【问题标题】：C bsearch causing segmentation faultC bsearch 导致分段错误
【发布时间】：2012-11-21 21:17:39
【问题描述】：

我有函数调用 userInteractive(*anangramInfo)，它传入结构 anangramInfo 的指针，这个结构包含指向实际字谜的指针“anagramPointer”。所以，我从用户那里得到了标准输入。然后使用它作为 bsearch 的键来查找指针，这些字谜具有相同的 anangram.sorted。然后将指针向左移动，直到键与 anagram.sorted 不匹配，然后移回右侧 printf anagram.word，直到 anagram.sorted 不匹配。但这导致了我的分割问题

#include <string.h>
#include <errno.h>
#include <ctype.h>
#include "anagrams.h"
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>

#define SIZE 80

//struct
struct anagram {
    char word[SIZE];
    char sorted[SIZE];
};

struct anagramInfo {
    struct anagram *anagramPtr;
    int             numOfAnagrams;
};



/* qsort struct comparision function (product C-string field) */ 
int sortedMemberCompare( const void *ptr1, const void *ptr2 )
{ 
    struct anagram *ia = (struct anagram *)ptr1;
    struct anagram *ib = (struct anagram *)ptr2;

    return strcmp(ia->sorted, ib->sorted);
    /* strcmp functions works exactly as expected from
    comparison function */ 
}

void userInteractive( struct anagramInfo *anagramInfoPtr ){

    struct anagram a;
    char data[SIZE];
    char *pos;

    printf("Enter a word to search for anagrams [^D to exit]:\n");
    fgets(data, SIZE, stdin);

    //get ripe of the '\n'
    pos=strchr(data, '\n');
    *pos = '\0';

    strncpy(a.word,data,sizeof(data));

    //lowercase word
    int i;
    for(i=0;data[i] != '\0';i++)
      {
        data[i]=(char)tolower(data[i]);
      }

    /* sort array using qsort functions */ 
    qsort(data,strlen(data), 1, charCompare);

    strncpy(a.sorted,data,sizeof(data));

    //struct pointer to the elements
    struct anagram *ptr= (struct anagram *)bsearch(a.sorted,anagramInfoPtr-> anagramPtr ->sorted,anagramInfoPtr-> numOfAnagrams,sizeof(struct anagram),sortedMemberCompare);

    printf(ptr->word);

    while(strncmp(ptr ->sorted,a.sorted,sizeof(a.sorted))==0) {
        ptr--;
    }

    while(strncmp(ptr ->sorted,a.sorted,sizeof(a.sorted))==0) {
        printf(ptr ->word);
        printf(" ");
        ptr++;
    }

}

【问题讨论】：

也许有些字符串恰好有 80 个字符长？ ->> 请阅读 strncpy() 的手册页
如何初始化 anagramInfo 结构体？
我在主代码中初始化它
@maerics 为什么要转移到programmers.se？这是一个关于特定编程实现问题的问题，它恰好是 StackOverflow 的主题，而不是programmers.se。
不要在. 或-> 运算符周围放置空格；它们比其他任何东西都更紧密（或一样紧密）。一定要在逗号后加空格。请检查fgets() 的返回值，如果报告失败则不要继续。不要在未检查 bsearch() 是否找到某些东西的情况下打印来自 bsearch() 的返回值。您尚未提供 SSCCE (Short, Self-Contained, Correct Example)，因此可以编译代码并运行，但会违反错误检查，但无法显示您的问题，因为它不是我们的全部代码需要。

标签： c bsearch

【解决方案1】：

您的疏忽是 bsearch 没有在连续数组上搜索；它甚至没有在正确的地方搜索，因为您正在搜索排序的字符串，但您应该搜索排序的字谜成员：

struct anagram *ptr = (struct anagram *)bsearch(&a,
            anagramInfoPtr-> anagramPtr,
            anagramInfoPtr-> numOfAnagrams,
            sizeof(struct anagram),
            sortedMemberCompare);

那么，当bsearch失败并返回NULL时，你不要检查ptr是NULL并使用它；因此是段错误。（您也不要在 Ctrl-D 上检查NULL）。

测试

这是一个修改后的版本 - 我添加了 charCompare 函数并用五个固定字符串创建了一个 main() 以使其正常工作。

#include <string.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>

#define SIZE 80

//struct
struct anagram {
    char word[SIZE];
    char sorted[SIZE];
};

struct anagramInfo {
    struct anagram *anagramPtr;
    int             numOfAnagrams;
};

/* qsort struct comparision function (product C-string field) */
int sortedMemberCompare( const void *ptr1, const void *ptr2 )
{
    struct anagram *ia = (struct anagram *)ptr1;
    struct anagram *ib = (struct anagram *)ptr2;

    return strcmp(ia->sorted, ib->sorted);
    /* strcmp functions works exactly as expected from
    comparison function */
}

int charCompare(const void *a, const void *b)
{
        const char *aa = a, *bb = b;
        if (*aa < *bb)
                return -1;
        if (*aa == *bb)
                return 0;
        return 1;
}

void userInteractive( struct anagramInfo *anagramInfoPtr ){

    struct anagram a;
    char data[SIZE];
    char *pos;

    printf("Enter a word to search for anagrams [^D to exit]:\n");
    fgets(data, SIZE, stdin);

    //get ripe of the '\n'
    pos=strchr(data, '\n'); // In Windows this might be a "\r" maybe?
    if (NULL == pos)
       exit(0);

    *pos = '\0';

    strncpy(a.word,data,sizeof(data));

    //lowercase word
    int i;
    for(i=0;data[i] != '\0';i++)
    {
        data[i]=(char)tolower(data[i]);
    }

    /* sort array using qsort functions */
    qsort(data, strlen(data), 1, charCompare);
    strncpy(a.sorted,data,sizeof(data));

    //struct pointer to the elements
    struct anagram *ptr = (struct anagram *)bsearch(&a,
                anagramInfoPtr-> anagramPtr,
                anagramInfoPtr-> numOfAnagrams,
                sizeof(struct anagram),
                sortedMemberCompare);

    // You must check that ptr is not NULL.
    if (NULL == ptr)
    {
        printf("Not found\n");
        return;
    }

    printf("Found: %s\n", ptr->word);

    /* Here there is a subtle error. If you find the FIRST item of the list,
       this code will position itself BEFORE THE BEGINNING of the list, and
       run a strncmp() against who knows what. Other segfault lurking! */

    while(ptr > anagramInfoPtr -> anagramPtr && strncmp(ptr ->sorted,a.sorted,strlen(a.sorted))==0) {
        ptr--;
    }

    // while(strncmp(ptr ->sorted,a.sorted,sizeof(a.sorted))==0) {
    //    ptr--;
    //}

    // Here too you should check you do not pass max number of anagrams,
    // if you hit the last item of the lot.
    while(strncmp(ptr ->sorted,a.sorted,sizeof(a.sorted))==0) {
        printf(ptr ->word);
        printf(" ");
        ptr++;
    }

}

int main()
{
        int i;
        struct anagramInfo *root;

        root = malloc(sizeof(struct anagramInfo));

        root->numOfAnagrams     = 5;
        root->anagramPtr        = malloc(root->numOfAnagrams * sizeof(struct anagram));

        strcpy(root->anagramPtr[0].word, "oriental");
        strcpy(root->anagramPtr[1].word, "teaching");
        strcpy(root->anagramPtr[2].word, "senator");
        strcpy(root->anagramPtr[3].word, "admirer");
        strcpy(root->anagramPtr[4].word, "rescued");

        for (i = 0; i < root->numOfAnagrams; i++)
        {
            int j;
            for (j = 0; root->anagramPtr[i].word[j]; j++)
               root->anagramPtr[i].word[j] = tolower(root->anagramPtr[i].word[j]);
            strcpy(root->anagramPtr[i].sorted, root->anagramPtr[i].word);
            qsort(root->anagramPtr[i].sorted, strlen(root->anagramPtr[i].sorted), 1, charCompare);
        }
        qsort(root-> anagramPtr, root-> numOfAnagrams, sizeof(struct anagram), sortedMemberCompare);
        for (;;)
            userInteractive(root);
    }

运行

$ gcc -W -Wall -o anagram anagram.c
$ ./anagram
Enter a word to search for anagrams [^D to exit]:
treason
Found: senator

Enter a word to search for anagrams [^D to exit]:
cheating
Found: teaching

Enter a word to search for anagrams [^D to exit]:
relation
Found: oriental

Enter a word to search for anagrams [^D to exit]:

$

【讨论】：