How to use strstr() function?

I have two files blacklist.txt and email.txt. The blacklist.txt contains some domain names. The email.txt also contains a few domain names. I have to compare both files and find the domain names of blacklist.txt into email.txt using the strstr() function. Following is the code I have written. The problem with this code is it returns me the output NULL instead of the matched text/domain name.

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

#define MAXCHAR 1000

int main() {
    FILE *fp, *fp1;
    char str[MAXCHAR];
    char str2[MAXCHAR];
    char *result;

    fp = fopen("blacklist.txt", "r");
    fp1 = fopen("email.txt", "r");
    if (fp == NULL || fp1 == NULL) { 
        printf("\n Cannot open one of the files %s %s \n", fp, fp1); 
        //return = 0; 
        if (fp != NULL) fclose(fp); 
        if (fp1 != NULL) fclose(fp1); 
    } 
    
    while (fgets(str, MAXCHAR, fp) != NULL || fgets(str2, MAXCHAR, fp1) != NULL)
    //printf("%s, %s", str,str2);
    fclose(fp);
    fclose(fp1);
    result = strstr(str, str2);
    printf("The substring starting from the given string: %s", result);
    return 0;
}

Solution

Here are some remarks on your code:

printing the error message has undefined behavior because you pass the FILE* pointers instead of the file names.
your program has undefined behavior because the body of the while loop is missing.
Since one cannot assume that both files be sorted, each line from blacklist.txt should be tested against all lines in email.txt.
if we can assume that both lines end with a newline, a match with strstr() means the line from the second file s a suffix of the line from the first file. It is a domain match if the return value is the start of the buffer and it is a match for a subdomain if the previous character is a ..

Here is a modified version:

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

#define MAXCHAR 1000

int main() {
    FILE *fp, *fp2;
    char str[MAXCHAR];
    char str2[MAXCHAR];

    // open and check blacklist.txt
    fp = fopen("blacklist.txt", "r");
    if (fp == NULL) {
        printf("Cannot open %s\n", "blacklist.txt");
        return 1;
    }
    // open and check email.txt
    fp2 = fopen("email.txt", "r");
    if (fp2 == NULL) {
        printf("Cannot open %s\n", "email.txt");
        fclose(fp);
        return 1;
    }
    
    // for each line in blacklist.txt
    while (fgets(str, MAXCHAR, fp) != NULL) {
        // restart from the beginning of email.txt
        rewind(fp2);
        // for each line of email.txt
        while (fgets(str2, MAXCHAR, fp2) != NULL) {
            // check for a domain match
            char *p = strstr(str, str2);
            if (p != NULL && (p == str || p[-1] == '.')) {
                // compute the length of the domains (excluding the newline)
                int n = strcspn(str, "\n");
                int n2 = strcspn(str2, "\n");
                // output the message with the matching domains
                printf("domain match on %.*s for %.*s\n", n2, str2, n, str);
            }
        }
    }
    fclose(fp);
    fclose(fp2);
    return 0;
}