strcasestr still not working

Question

So I read through other questions and they were told to put #define _GNU_SOURCE before any include and it would work but it doesn't work for me. I also tried adding #define _GNU_SOURCE char *strcasestr(const char *haystack, const char *needle); but still doesn't work. I couldn't find anything else about this, maybe anyone can help? Thanks in advance.

Error: implicit declaration of function 'strcasestr'

/**
 *
 * Description:  This is code for Lab 3 Task 2.
 *               Reads data from file and gives opportunity to search by cities
 */
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

    printf("Please input the city you want to find employees in:");
    scanf("%s", input);
    maxline = i;
    for (i = 0; i <= maxline; i++) {
        if (strcasestr(employee[i].city, input) != 0) { // PROBLEM
            printf("%d %s %s %s\n", &employee[i].ID, employee[i].fn,
                                    employee[i].ln, employee[i].city);
            amount++;
        }
    }
    printf("%d matches out of %d members", amount, maxline);
    return 0;
}

Answer 1

The strcasestr function isn't available as part of the standard Windows build environment. It's not a part of the C standard library and ships only with certain platforms and build environments.

You can, however, code up your own version. Here's a simple one based on the naive string matching algorithm. You can possibly do better using the Rabin-Karp, Boyer-Moore, or Knuth-Morris-Pratt algorithms:

char* myStrcasestr(const char* haystack, const char* needle) {
    /* Edge case: The empty string is a substring of everything. */
    if (!needle[0]) return (char*) haystack;

    /* Loop over all possible start positions. */
    for (size_t i = 0; haystack[i]; i++) {
        bool matches = true;
        /* See if the string matches here. */
        for (size_t j = 0; needle[j]; j++) {
            /* If we're out of room in the haystack, give up. */
            if (!haystack[i + j]) return NULL;

            /* If there's a character mismatch, the needle doesn't fit here. */
            if (tolower((unsigned char)needle[j]) != 
                tolower((unsigned char)haystack[i + j])) {
                matches = false;
                break;
            }
        }
        if (matches) return (char *)(haystack + i);
    }
    return NULL;
}