stdlib.h"

void getCommon(char str1[],char str2[],char * str3);
int stringLength(char * str);

void main(){
char str1[50];
char str2[50];
char str3[50];
gets(str1);
gets(str2);
getCommon(str1,str2,str3);
printf("%s\n",str3);
// printf("%d\n",stringLength(str2));
}
int stringLength(char * str){
int len = 0;
while(*str != '\0'){
len++;
str++;
}
return len;
}
void getCommon(char str1[],char str2[],char * str3){
int len1,len2;
int i,j,k;
int max=0;
int start = -1;

len1 = stringLength(str1);
len2 = stringLength(str2);
for(i = 0; i < len1; i++){
for(j = 0; j < len2; j++){
if(str1[i] == str2[j]){
for(k = 0; (str1[i+k] == str2[j+k] && str1[i+k] != '\0'); k++);
if(max < k){
max = k;
start = i;
}
}
}
}

if(start == -1){
str3[0] = '\0';;
}else{
memcpy(str3,&str1[start],max);
str3[max] = '\0';
}
}