bsearch() 总是返回空指针

Question

我正在尝试在预先排序的数组中查找用户输入的字符串。如果我编写自己的二分查找函数，输入就会被正确找到。如果我使用 C bsearch，我总是得到一个 NULL 指针。

这是相关的代码片段：

printf(bsearch(&input, *words, curr_idx + 1, max_len,
               (int (*)(const void *, const void *))strcmp) ?
                        "YES" : "NO");

char input[max_len] 是 scanf("%s", input); uppercase(input);

的结果

char **words 是预排序的大写字符串数组

int curr_idx 是 words

的最大索引

int max_len 是 words 中单词的最大长度（以字节为单位）（当前为 18）

我已经尝试输入我知道在数组中的字符串，以及我知道不在数组中的字符串，并且每种情况 returns 一个 NULL 指针。

在gdb中设置断点并检查input和words的内容，似乎没有任何错误：

(gdb) print (char*)input
 = 0x7fffffffe610 "STONE"

(gdb) print words[150980]
 = 0x555555bf45a0 "STONE"

编辑以添加 MCVE：

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

char **words;
char *dictionary[5] = { "STOMPS", "STONABLE", "STONE", "STONEBOAT", "STONEBOATS" };
int curr_idx = 4;
int max_len = 18;

int compare(const void *a, const void *b)
{
    return strcmp((const char *)a, (const char *)b);
}

void uppercase(char *input)
{
    char *t = input;
    while (*t) {
        *t = toupper((unsigned char)*t);
        t++;
    }
}

int main()
{
        words = malloc((curr_idx + 1) * sizeof(char *));
        int i;
        for (i = 0; i < 5; i++){
               // words[i] = malloc(sizeof(char) * max_len);
               words[i] = dictionary[i];
        }

        char input[max_len];

    printf("Enter a word: ");
    scanf("%s", input);
    uppercase(input);
    printf(bsearch(input, words, curr_idx + 1, sizeof(*words), compare) ?
               "YES\n" :
               "NO\n");
}

malloc() 位是不必要的，但旨在尽可能接近地复制原始程序。

Answer 1

这是您的代码的简化版本：

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

int compare(const void *a, const void *b)
{
    return strcmp(a, b);
}

int main(void)
{
    const char *words[] = { "A" };
    puts(bsearch("A", words, 1, sizeof *words, compare) ?
            "YES" :
            "NO");
}

问题是 bsearch 使用指向当前数组元素的指针调用您的 compare 函数（作为第二个参数，即；第一个参数始终是作为第一个参数提供给 bsearch 的键指针。

您的数组元素是指针 (char *)，因此 compare 接收到指向 char 的指针。要使 strcmp 工作，您需要取消引用该指针：

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

int compare(const void *a, const void *b)
{
    const char *const *pstr = b;
    return strcmp(a, *pstr);
}

int main(void)
{
    const char *words[] = { "A" };
    puts(bsearch("A", words, 1, sizeof *words, compare) ?
            "YES" :
            "NO");
}

Answer 2

你的比较函数有误。您正在比较字符指针数组中的条目，因此向比较函数传递了两个伪装成 const void * 的 char ** 值。您的比较代码需要做出相应的反应。

int compare(const void *v1, const void *v2)
{
    const char *s1 = *(char **)v1;
    const char *s2 = *(char **)v2;
    // printf("[%s] <=> [%s] = %d\n", s1, s2, strcmp(s1, s2));
    return strcmp(s1, s2);
}

然后还需要正确调用 bsearch 函数：

char *key = input;
bsearch(&key, words, 5, sizeof(*words), compare);

这个比较函数相对于其他一些函数的优势在于，同一个比较器可以与 qsort() 一起使用来对数组中的数据进行排序，从而确保比较是……错误的……可比的。您可以设计其他驱动 bsearch() 的方法（参见 melpomene's ），因为 bsearch() 总是将键作为第一个参数传递给比较函数。但是在我看来，使用相同的比较器进行排序和搜索（而不是需要两个不同的函数）的能力似乎超过了不总是取消引用键的好处。

工作代码：

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

static char **words;
static char *dictionary[] = { "STOMPS", "STONABLE", "STONE", "STONEBOAT", "STONEBOATS" };
static int curr_idx = 4;
static int max_len = 18;

#if 0
int compare(const void *a, const void *b)
{
    return strcmp((const char *)a, (const char *)b);
}

#endif

static int compare(const void *v1, const void *v2)
{
    const char *s1 = *(char **)v1;
    const char *s2 = *(char **)v2;
     printf("%s(): [%s] <=> [%s] = %d\n", __func__, s1, s2, strcmp(s1, s2));
    return strcmp(s1, s2);
}

static void uppercase(char *input)
{
    char *t = input;
    while (*t)
    {
        *t = toupper((unsigned char)*t);
        t++;
    }
}

int main(void)
{
    words = malloc((curr_idx + 1) * sizeof(char *));
    int i;
    for (i = 0; i < curr_idx + 1; i++)
    {
        // words[i] = malloc(sizeof(char) * max_len);
        words[i] = dictionary[i];
    }

    char input[max_len];

    char fmt[10];
    snprintf(fmt, sizeof(fmt), "%%%ds", max_len - 1);
    while (printf("Enter a word: ") > 0 && scanf(fmt, input) == 1)
    {
        uppercase(input);
        char *key = input;
        char **result = bsearch(&key, words, curr_idx + 1, sizeof(*words), compare);
        if (result != 0)
            printf("Key [%s] found at %p [%s]\n", input, result, *result);
        else
            printf("Key [%s] not found\n", input);
    }
    putchar('\n');

    return 0;
}

示例运行（程序名称bs11）：

$ ./bs11
Enter a word: abracadabra
compare(): [ABRACADABRA] <=> [STONE] = -18
compare(): [ABRACADABRA] <=> [STONABLE] = -18
compare(): [ABRACADABRA] <=> [STOMPS] = -18
Key [ABRACADABRA] not found
Enter a word: STOMPS
compare(): [STOMPS] <=> [STONE] = -1
compare(): [STOMPS] <=> [STONABLE] = -1
compare(): [STOMPS] <=> [STOMPS] = 0
Key [STOMPS] found at 0x7fa1e4402a70 [STOMPS]
Enter a word: stona
compare(): [STONA] <=> [STONE] = -4
compare(): [STONA] <=> [STONABLE] = -66
compare(): [STONA] <=> [STOMPS] = 1
Key [STONA] not found
Enter a word: STONABLE
compare(): [STONABLE] <=> [STONE] = -4
compare(): [STONABLE] <=> [STONABLE] = 0
Key [STONABLE] found at 0x7fa1e4402a78 [STONABLE]
Enter a word: stonable
compare(): [STONABLE] <=> [STONE] = -4
compare(): [STONABLE] <=> [STONABLE] = 0
Key [STONABLE] found at 0x7fa1e4402a78 [STONABLE]
Enter a word: stonc
compare(): [STONC] <=> [STONE] = -2
compare(): [STONC] <=> [STONABLE] = 2
Key [STONC] not found
Enter a word: STONE
compare(): [STONE] <=> [STONE] = 0
Key [STONE] found at 0x7fa1e4402a80 [STONE]
Enter a word: stobneage
compare(): [STOBNEAGE] <=> [STONE] = -12
compare(): [STOBNEAGE] <=> [STONABLE] = -12
compare(): [STOBNEAGE] <=> [STOMPS] = -11
Key [STOBNEAGE] not found
Enter a word: STONEBOAT
compare(): [STONEBOAT] <=> [STONE] = 66
compare(): [STONEBOAT] <=> [STONEBOATS] = -83
compare(): [STONEBOAT] <=> [STONEBOAT] = 0
Key [STONEBOAT] found at 0x7fa1e4402a88 [STONEBOAT]
Enter a word: stoneboatatdawn
compare(): [STONEBOATATDAWN] <=> [STONE] = 66
compare(): [STONEBOATATDAWN] <=> [STONEBOATS] = -18
compare(): [STONEBOATATDAWN] <=> [STONEBOAT] = 65
Key [STONEBOATATDAWN] not found
Enter a word: STONEBOATS
compare(): [STONEBOATS] <=> [STONE] = 66
compare(): [STONEBOATS] <=> [STONEBOATS] = 0
Key [STONEBOATS] found at 0x7fa1e4402a90 [STONEBOATS]
Enter a word: zoo
compare(): [ZOO] <=> [STONE] = 7
compare(): [ZOO] <=> [STONEBOATS] = 7
Key [ZOO] not found
Enter a word: ^D
$

bsearch() 总是返回空指针

bsearch() always returning null pointer

c

bsearch