无法使用 C 中的函数指针进行字符串比较

int compare_first_characters(const void *p, const void *q) {
    return strcmp(*(char **)p, *(char **)q);
}

1. 这完全是无稽之谈，并调用未定义的行为。 不等同于*(char **)p(char *)p
2. strcmp比较整个字符串，而不是第一个字符。

如果要比较字符串

int compare_first_characters(const void *p, const void *q) {
    return !strcmp(p, q);
}

如果只有第一个字符

int compare_first_characters(const void *p, const void *q) {
    const char *a = p, *b = q;
    return *a == *b;
}

https://godbolt.org/z/MMEra35vY

调用的两个比较函数均无效。bsearch

该函数按如下方式声明bsearch

void *bsearch(const void *key, const void *base,
              size_t nmemb, size_t size,
              int (*compar)(const void *, const void *));

这必须通过指向它的指针通过引用传递。key

但是，您调用的函数如下

const void *found = bsearch(key, words, n, sizeof(words[0]), compare_first_characters);
                            ^^^  

因此，在第一个比较函数中，这种双重取消引用

int compare_first_characters(const void *p, const void *q) {
    return **(char **)p - **(char **)q;
}

对应于根据其声明实际具有类型的参数的指针keyconst char *

const char *key = "g";

调用未定义的行为。你错了，使用第一个比较功能，程序可以正常工作。例如，尝试使用另一个字符串作为键，例如 ."y"

您必须编写函数调用，例如

const void *found = bsearch( &key, words, n, sizeof( words[0] ), compare_first_characters );
                             ^^^^

请注意，该函数比较整个字符串。因此，要使用第二个比较函数，您需要指定一个字符串文本，该文本等于数组的字符串之一：在这种情况下，两个比较函数都可以正常工作。strcmp

相对于第一个比较函数，您应该将 void 指针转换为无符号字符类型。const unsigned char **

这是您更新的程序。

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

int compare_first_characters1( const void *p, const void *q ) {
    return **( const unsigned char ** )p - **( const unsigned char ** )q;
}

int compare_first_characters2( const void *p, const void *q ) {
    return strcmp( *( char ** )p, *( char ** )q );
}

int main( void )
{
    const char *words[] = { "red", "blue", "green", "yellow" };
    int n = sizeof( words ) / sizeof( *words );
    const char *key1 = "y";

    qsort( words, n, sizeof( words[0] ), compare_first_characters1 );
    printf( "Sorted by first letter: \n" );
    for (int i = 0; i < n; i++)
    {
        printf( "%s ", words[i] );
    }
    printf( "\n" );
    printf( "Looking for a word that starts with '%s': ", key1 );
    const void *found = bsearch( &key1, words, n, sizeof( words[0] ), compare_first_characters1 );
    printf( "found '%s'\n", found ? *( const char ** )found : "none" );

    putchar( '\n' );

    const char *key2 = "yellow";
    qsort( words, n, sizeof( words[0] ), compare_first_characters2 );
    printf( "Sorted by first letter: \n" );
    for (int i = 0; i < n; i++)
    {
        printf( "%s ", words[i] );
    }
    printf( "\n" );

    printf( "Looking for a word that starts with '%s': ", key2 );
    found = bsearch( &key2, words, n, sizeof( words[0] ), compare_first_characters2 );
    printf( "found '%s'\n", found ? *( const char ** )found : "none" );
}

程序输出为

Sorted by first letter:
blue green red yellow
Looking for a word that starts with 'y': found 'yellow'

Sorted by first letter:
blue green red yellow
Looking for a word that starts with 'yellow': found 'yellow'

顺便说一句，由于数组中的所有字符串都与第一个字符不同，因此您可以使用第一个比较函数对数组进行排序，然后搜索目标字符串，例如，或者在调用中使用第二个比较函数。"yellow""green"bsearch

这是更新以前的程序。

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

int compare_first_characters1( const void *p, const void *q ) {
    return **( const unsigned char ** )p - **( const unsigned char ** )q;
}

int compare_first_characters2( const void *p, const void *q ) {
    return strcmp( *( char ** )p, *( char ** )q );
}

int main( void )
{
    const char *words[] = { "red", "blue", "green", "yellow" };
    int n = sizeof( words ) / sizeof( *words );
    const char *key = "green";

    qsort( words, n, sizeof( words[0] ), compare_first_characters1 );
    printf( "Sorted by first letter: \n" );
    for (int i = 0; i < n; i++)
    {
        printf( "%s ", words[i] );
    }
    printf( "\n" );
    printf( "Looking for a word that starts with '%s': ", key );
    const void *found = bsearch( &key, words, n, sizeof( words[0] ), compare_first_characters1 );
    printf( "found '%s'\n", found ? *( const char ** )found : "none" );
}

它的输出是

Sorted by first letter:
blue green red yellow
Looking for a word that starts with 'green': found 'green'