C-Program to Search a substring in a given string

#include<stdio.h>

#include<string.h>

int main()

{

int i,j,flag=0,len1,len2;

char str[100],substr[100];

printf("Enter a string\n");

gets(str);

printf("Enter a substring to be searched\n");

gets(substr);

len1=strlen(str);

len2=strlen(substr);

for(i=0;i<=len1-len2;i++)

{

    for(j=i;j<i+len2;j++)

    {

        flag=1;

        if(str[j]!=substr[j-i])

        {

            flag=0;

            break;

        }

    }

    if(flag==1)

    break;

}

if(flag==1)

{

    printf("Search is Successful\n");

}

else

{

        printf("Search is not Successful\n");

}

return 0;

}