Check if a string is a substring of another string in c

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




int checkpan(char str[100], char c[100], char n[100]);

int main(){
int flag;
char str[100],c[100],r[100],n[100];

printf("Enter the String:   ");
gets(str);

printf("\nEnter the String 2:   ");
gets(c);

printf("\nEnter the word to replace the substring:   ");
gets(n);

flag = checkpan(str,c,n);


if (flag == 1)
printf("Substring found\n");

else
printf("Substring not found\n");

return 0;

}


int checkpan(char str[100], char c[100],char n[100]){

int i = 0, j = 0, len = 0, len2 = 0, flag=0;
char r[100];

for (i = 0; str[i] != '\0'; i++)
len++;
for (j = 0; str[j] != '\0'; j++)
len2++;

strcpy(str, r);

for (j = 0; j < len2; j++)
{
for (i = 0; i < len; i++){

if (c[j] == str[i]){
flag = 1;
r[j] = n[0];
break;
}

}
if (flag == 1)
break;
}
printf("%s", r);

if (flag == 1){
return 1;

}
else
return 0;
}