c - strcastr 仍然无法正常工作

Question

所以我通读了其他问题，他们被告知在任何 include 之前放置 #define _GNU_SOURCE 并且它可以工作，但它对我不起作用。我也尝试添加 #define _GNU_SOURCE char *strcasetr(const char *haystack, const char *needle); 但仍然不起作用。我找不到关于此的任何其他信息，也许有人可以提供帮助吗？提前致谢。

错误:函数“strcastr”的隐式声明

/**
 *
 * 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

strcasetr 函数在标准 Windows 构建环境中不可用。它不是 C 标准库的一部分，仅随特定平台和构建环境一起提供。

但是，您可以编写自己的版本。这是一个基于原始字符串匹配算法的简单算法。使用 Rabin-Karp、Boyer-Moore 或 Knuth-Morris-Pratt 算法可能会做得更好:

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;
}